Carbon Dioxide Sequestration Performance of Nostoc linckia Cultivated in a Modular Photobioreactor at the Interior-Landscape Interface

Abstract

This research addresses the need for climate-resilient architecture through a biotechnological intervention at the indoor-landscape interface. It presents a modular system utilizing the cyanobacterium Nostoc linckia to regulate air quality in enclosed spaces. Grounded in biophilic design principles, the study conceptualizes photosynthetic systems as modular living interfaces that integrate metabolic processes, environmental performance, and spatial experience. Functioning as an active environmental buffer, the system relates measurable carbon sequestration performance to spatial integration and esthetic qualities. Experimental performance was evaluated using a closed atmospheric test chamber with three different CO₂ regimes: low (400–1000 ppm), medium (1000–2000 ppm), and high (2000–5000 ppm). Biomass productivity was assessed via optical density measurements at 570 nm and 650 nm and dry weight analysis. The results show that the system maintains effective carbon sequestration and biomass growth in all regimes, demonstrating its capacity to adapt to fluctuating atmospheric loads, with sequestration efficiency increasing 2.15-fold under elevated CO₂ availability. Furthermore, experimental data were used to model scaling scenarios across various workspace typologies, projecting an annual CO₂ sequestration of 1.9–27.0 kg/year and biomass production of 1.0–14.8 kg/year. These findings define the photobioreactor as a circular interface and demonstrate that biotechnological modules can contribute to ecological regenerative cycles by transforming interior-derived carbon into productive biomass for reuse at the landscape scale, validating the system as a viable circular environmental infrastructure.

1. Introduction

Contemporary architectural research increasingly prioritizes climate-resilient and cyclical approaches. Addressing these problems requires viewing buildings not as isolated entities, but as holistic systems integrating interior, exterior, and urban environments. This inter-scale interaction is increasingly focused on the modular approach, partly because the modular structure allows for small-scale interventions within a broad socio-ecological framework [1,2]. In this context, the study adopts an inclusive perspective that combines interior architecture, landscape architecture, and biotechnology, beyond considering different architectural scales holistically. With this perspective, the study investigates cyanobacteria-based photobioreactors as active agents in CO₂ sequestration at the interior-landscape interface.

A critical consequence of this challenge is the accumulation of metabolic CO₂ within enclosed spaces, which conventional dilution-based ventilation strategies are increasingly unable to resolve. Photobioreactor systems integrating photosynthetic microorganisms offer a promising biological solution to this problem by actively fixing indoor CO₂ into productive biomass. The escalating impacts of climate change, most notably within urban environments, underscore a pressing imperative to maintain carbon sequestration and air quality at sustainable levels. This urgency demands integrated design approaches that fuse natural systems with biotechnological advancements, ensuring that technical integration respects both biophilic parameters and human sensory experience. Photobioreactors, considered as biotechnological components in this study, are often viewed as technical equipment in engineering literature, yet they are known to offer numerous benefits when integrated into architecture. However, the spatial integration of such infrastructure often neglects critical design factors, particularly those related to human scale and sensory interaction. This functionalist perspective overlooks the experiential potential emphasized by biophilic design. While existing research in this field highlights the importance of creating healthier and more sustainable indoor environments [3,4], studies directly linking photobioreactors containing photosynthetic microorganisms with spatial design remain quite limited. Existing PBR research predominantly addresses engineering performance metrics such as biomass productivity, light utilization efficiency, and reactor geometry [5,6,7], without incorporating spatial design criteria, human-scale integration, or biophilic parameters. Studies that bridge biotechnological performance with interior architectural design methodology remain largely absent from the literature [1,2,8]. Notably, research linking cyanobacteria-based photobioreactors to air quality improvement, human-nature interaction, and cross-scale integration remains limited. Addressing this gap, the present study proposes a multidimensional approach, positioning biological processes not only as environmental interventions but also as potential architectural elements that aim to contribute to experiential and ecological objectives simultaneously. The primary aim of this study is to quantify the CO₂ sequestration performance of Nostoc linckia-based photobioreactors across three atmospheric load regimes (400–1000 ppm, 1000–2000 ppm, and 2000–5000 ppm) and to translate the experimentally derived sequestration kinetics into evidence-based architectural scaling scenarios at the interior-landscape interface. In doing so, the study seeks to address the absence of experimentally validated, biophilic-compatible biotechnological systems capable of functioning as active carbon sinks within architectural design frameworks, thereby contributing to both indoor air quality management and circular biomass production at the landscape scale.

To systematically examine the potential of photobioreactors in enhancing indoor air quality, Nostoc linckia was selected as the model organism due to its high photosynthetic efficiency and commercial viability. Distinctively, its biomass can be utilized as an agricultural biofertilizer due to its nitrogen-fixing metabolism [9]. This capability enables sustainable, circular solutions at the interior-landscape interface. Beyond its biological advantages, the characteristics of this species make it suitable for architectural experimentation, as its growth behavior and visual presence can be incorporated into modular design strategies without compromising spatial accessibility. In order to empirically validate these potentials, the study adopts an experimental approach testing Nostoc linckia-based photobioreactors under controlled enclosed space conditions. Experiments conducted in a closed atmospheric test chamber examine sequestration performance across varying CO₂ concentration levels. By employing scientific data not as an isolated biological analysis but as an evidence-based framework, this research demonstrates how modular biotechnological systems can be calibrated for maximal sustainability performance and meaningfully integrated into architectural design processes. Building upon experimental validation, this research further demonstrates the scalability of the proposed system through architectural application scenarios, translating laboratory-derived performance data into dimensioning strategies for real-world interior environments.

2. Theoretical Framework

This section establishes the theoretical framework that forms the basis of the study across four interconnected dimensions. First, the environmental buffering role of the interior-landscape interface is examined in relation to indoor CO₂ accumulation. This is followed by a discussion of biophilic design principles as the conceptual basis for integrating biotechnological systems into architectural space. The performance characteristics of photobioreactor systems are then reviewed, and finally, Nostoc linckia is positioned as a model organism for circular design strategies. Together, these dimensions provide the scientific rationale for the experimental approach adopted in this study.

2.1. Environmental Buffering and CO₂ Accumulation in Interior-Landscape Interface

The interface between interior and landscape is a critical spatial condition in architectural design. It functions as a physical transition zone and a mediator between inside and outside in terms of perception, ecology, and technology. Rather than belonging exclusively to either domain, these hybrid conditions represent “in-between spaces” spatial formations that carry attributes of both interiority and exteriority [10]. Various concepts are used in the literature to discuss this interdisciplinary spatiality, such as interior-exterior hybridity, thresholds, and layered spatial continuities [11,12,13]. These intermediate zones are more than just an architectural enhancement; their function as environmental buffers is to regulate air quality and reduce thermal loads on the building envelope [14].

However, the regulatory capacity of these traditional buffer zones is increasingly challenged by the dual crisis of rising atmospheric carbon dioxide (CO₂) and deteriorating outdoor air quality. While CO₂ accumulation is widely recognized as the primary driver of climate change [15], its impact on the built environment creates a specific ventilation paradox: relying on natural ventilation to dilute indoor pollutants is becoming ineffective due to high outdoor pollution levels. This condition leads to the rapid accumulation of metabolic CO₂ within enclosed spaces, transforming the interior from a shelter into a zone of potential health risk.

The implications of exposure to excess CO₂ constitute a substantial concern for occupant well-being. Beyond general health risks, elevated concentration levels are directly linked to reduced cognitive performance, fatigue, and physiological stress, which significantly undermine productivity in urban environments [16,17]. In this context, the interior-landscape interface must be redefined. It can no longer function solely as a passive “box-within-a-box” spatial volume [18]; rather, it must operate as an active, layered organizational system. Consequently, this study posits the interface as an adaptable ecological membrane, one capable of incorporating biotechnological elements, such as photobioreactor modules, to actively metabolize pollutant loads while supporting perceptual continuity across scales. In this context, the following section examines how biophilic design theory provides a conceptual basis for integrating active biotechnological systems into the indoor-landscape interface.

2.2. Biophilic Design Integration: Biotechnological Systems as Living Components

The concept of biophilic design, rooted in Edward O. Wilson’s notion of biophilia, posits that humans possess an innate, evolutionarily adapted inclination to connect with natural systems. This connection is inextricably linked to psychological well-being, cognitive performance, and physiological health [19,20,21]. While traditional biophilia emphasizes elements such as sunlight, vegetation, and water [12], biotechnological systems like photobioreactors introduce a novel paradigm: design tools inspired by the behaviors and rhythms of living organisms [22]. When evaluated through this lens, photobioreactors transcend their role as mere equipment, emerging as active spatial agents. The dynamic qualities of photosynthetic organisms, including growth patterns, chromatic variation, and responsiveness, blur the boundaries between artificial and biological systems, fostering a “technological nature” that supports a connection with living environments [23].

The integration of these systems with biophilic design theory [12] offers an integrated and multi-layered spatial experience that combines different design categories rather than a single functionality. First, the physical presence of the system is directly related to the components defined in the literature under the heading of environmental factors. The system incorporates plants and living organisms into the space through its photosynthetic living culture, while also bringing water elements and a natural air experience into the interior through the liquid culture medium and active ventilation. This physical presence transforms the system from a static structural element into a living interface that supports the experience of nature in space, as described in the literature. Secondly, the temporal and dynamic behaviors of the system find a theoretical counterpart in the category of natural patterns and processes. Fluid movements within the liquid and color changes associated with biomass provide a form of stimulation that breaks monotony, defined in the literature as sensory variability. In addition, the traceable development process of microorganisms coincides with the growth and flowering characteristics that are said to support the establishment of a psychological bond between the user and natural cycles. Furthermore, the system’s ability to metabolically adapt to changing pollutant loads functions as an element of dynamic equilibrium and tension in the spatial design. The formal and atmospheric characteristics of the system are positioned at the intersection of light and space with natural shapes and forms. Modular units employ biophilic design principles based on nature. Through interaction with artificial or natural light, the reactors function as volumetric lighting elements and dynamically adjust the atmosphere within the space [4,12,24].

In this context, the experimental study presented in this work gains significance. By simulating variable CO₂ scenarios under controlled conditions, the research provides empirical insights into the functional applicability of photobioreactors. This is a prerequisite for photobioreactors to function as biophilic components. In this sense, confirming the metabolic stability of Nostoc linckia informs spatial strategies that aim to integrate resilient biological processes into the built environment, supporting human-nature interaction through both functional and experiential dimensions. To evaluate whether photobioreactors can fulfill this role, the following section reviews the performance characteristics and carbon sequestration efficiency of PBR systems as documented in the current literature.

2.3. Photobioreactor (PBR) Performance and Carbon Sequestration Efficiency

Photobioreactors (PBRs) are controlled cultivation systems designed to optimize the growth of photosynthetic microorganisms by regulating critical parameters such as light exposure, gas exchange, and nutrient availability. In these units, CO₂ acts as the primary carbon source; it is delivered into a liquid medium where microorganisms convert it into biomass through photosynthesis [5]. Conventional CO₂ sequestration approaches, including chemical scrubbing, HVAC-integrated filtration, and nature-based solutions such as green walls, present practical limitations for indoor architectural integration, ranging from high energy demands and chemical consumables to insufficient active carbon fixation capacity [25]. In this context, PBRs offer a low-energy, ambient-temperature alternative that is more compatible with the spatial and ecological requirements of built environments. By maintaining stable conditions for photosynthetic activity, these systems enable measurable CO₂ uptake in compact volumes, making them suitable for integration into built environments where spatial and ecological performance must intersect [6,9].

The shift towards biological sequestration is driven by the inadequacy of mechanical ventilation alone, which may be insufficient to sustainably reduce indoor CO₂ levels under high occupancy conditions [26]. Within this domain, photosynthetic microorganisms are identified as efficient CO₂ converters due to their versatile metabolic pathways and ecological adaptability [9]. While species such as Chlorella, Scenedesmus, and Spirulina are frequently cited for their high tolerance to CO₂-rich environments [26,27], cyanobacteria species like Nostoc and Anabaena present a distinct advantage for architectural applications. Their ability to fix atmospheric nitrogen alongside carbon sequestration allows them to thrive in variable conditions with relatively low maintenance requirements, positioning them as promising candidates for resilient, compact PBR units [9].

Current research emphasizes that capture efficiency is influenced not only by the biological agent but also by the structural qualities of the reactor. Factors such as mass transfer efficiency, light-path distribution, and hydrodynamic behavior significantly affect carbon fixation rates [6]. Consequently, optimizing reactor geometry, whether tubular, flat-panel, or modular column configurations, is critical for maximizing CO₂ uptake performance within the spatial constraints of interior and interface designs. The theoretical dimensions outlined above collectively inform the experimental design and analytical framework presented in the following section.

2.4. Nostoc linckia as a Model Organism for Circular Design

Accelerating climate change requires architectural interventions that go beyond mitigation to incorporate regenerative biological processes. In this context, the filamentous cyanobacterium Nostoc linckia serves as a suitable model organism for circular design strategies. While microalgae and cyanobacteria are widely recognized for their efficiency in lipid production and bioenergy applications, Nostoc offers a distinct advantage for regenerative landscape strategies through its dual metabolic profile. It can combine high photosynthetic efficiency for CO₂ sequestration with a significant ability to fix atmospheric nitrogen. This capacity enables the organism to bridge the gap between atmospheric remediation and landscape restoration, offering valuable potential for improving designs and integrating them with the environment in which biological elements are produced [28,29,30].

From an evolutionary perspective, cyanobacteria, particularly Nostoc species, were the primary drivers of Earth’s oxygenated atmosphere, establishing the fundamental foundations of the carbon and nitrogen cycles [28]. Taking advantage of this evolutionary resilience offers a meaningful design pathway for connecting natural systems with the built environment. Nostoc linckia’s ability to sequester CO₂ and fix nitrogen allows it to function as both a carbon capture mechanism and a fertilizer source across various design scales [29,31].

From a circular perspective, the biomass generated following carbon capture is redefined not as waste but as a resource for soil health. Literature confirms that various photosynthetic species, including Acutodesmus dimorphus, Spirulina platensis, Chlorella vulgaris, Scenedesmus dimorphus, and Anabaena azolla, have been successfully utilized as bio-fertilizers with encouraging results [9]. However, Nostoc linckia is particularly valuable for low-input urban landscapes due to its nitrogen-fixing capability, which can enhance soil fertility without synthetic additives. According to the literature, it can be said that Nostoc linckia, used as a biofertilizer, promotes plant growth, particularly as an environmentally friendly and low-cost alternative to chemical fertilizers [29,32].

Nostoc linckia has been found to be used to increase seed germination in plants, support root/stem development, and raise the vitality index. In this context, attempts have been made to encourage germination by soaking seeds in cyanobacterial filtrate or to support root and stem development by planting germinated seeds in media containing cyanobacterial cell suspensions [32]. Sources confirm that it has been tested specifically on rice and tomatoes. Studies on other subspecies of the Nostoc genus have observed that they improve soil structure and productivity in crops such as corn and soybeans, which highlights the potential of Nostoc-based biofertilizers for grain systems, which may include N. linckia [9,29,32,33]. These findings support the assumption that biomass harvested from architectural photobioreactors can be realistically evaluated as a biofertilizer input in circular design scenarios. In this regard, using cyanobacteria as biofertilizers supports agricultural production and landscape strategies that redefine the biological functionality of soil in urban green infrastructure [29,34].

In addition to its metabolic properties, it also offers distinct esthetic characteristics, with its rich pigmentation and fibrous texture providing a unique visual environment that can be used for architectural effects. Consequently, Nostoc linckia is proposed here as a scalable, modular carbon sequestrant that simultaneously supports biophilic experience and soil regeneration cycles.

3. Materials and Methods

3.1. Experimental Setup and Environmental Control

This study employed a systematic experimental method to investigate the CO₂ capture capacity of photobioreactors within an indoor-landscape interface. To establish a scalable correlation with real-world architectural environments, a volumetric scaling approach was adopted. Based on literature estimating the typical single-person workspace volume between 10 and 25 m³ [35,36,37], a 1/100 volumetric ratio was applied. This ratio was selected to create a physically manageable sealed volume (0.25 m³) that is proportionally representative of standard single-person workspace volumes (10–25 m³) as documented in anthropometric and habitation standards [35,36,37]. The 1:100 ratio preserves the volumetric relationship between the biological module (1 L) and the represented space, enabling direct translation of laboratory-derived sequestration rates into architectural dimensioning. Consequently, a sealed 0.25 m³ acrylic atmospheric chamber was constructed to represent this volume (Figure 1a).

To ensure the isolation of the experiment, the chamber was sealed leak-tight and environmental variables were controlled. In order to avoid passive CO₂ loss, a control test was conducted using the same atmospheric chamber environment filled with distilled water instead of culture medium, starting at 5000 ppm. The CO₂ concentration was monitored under the same conditions for 24 h, and no measurable change was observed, confirming that the CO₂ decrease in the experiments was due to biological activity rather than system leakage. This result further confirms that the chamber functioned as an airtight system without passive CO₂ loss during the monitoring period. The CO₂ used in all experiments was supplied from a commercial-grade compressed CO₂ cylinder. The setup included a Trotec BZ30 data logger (Trotec GmbH, Heinsberg, Germany) (Figure 1c), equipped with an NDIR (Non-Dispersive Infrared) CO₂ sensor (range: 0–9999 ppm; accuracy: ±75 ppm or ±5% of reading) and a temperature sensor (accuracy: ±0.5 °C). The device features automatic baseline calibration, and sensor accuracy was verified against observational records prior to experiments. Following CO₂ injection, the air pump was operated to ensure homogeneous gas distribution within the chamber before initiating data acquisition. The air pump operates exclusively within the sealed interior of the chamber in a closed loop to ensure homogeneous gas circulation and to circulate chamber air through the reactor module. It is not connected to the inlet tube or the external environment. The inlet port was sealed immediately after CO₂ injection and remained closed throughout the measurement period. The gas circuit formed a fully closed recirculation loop between the atmospheric chamber and the photobioreactor module, and no gas was vented to the external environment during the experiments. CO₂ concentration was measured exclusively within the chamber atmosphere rather than at the photobioreactor outlet, as the continuous air circulation ensures that chamber measurements directly reflect net biological fixation. Illumination was provided by a desk lamp (Figure 1h) positioned to deliver 5000 lux to the front surface of the reactor and 500 lux to the rear, maintaining an average illuminance of 2750 lux to support photosynthetic activity. This illumination level has been selected to meet the light saturation point reported for Nostoc linckia photosynthesis under BG11 environmental conditions and is consistent with values documented in the literature for this species [29,38]. The selected level also represents the upper range of office environment lighting conditions and ensures the applicability of experimental performance data to architectural scenarios where high-efficiency LED integration is incorporated into furniture modules. Illumination measurements were verified using a UNI-T UT383 portable lux meter (Uni-Trend Technology Co., Ltd., Dongguan, China) (range: 0–199,900 lux; accuracy: ±4% of reading; Figure 1g).

3.2. Preparation of Biological Material

The cyanobacterium Nostoc linckia (EGE-MACC-4), obtained from the Ege University Microalgae Culture Collection, was selected as the model organism. Cultures were cultivated in BG11 medium [39]. The inoculation and propagation of the cultures were conducted at ACTV Biotechnology Laboratory, Istanbul, Turkey, where all experimental procedures were carried out. For the experimental module, a 1 L autoclavable glass photobioreactor (Figure 1e) was utilized. The vessel was sterilized before inoculation to prevent contamination. The glass vessel employed was an ISOLAB borosilicate glass autoclave bottle (ISOLAB Laborgeräte GmbH, Eschau, Germany) (1000 mL), manufactured from BORO 3.3 glass in accordance with ISO 4796 standards. This vessel provides the optical transmittance, chemical resistance, and thermal stability required for photosynthetic cultivation and repeated sterilization processes. This modular volume was specifically chosen to represent a scalable unit capable of integration into larger architectural arrays.

3.3. Experimental Design and Procedure

The experimental design aimed to assess the performance of Nostoc linckia under varying atmospheric loads, representing different indoor air quality scenarios. Three separate, sequential experiments were conducted at distinct initial CO₂ concentration ranges, defined based on their impact on human health as outlined in

Table 1. Experimental CO₂ concentration levels and associated health effects.

Experimental Regime	Concentration (ppm)	Associated Health and Comfort Effects
Level 1 (Low Load)	400–1000	Typical indoor air quality, generally considered acceptable for comfort [40,41].
Level 2 (Medium Load)	1000–2000	Onset of lethargy, complaints related to lack of fresh air, and potential decrease in cognitive performance [40].
Level 3 (High Load)	2000–5000	Significant physiological symptoms: headache, fatigue, loss of focus, increased heart rate, and nausea [42].

.

In each experiment, CO₂ was injected as a single bolus until the target upper limit was reached, after which the inlet port was sealed and no additional CO₂ was supplied during the monitoring period. The reduction in CO₂ levels was monitored until the concentration dropped to the lower limit of the respective range. The same culture was used across all three regimes, with fresh BG11 medium added to adjust the optical density to the same initial value (570 nm and 650 nm) at the start of each regime. Optical density and dry weight measurements were recorded at the beginning of the first experiment and at the end of the final experiment. Throughout the entire period, temperature and humidity were recorded at 3 min intervals to monitor microclimatic changes, and hourly data were plotted on a graph for clarity. Hourly CO₂ absorption rates were derived by extracting concentration values at the start of each hour from the continuous recordings.

Data normality was assessed via Shapiro–Wilk tests (high-load: W = 0.992, p = 0.828; medium-load: W = 0.956, p = 0.010; low-load: W = 0.969, p = 0.264). As the medium-load regime deviated significantly from normality (p < 0.05), non-parametric analyses were applied consistently across all three regimes to ensure comparability [43]. To enable rigorous statistical comparison across experimental regimes, hourly CO₂ absorption rates were calculated as the difference between consecutive measurements, yielding independent data points for each one-hour interval. This approach transformed the continuous monitoring data into discrete hourly sequestration rates (ppm/h), providing n = 106 observations for the high-load regime, n = 76 for the medium-load regime, and n = 45 for the low-load regime. Differences in hourly CO₂ absorption rates among the three regimes were compared using the Kruskal–Wallis test [44], followed by pairwise Mann–Whitney U tests with Bonferroni correction [45].

Ultimately, the kinetic data derived from these modular tests serve as the quantitative baseline for dimensioning and calculating the CO₂ sequestration capacity of the architectural design proposals developed in this study. The experiments were planned as an analytical study comparing three sequential CO₂ regimes under controlled environmental conditions. Within the framework of architectural system modeling, the analysis process aims to comparatively evaluate the system’s performance behavior regarding CO₂ reduction and biomass growth under different atmospheric loads. This stratified experimental design enables the derivation of concentration-dependent sequestration rates, which serve as the quantitative basis for the architectural scaling scenarios presented in Section 5.

4. Experimental Results and Findings

This section presents the findings of experimental studies conducted with the species Nostoc linckia. The data presented demonstrate the carbon sequestration performance of the system and its impact on environmental conditions, species growth, and energy consumption at different initial CO₂ concentrations, as measured in three sequential experiments. Throughout the experimental series, environmental boundary conditions were rigorously controlled to isolate biological variables. The internal temperature of the chamber remained relatively stable between 21.6 °C and 22.6 °C across all trials, confirming that the lighting system imposed minimal thermal load on the controlled volume. Conversely, relative humidity (RH) exhibited a consistent upward trend in all experiments, rising from an initial range of 63–65% to approximately 73–75%. This increase is attributed primarily to evaporation from the liquid culture medium, facilitated by the active aeration system. Biological viability was confirmed by optical density (OD) measurements at 570 nm and 650 nm at all carbon levels and dry weight measurements. Nostoc linckia showed significant growth at all carbon levels.

In Experiment 1, conducted with Nostoc linckia, the initial CO₂ concentration was established at 5000 ppm and was reduced to 2000 ppm after approximately 104.5 h. This time frame and concentration range correspond to the high-CO₂ scenario and are illustrated in Figure 2.

This substantial decline in CO₂ concentration indicates that the Nostoc linckia is capable of effective carbon assimilation under conditions of elevated carbon concentration. Crucially, the kinetic data reveals an adaptive response in this phase; the rate of carbon uptake was notably accelerated under high concentrations compared to lower regimes, indicating that the modular unit operates with greater photosynthetic efficiency when carbon availability is high. The initial optical density (OD) at 570 nm was 0.506, which increased to 1.345 at the conclusion of the experiment, while OD at 650 nm increased from 0.540 to 1.393. Correspondingly, the dry biomass weight increased from 0.12 mg/mL to 0.53 mg/mL. These increases indicated a significant increase in cyanobacteria density and a high level of photosynthetic activity in the system. The temporal CO₂ reduction followed a consistent exponential decay pattern throughout the experimental period.

Following the high-load test, Experiment 2 assessed the system’s performance starting from an initial CO₂ concentration of 2000 ppm. After 75.0 h, the value decreased to 1000 ppm (Figure 3).

Throughout the process, the ambient humidity increased from 65.0% to 73.1%, and the temperature remained between 21.6 °C and 22.2 °C. The initial optical density (OD) at 570 nm increased from 0.506 to 1.443, while OD at 650 nm increased from 0.530 to 1.577. In parallel with optical measurements, dry biomass density exhibited a rise from 0.12 mg/mL to 0.54 mg/mL. The results demonstrated that Nostoc linckia retained its capacity for effective carbon sequestration and biomass production even at moderate CO₂ concentrations, with the magnitude of CO₂ reduction and OD increase being lower than in the high-CO₂ case but following the same qualitative trend. Within Experiment 3, the initial concentration of CO₂ was established at 1000 ppm, and following a duration of approximately 44.1 h, a decline to 400 ppm was observed (Figure 4). As in Experiment 1, the CO₂ reduction followed a consistent exponential decay pattern under moderate atmospheric load.

Throughout the experiment, the ambient humidity increased from 65.0% to 73.0%, while the ambient temperature remained between 21.5 °C and 22.5 °C. The initial optical density (OD) at 570 nm increased from 0.506 to 0.967, and an increase from 0.535 to 1.032 was observed at 650 nm. This growth was further quantified by dry weight analysis, showing a net increase from 0.12 mg/mL to 0.31 mg/mL. The findings demonstrated that the system has effective carbon capture and biomass production capacity even at low CO₂ concentrations. It can be asserted that the CO₂ scenario starting at 1000 ppm is similar to the scenario more commonly observed in enclosed spaces where daily life takes place. When the three experiments were evaluated together, a consistent correlation was observed between CO₂ reduction and OD-based biomass increase at all concentration levels. The CO₂ reduction again followed an exponential decay pattern, with the steepest relative decay rate recorded in Experiment 3 across the three regimes.

Total energy consumption was calculated based on the operational duration of each experiment, utilizing the constant power ratings of the air pump (5.3 W) and lighting system (9 W). As summarized in

Table 2. Summary of experiments and results for the photobioreactor experiments.

Experimental Regime	Duration (h)	Initial Chamber Conditions (CO2; T; RH)	Final Chamber Conditions (CO2; T; RH)	Initial OD (Abs: Value)	Final OD (Abs: Value)	Dry Biomass (mg/mL)
High Load (5000–2000 ppm)		5000 ppm	2000 ppm	570 nm: 0.506	570 nm: 1.345	Initial: 0.12
	104.5	21.6 °C	22.6 °C	650nm: 0.540	650 nm: 1.393	Final: 0.53
		63.1%	75.4%
Medium Load (2000–1000 ppm)		2000 ppm	1000 ppm	570 nm: 0.506	570 nm: 1.443	Initial: 0.12
	75.0	21.6 °C	22.2 °C	650 nm: 0.530	650 nm: 1.577	Final: 0.54
		65.0%	73.1%
Low Load (1000–400 ppm)		1000 ppm	400 ppm	570 nm: 0.506	570 nm: 0.967	Initial: 0.12
	44.1	22.0 °C	22.0 °C	650 nm: 0.535	650 nm: 1.032	Final: 0.31
		65.0%	73.0%

, the total energy consumption correlated directly with operational duration, with the high-CO₂ regime requiring the highest input due to the extended process, despite constant power ratings. This indicates that the high-CO₂ regime required the highest total energy input due to the extended operational duration but also achieved the largest absolute CO₂ reduction. These values provide an upper-bound baseline for a single-module operation in a laboratory setting; however, in architectural applications, energy efficiency would likely be optimized through shared air circulation systems and the integration of natural daylight.

The comparative examination of the three experiments (Table 2 and Figure 5) reveals that while the initial CO₂ concentration dictated the operational duration and total energy demand, the system exhibited a consistent CO₂ removal trend and reproducible biological growth behavior across all experimental regimes. The high-concentration scenario (5000 ppm) demonstrated the system’s capacity for substantial absolute carbon mass reduction, whereas the medium (2000 ppm) and low (1000 ppm) concentration scenarios confirmed its agility in restoring ambient air quality levels. Notably, the highest final absorbance values were observed under the 2000 ppm initial CO₂ condition, indicating the highest relative growth efficiency of Nostoc linckia at moderate CO₂ levels within the tested range.

Statistical analysis of hourly absorption rates revealed significant differences in system performance across the three experimental regimes (Kruskal–Wallis H = 137.35, p < 0.001) [44]. Descriptive analysis showed that the high-load regime (5000–2000 ppm) exhibited a mean hourly sequestration rate of 28.71 ± 7.89 ppm/h, which was significantly higher than both the medium-load regime (13.33 ± 4.15 ppm/h) and the low-load regime (13.61 ± 6.48 ppm/h). Pairwise Mann–Whitney U tests with Bonferroni correction (adjusted α = 0.0167) confirmed significant differences between high-load and medium-load regimes (U = 7769.5, p < 0.001, r = 0.93) and between high-load and low-load regimes (U = 4424.5, p < 0.001, r = 0.86), both indicating very large effect sizes [45]. No significant difference was observed between medium-load and low-load regimes (p = 0.819), suggesting a functional threshold around 2000 ppm, below which the sequestration rate stabilizes. These findings demonstrate that Nostoc linckia exhibits concentration-dependent carbon uptake kinetics, with sequestration efficiency increasing 2.15-fold under elevated CO₂ availability.

Collectively, these quantitative results demonstrate that the modular Nostoc linckia unit is capable of maintaining measurable CO₂ sequestration under both extreme and typical indoor conditions, thereby supporting its operational viability for architectural integration. These experimentally derived sequestration rates and biomass productivity metrics provide the empirical foundation for translating modular performance into architectural scaling scenarios, as detailed in the following section.

5. System Validation: From Experimental Data to Architectural Application

5.1. Scaling Methodology

The transition from the experimental setup to architectural application is grounded in a volumetric scaling strategy. The experimental data presented in Section 3 were obtained using a 0.25 m³ sealed atmospheric chamber. This control volume was selected to represent a 1/100 scale model of typical single-person workspace volumes, which range between 10 and 25 m³ in standard office typologies based on anthropometric and habitation standards [35,36,37]. This scaling approach allows for the direct translation of laboratory performance data into architectural dimensioning, establishing the 1 L photobioreactor module as the fundamental functional unit of the proposed system.

To quantify the system’s impact on indoor air quality, the governing CO₂ dynamics were modeled using the standard single-zone mass balance equation, as established in ventilation literature [46,47,48]. The model integrates the photobioreactor system not merely as a passive element but as an active variable within the room’s atmospheric equation. The expanded mass balance equation for a ventilated space integrated with photobioreactor modules is expressed as:

$$V\times \left({\displaystyle \frac{d{C}_{in}}{dt}}\right)= G_{occ}- Q\times \left(C_{in} - C_{out}\right)- n \times r_{seq}$$

where the parameters are defined as follows:

V: Volume of the occupied zone (m³)

d$C_{in}$/dt: Time-dependent rate of change in indoor CO₂ concentration (ppm/h)

$G_{occ}:$ Total metabolic CO₂ generation rate from occupants (ppm·m³/h)

Q: Mechanical ventilation rate (m³/h)

$C_{in}$ and $C_{out}$: Indoor and outdoor CO₂ concentrations, respectively (ppm)

n: Number of active photobioreactor modules

$r_{seq}$: Carbon sequestration rate per module (ppm×m³/h).

The removal rate represents the mean hourly CO₂ concentration decrease, derived by extracting values at hourly intervals from continuous 3 min recordings as described in Section 3.3. The critical parameter $r_{seq}$, represents the absolute carbon removal capacity of a single module. It is derived from the experimental concentration decay rates ($R_{exp}\times$ using the relation $r_{seq}= R_{exp}\times V_{chamber}$ where $V_{chamber}$ is the fixed volume (0.25 m³)) of the experimental setup.

Table 3. Experimentally derived CO₂ sequestration rates and biomass production.

Experimental Regime	Duration (h)	Removal Rate ${\mathit{R}}_{\mathit{e}\mathit{x}\mathit{p}}$ (ppm/h)	Sequestration Rate ${\mathit{r}}_{\mathit{s}\mathit{e}\mathit{q}}$ (mL/h) *	Biomass Production (mg/d) *
High Load (5000–2000 ppm)	104.5	28.71	7.18	94.2
Medium Load (2000–1000 ppm)	75.0	13.33	3.33	134.4
Low Load (1000–400 ppm)	44.1	13.61	3.40	103.4

* $r_{seq}$ calculated as $R_{exp}$ × 0.25 m³/. Note that 1 ppm m³//h ≅ 1 mL/h.

presents the derived $r_{seq}$ values for each tested CO₂ regime.

This mathematical model highlights a fundamental distinction in air quality management strategies. In standard HVAC systems, the term $Q.\left(C_{in}- C_{out}\right)$ represents a dilution mechanism [46], where indoor contaminants are displaced by outdoor air but not degraded. In contrast, the term $- n . r_{seq}$ introduces an active biological sink into the system. Unlike mechanical ventilation, which manages concentration through displacement, the photobioreactor system permanently removes CO₂ from the air volume by fixing it into organic biomass [9,34]. Consequently, the integration of this biological layer shifts the indoor environment from a purely dissipative system to a regenerative one, capable of altering the fundamental decay rate of metabolic pollutants independent of the ventilation rate.

5.2. CO₂ Sequestration Kinetics and Biomass Productivity

Analysis of the three experimental regimes reveals concentration-dependent carbon uptake behavior consistent with Monod-type saturation kinetics observed in photosynthetic microorganisms [7,49]. Table 3 summarizes the experimentally derived sequestration rates and corresponding biomass productivity.

Beyond kinetic rates, the critical distinction of this biological carbon capture system lies in its productive output. As carbon constitutes nearly half of the DW of cyanobacterial biomass, approximately 1.83 kg of CO₂ is fixed for every 1 kg of biomass produced [6,50,51]. Under the experimental conditions tested, each 1 L module demonstrated a daily productivity of 94–135 mg of dry biomass, accumulating to 2.8–4.1 g per month and 34–49 g annually. This biomass represents carbon permanently removed from the atmospheric cycle, a fundamental difference from the temporary displacement achieved through mechanical ventilation. Additionally, Nostoc linckia’s nitrogen-fixing capacity enables the harvested biomass to be directly applied as biofertilizer [29,31], establishing a circular relationship where indoor air quality management directly supports soil health at the landscape scale.

5.3. Architectural Application Scenarios

Building upon the experimentally validated sequestration kinetics presented in Section 5.2, this section translates the modular performance data into architectural application scenarios. The objective is to demonstrate how arrays of photobioreactor modules can be dimensioned and integrated within real-world interior environments to achieve measurable air quality improvements while generating productive biomass for landscape regeneration. Five workspace typologies were defined to demonstrate the scalability of the modular system across different spatial configurations. The selected volume ranges (25–300 m³) represent common interior environments based on anthropometric and habitation standards documented in human-space relationship studies [35,36,37].

The module quantities in

Table 4. Architectural scenario parameters and recommended module configurations.

Scenario	Volume (m3)	Occupants	Modules (n)	Typical CO2 Range (ppm)
A: Single-Person Office	25	1	30	800–1500 [52,53]
B: Shared Office	50	3–4	60	1000–2000 [53,54]
C: Meeting Room	75	8–12	100	1500–3000 * [55]
D: Open-Plan Office	150	10–15	200	1000–1800 [53]
E: Atrium/Interface	300	15–25	300	600–1200 [56]

* Peak concentration during occupied periods.

are derived from the experimental scaling relationship established in Section 5.1: since one 1 L module operating in a 0.25 m³ chamber represents a 1/100 scale model of a 25 m³ workspace, proportional scaling yields a baseline of 1 module per m³ for equivalent sequestration density. The ranges presented (1.0–1.3 modules/m³) account for variations in occupancy patterns, ventilation conditions, and target CO₂ reduction rates. The CO₂ reduction rate for each scenario is calculated by applying the mass balance equation:

$${\displaystyle \frac{d{C}_{in}}{dt}}= -{\displaystyle \frac{\left(n \times r_{seq}\right)}{V}}$$

where n is the number of active modules, and V is the space volume. Using the medium-load sequestration rate ($r_{seq}$ = 3.33 ppm·m³/h) as representative of typical indoor conditions,

Table 5. Projected system performance and annual biomass production under controlled laboratory baseline conditions.

Scenario	Volume (m3)	Modules (n)	Instantaneous Reduction (ppm/h)	Daily CO2 Fixation (g/day)	Annual CO2 Fixation (kg/year)	Annual Biomass Production (kg/year)
A: Single-Person Office	25	30	4.0	5.2–7.4	1.9–2.7	1.0–1.5
B: Shared Office	50	60	4.0	10.3–14.8	3.8–5.4	2.0–3.0
C: Meeting Room	75	100	4.5	17.2–24.7	6.3–9.0	3.4–4.9
D: Open-Plan Office	150	200	4.5	34.4–49.4	12.6–18.0	6.9–9.9
E: Atrium/Interface	300	300	3.3	51.6–74.1	18.8–27.0	10.3–14.8

presents the projected performance metrics for each scenario.

The performance metrics presented in Table 5 demonstrate a notable operational distinction between the proposed biotechnological interface and standard ventilation strategies. Conventional HVAC systems address Indoor Air Quality (IAQ) through the process of dilution, which involves the volumetric displacement of contaminants [46,57]. In contrast, the photobioreactor system functions as a continuous metabolic sink. Consequently, the system’s efficacy is best evaluated not solely by the instantaneous reduction in concentration (ppm/h), but also by its cumulative capacity to physically sequester carbon mass from the built environment.

The projected annual sequestration values (up to 27.0 kg CO₂ for Scenario E) are grounded in the specific stoichiometry of algal photosynthesis [6,50]. As established in bioengineering literature, the fixation of 1 unit of dry biomass requires the intake of approximately 1.83 units of CO₂ [50,51]. This ratio explains why the mass of sequestered CO₂ in Table 5 exceeds the DW of the produced biomass; the process involves the extraction of carbon atoms for cellular synthesis while releasing the heavier oxygen component back into the atmosphere [6]. Unlike mechanical filters that capture particulates but leave gaseous pollutants unchanged, this process constitutes biogenic fixation, effectively transforming a volatile metabolic waste product into stable organic matter [26]. It should be noted that the projected values in Table 5 are derived from controlled experimental conditions and are subject to variability under real-world application scenarios. Key factors that may influence system performance include fluctuations in light availability, ambient temperature variations, and culture maintenance intervals, all of which are known to significantly affect photosynthetic efficiency and CO₂ sequestration rates in photobioreactor systems [7,58]. Additionally, occupancy fluctuations will alter the metabolic CO₂ generation term, and ventilation rates (Q) will modulate the dilution component of the mass balance equation. Consequently, the values presented in Table 5 should be interpreted as baseline performance estimates under optimized operational conditions rather than absolute projections.

5.4. Circular Integration: From Metabolic Output to Landscape Input

The holistic value of the system extends beyond air quality remediation to the generation of a regenerative material stream. The “Annual Biomass Production” calculated in Table 5 represents a critical resource for the interior-landscape interface. Unlike microalgae species typically employed for air remediation (e.g., Chlorella or Spirulina), the Nostoc linckia utilized in this study possesses a nitrogen-fixing metabolism, which renders the harvested biomass a high-value biofertilizer [29,31].

In a circular design context, the biomass outputs identified in the scenarios (ranging from 1.0 to 14.8 kg/year offer a direct pathway to restore urban soil health. Literature confirms that cyanobacterial biofertilizers, including Nostoc species, can fix 18–45 kg of bioavailable nitrogen per hectare annually [9], significantly enhancing soil structure and plant growth as a sustainable alternative to synthetic chemical fertilizers [34,59,60]. Consequently, the architectural interface proposed here acts as a metabolic converter: it captures the “waste” carbon of the indoor environment (Zone A) and transforms it into a “nutrient” input for the exterior landscape (Zone C), thereby closing the material loop as visualized in the subsequent discussion. Scaling from the biomass production values presented in Table 5, even modest modular arrays are estimated to generate a meaningful contribution of bioavailable nitrogen for urban soil supplementation at the landscape scale.

6. Discussion

According to the experimental results, the developed photobioreactor module can be described as an adaptable biological carbon buffer system that operates within the 1000–5000 ppm range, provides optimal biological productivity at approximately 2000 ppm, and can safely process excessive carbon loads at 5000 ppm. The findings of this study provide important experimental evidence regarding the scalability and effectiveness of Nostoc linckia-based photobioreactors as active components of indoor-landscape interfaces. The scalable and modular system framework summarized within the scope of this research is presented in Figure 6 as a circular biophilic interface. This design model connects the experimental results, the interior space (Zone A), the interface space incorporating biotechnology (Zone B), and the landscape (Zone C). As demonstrated in this paper, the modular integration scenarios detailed in Section 5 aim to form the basis for the study’s advanced phases. The system proposal summarized here can be easily adapted to spaces with low maintenance requirements. Based on this study, carbon sequestration and biomass calculation from the modules are feasible under the appropriate conditions referenced in the experiment.

The applicability of the proposed system in architectural scenarios is directly related to the volumetric capacity required for managing human-generated metabolic loads. Standard mechanical ventilation systems are primarily based on the principle of dilution, whereby pollutants are diluted with fresh air to ensure acceptable indoor air quality [57]. While not intended to replace standard HVAC ventilation, the proposed photobioreactor system functions in parallel as a distributed ecological buffer, complementing dilution-based air exchange by fixing a fraction of indoor CO₂ within the architectural system.

Conventionally, metabolic CO₂ is treated as ‘waste’ and discharged into the atmosphere to maintain acceptable indoor air quality. Notably, this study contributes to an emerging reconsideration of the conventional linear waste management approach by positioning photobioreactor modules based on Nostoc linckia as an interface integrated into the existing ventilation cycle. When the conceptual framework of this interface is grounded in accurate metabolic data, the current literature reveals the dynamic structure of carbon loads in indoor environments. Controlled room experiments by Sakamoto et al. [61] show that the hourly CO₂ emission rates of individuals engaged in sedentary activities (tablet/phone use, etc.) ranged from 12.9 to 15.1 L/h in the morning sessions and from 14.5 to 16.1 L/h in the afternoon sessions due to the thermogenic effect of diet. Similarly, Persily and de Jonge [46] state that during standard office activities, the average CO₂ production rate of an adult (body surface area 1.8 m²) is approximately 0.0052 L/s (~18.7 L/h), but this value may increase depending on physical activity intensity and body mass. Correspondingly, it cannot be expected that a single 1 L module alone will neutralize a person’s instantaneous metabolic output; instead, the system is conceived as a ‘cumulative modular matrix’ integrated into spatial elements such as library units, partitions, or seating. Given the literature-based average CO₂ production of a sedentary individual, the system’s efficacy relies on the collective surface area and volume of this matrix rather than a solitary unit. Our experimental results demonstrate the system’s ability to ‘accelerate metabolic uptake’ under high carbon loads (5000 ppm), suggesting that in high-occupancy scenarios, the modular matrix operates in synergy with mechanical ventilation. In this configuration, it functions as an active metabolic buffer, maintaining Indoor Air Quality (IAQ) within the optimal comfort range of 400–1000 ppm, thereby acting as a complementary biotechnological layer that reduces the load on conventional HVAC systems.

Furthermore, the present study aligns with recent research suggesting that dividing photosynthetic elements into micro-scale units enhances the system’s adaptability to environmental variables [62]. As Pierobon et al. [63] have highlighted, the modular design of photobioreactors is critical for the optimization of light distribution and flow control. The results of this study corroborate this hypothesis, demonstrating that a compact modular volume allows Nostoc linckia to reach metabolic saturation efficiently. This scalability offers a significant advantage over monolithic systems [8]. Modular PBR elements offer flexible solutions regarding maintenance and energy efficiency. Consequently, the proposed system facilitates a decentralized air remediation configuration, whereby individual units can be serviced without compromising the integrity of the building-scale infrastructure. Beyond functioning as a functional air purification device, the modular photobioreactor is considered an esthetic spatial component that introduces visible biological growth, color variations, and gradual metabolic rhythms. This approach transforms the static architectural space into a living environment where the biophilic experience is sustained.

A significant contribution of this research is the adaptive photosynthetic response observed under high CO₂ loads (Experiment 1). Equally, the sustained sequestration capacity demonstrated at moderate concentrations (Experiment 2) and the system’s functional performance at near-ambient indoor conditions (Experiment 3) confirm the operational viability of the module across the full range of realistic indoor CO₂ scenarios. The carbon sequestration rate in the 5000–2000 ppm range suggests that the system functions as a dynamic biological agent rather than a passive mechanical filter. This supports the argument presented by Han et al. [26] regarding the potential of microalgae systems to serve as effective remediation tools. Unlike static HVAC filters, the biological module has the capacity to “up-regulate” performance during pollution increases. This adaptive behavior curve positions the system as a “functional regulatory buffer” during intense indoor air quality situations.

As well as improving air quality, the selection of Nostoc linckia strengthens the interplay between resource cycling and regenerative design across scales. This performance becomes particularly significant when compared to conventional indoor plants: while typical houseplants under interior lighting conditions (1–50 µmol m⁻² s⁻¹) exhibit net CO₂ assimilation rates generally below 3.9 mg/h, and in some cases even release CO₂ through respiration [64], the photobioreactor module presented here achieves sequestration rates of 6.6–14.1 mg CO₂/h (equivalent to 3.33–7.18 mL/h) per liter of culture, representing a 1.7- to 3.6-fold improvement in carbon capture efficiency within a comparable volumetric footprint. The results of this study contribute to the existing literature in several distinct ways. While most indoor air remediation studies employing photobioreactors have focused on green microalgae such as Chlorella, this study demonstrates that the filamentous cyanobacterium Nostoc linckia can serve as an effective and architecturally viable CO₂ sequestrant across a wide range of atmospheric loads. Furthermore, the adaptive concentration-dependent response observed in this study, in which sequestration rates increased 2.15-fold under elevated CO₂ availability, has not been previously reported for this species in enclosed architectural contexts. Perhaps most significantly, by linking experimentally derived kinetics to a validated mass balance model and translating these into architectural scaling scenarios, this study establishes a methodological bridge between laboratory-scale biological performance and real-world spatial design applications, a connection that remains largely absent from existing literature [1,2,8].

This study elevates the photobioreactor interface from a mere air purification device to a regenerative circular design model aligned with the UN Sustainable Development Goals (specifically SDG 11: Sustainable Cities and Communities and SDG 13: Climate Action). The selection of Nostoc linckia is strategic; its dual capacity for atmospheric carbon sequestration and nitrogen fixation enables the transformation of indoor ‘waste’ carbon into a valuable agricultural input. Returning the harvested biomass to the urban landscape (Zone C) as a bio-fertilizer alternative to synthetic additives establishes a closed material loop. In line with ‘cradle-to-cradle’ principles, this model contributes to transforming buildings into productive ecological systems that nourish urban green infrastructure.

In accordance with the findings of Touloupakis et al. [29] and Lucato [31], the nitrogen-fixing metabolism of Nostoc renders it a valuable resource for biofertilizer production. In developing an accessible and easily implementable modular system, the study successfully extended this biological potential to the domain of architectural practice. As emphasized by Alobwede et al. [34], the integration of algal species as soil amendments is pivotal to the realization of a circular economy fertilization model. Consequently, the system purifies indoor air high in CO₂ and directs the biomass generated in this process to urban green infrastructure. Thereby, the system impacts both human health and progressively transforms indoor-derived carbon into a regenerative resource for soil health at the landscape scale.

While the experimental energy consumption represents a controlled laboratory baseline dependent on standalone equipment, the transition to architectural application offers opportunities for optimization through functional integration. Since the proposed system is conceived as a ‘distributed modular matrix’ integrated into furniture elements (e.g., partitions, seating units, shelving), reliance on standard ceiling-mounted office lighting (~500 lux) would be insufficient for peak photosynthetic rates. Therefore, the architectural scaling strategy necessitates localized, high-efficiency LED integration embedded directly within the furniture modules. In this configuration, the energy consumed for the photobioreactor is not a parasitic load but functions as a dual-purpose architectural element, providing both the necessary Photosynthetic Photon Flux Density (PPFD) for the biomass and serving as the primary ambient lighting source for the interior environment. Furthermore, integrating the reactor’s aeration system with the building’s existing HVAC airflow eliminates the need for individual air pumps [8], significantly reducing the net operational carbon footprint compared to the experimental setup. This approach aligns the proposed system with net-zero energy goals, ensuring that the biological component does not become a parasitic energy load.

Elrayies [2] argues that the integration of microalgae into building envelopes is essential for a greener future. Consistent with this view, positioning reactors at the window interface to utilize photosynthetically active radiation (PAR) from natural daylight would drastically reduce the reliance on dedicated lighting. Crucially, in contrast to outdoor applications that necessitate exclusive energy-intensive illumination, deployment at the interior-landscape interface capitalizes on the ambient artificial light already supplied for occupant visual comfort. This synergy allows the system to scavenge otherwise dissipated photonic energy, effectively transforming a routine building operational load into a productive metabolic input. When including natural light and additional artificial light conditions, performance fluctuations may be expected; however, this study measured maximum potential under optimal conditions. Furthermore, integrating the reactor’s aeration system with a building’s existing HVAC airflow would eliminate the need for individual air pumps, a strategy supported by Kim et al. [8] for nature-based building enclosures. Consequently, the net energy footprint of an integrated system is expected to be significantly lower than the experimental baseline, transitioning the technology from an active load to a passive-active hybrid strategy. The experimental findings of this study establish a quantitative performance baseline under controlled conditions. While real architectural environments involve dynamic CO₂ loads, variable lighting, and long-term operational factors, the concentration-dependent adaptive behavior demonstrated across three experimental regimes suggests that the system is well-positioned to respond to fluctuating indoor conditions. Future research incorporating dynamic occupancy simulations and extended operational assessments will further validate the system’s performance potential in architectural deployment.

7. Conclusions

This study provides experimental evidence supporting the potential viability of Nostoc linckia photobioreactors to function as biotechnological interventions at the intersection of interior architecture and landscape architecture. Experimental results indicate that a modular 1 L system can effectively maintain biological stability and growth across low (400–1000 ppm), medium (1000–2000 ppm), and high (2000–5000 ppm) carbon dioxide concentrations. Crucially, the experimentally derived sequestration rates (3.33–7.18 mL/h per module) provided a quantitative foundation for architectural scaling. The analysis demonstrates that modular arrays can achieve annual CO₂ sequestration ranging from 1.9 kg/year for a single-person office to 27.0 kg/year for larger atrium spaces, while generating 1.0–14.8 kg/year of productive biomass.

Experimental results validate that Nostoc linckia can be integrated into architecture not merely as a passive finish, but as an active metabolic agent. The system biotechnologically purifies indoor air with high CO₂ content, and by recirculating the produced biomass into the landscape as a nitrogen-rich biofertilizer, it facilitates a two-way, circular ecological relationship between the interior and the landscape. This represents a meaningful step toward reframing dilution-based ventilation approaches as opportunities for active metabolic capture, offering a sustainable design approach validated by measurable environmental performance.

In conclusion, this study reimagines the photobioreactor as a multifunctional design element that blurs the boundaries between technological infrastructure and ecological layers. The validated scaling methodology establishes that modular arrays, dimensioned at approximately 1 module per cubic meter, can serve as distributed biological carbon sinks complementing conventional HVAC systems.

Future research should address long-term operational stability, automated maintenance protocols, and the integration of these systems into Building Information Modeling (BIM) frameworks to facilitate widespread architectural implementation; the data from this study provides a foundation for this. Furthermore, developing a full mathematical parameterization of the system’s CO₂ uptake kinetics represents a valuable direction for future work, as it will significantly advance its predictive modeling.