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Article

A Photosynthetic Bacterium Suitable for Treating High-Salinity Sea Cucumber Boiling Broth

by
Shaokun Dong
1,
Yusi Guo
1,
Jinrui Ji
1,
Pu Song
1,
Ning Ma
1,
Hongjin Qiao
1,* and
Jinling Cai
2,*
1
School of Life Sciences, Ludong University, Yantai 264025, China
2
College of Chemical Engineering and Materials Science, Tianjin University of Science & Technology, Tianjin 300457, China
*
Authors to whom correspondence should be addressed.
Fermentation 2025, 11(5), 284; https://doi.org/10.3390/fermentation11050284
Submission received: 30 March 2025 / Revised: 25 April 2025 / Accepted: 13 May 2025 / Published: 14 May 2025
(This article belongs to the Section Microbial Metabolism, Physiology & Genetics)
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Abstract

:
Photosynthetic bacteria exhibit significant bioremediation potential and resource recycling characteristics, rendering them valuable candidates for sustainable wastewater treatment applications. Sea cucumber boiling broth (SCBB) contains high concentrations of organic compounds and nutrient salts, whose indiscriminate discharge poses serious environmental risks. This study aimed to evaluate a photosynthetic bacterium capable of effectively treating SCBB, which was isolated from the intertidal sediment samples. The bacterial strain was identified using 16S rDNA sequencing, and optimal growth conditions, including light, pH, and temperature, were determined. Finally, a small-scale trial was conducted in a fed-batch fermenter. The results showed that 16S rDNA analysis placed this strain in the Chromatiaceae family, forming a distinct lineage from the closest related species Marichromatium purpuratum and M. gracile, and was tentatively named Marichromatium sp. DYYC01. The strain exhibited optimal growth under anaerobic conditions at 30 °C, light intensity of 100 μmol photons/m2/s, and pH 7.0. Batch culture experiments demonstrated maximum biomass accumulation (OD660 = 0.831) in SCBB medium with an initial COD loading of 3913 mg L⁻1, concomitant with significant nutrient removal efficiencies: 76.45% COD, 55.82% total nitrogen (TN), and 56.67% total phosphorus (TP). Scaling up to fed-batch fermentation enhanced bioremediation performance, achieving removal rates of 83.13% COD, 72.17% TN, and 73.07% TP with enhanced growth (OD660 = 1.2). This study reveals Marichromatium sp. DYYC01’s exceptional halotolerance in high-salinity organic wastewater treatment. The strain’s capacity for simultaneous biomass production and efficient nutrient recovery from hypersaline processing effluent positions it as a promising candidate for developing circular bioeconomy strategies.

1. Introduction

Sea cucumber (Apostichopus japonicus), as a high-value marine organism, contains diverse bioactive compounds, including autolytic enzymes [1]. Improper processing techniques may activate these enzymatic systems, triggering tissue autolysis that compromises product quality and nutritional integrity [2]. Current industrial processing predominantly employs high-temperature steam treatment, generating nutrient-rich waste broth containing proteins, polysaccharides, and nitrogen, phosphorus compounds [3,4]. Direct discharge of this effluent into coastal ecosystems can easily cause eutrophication of the water body and also be a waste of resources [5].
Photosynthetic bacteria (PSB) have emerged as multifunctional biocatalysts due to their phylogenetic diversity, versatile metabolic pathways, and biotechnologically valuable metabolites [6,7]. Their applications span organic waste remediation, sustainable aquaculture, agricultural biotechnology, and bioenergy production, with particular research emphasis on wastewater valorization [8,9]. PSB demonstrates exceptional bioremediation capabilities through high-efficiency organic pollutant degradation and remarkable tolerance to toxic substances such as heavy metal ions [10,11]. Conventional wastewater treatment paradigms predominantly focus on end-of-pipe pollutant degradation, often neglecting resource recovery while incurring substantial energy demands and greenhouse gas emissions [12,13]. This necessitates the development of eco-friendly alternatives that integrate waste-to-resource conversion, where PSB-based systems offer distinct advantages: low energy requirements, cost-effectiveness, and environmental adaptability [14,15]. Key technical merits include: (1) capacity for direct treatment of high-load organic wastewater; (2) compact system footprint with reduced infrastructure costs; (3) valorization potential of PSB biomass as organic fertilizer or probiotic agent through scale-up cultivation [16,17,18]. However, market deficiencies persist in marine-adapted PSB strains capable of processing hypersaline effluents [19], particularly given increasing industrial demand for high-salinity wastewater solutions. Therefore, screening a strain of marine photosynthetic bacteria that can effectively treat high salinity wastewater is currently needed in the market.
This study employed progressive enrichment techniques using sea cucumber boiling broth (SCBB) as culture medium. Through systematic screening, we isolated a marine PSB strain with hypersaline adaptation capabilities from the intertidal sediment samples collected at Yantai First Beach. Subsequent optimization experiments systematically investigated optimal growth parameters and SCBB treatment efficiency. Our findings provide critical strain resources and technical parameters for developing a “waste-to-resource” remediation strategy. The results not only expand the application boundaries of PSB in high salinity bioremediation but also innovatively repurpose marine processing byproducts as microbial culture substrates.

2. Materials and Methods

2.1. Culture Medium

The sea cucumber boiling broth (SCBB) was sourced from an aquatic product processing facility (Yantai, China). Primary SCBB underwent vacuum filtration (0.45 μm cellulose membrane) to eliminate particulate matter, with initial chemical oxygen demand (COD) of 1420–15,600 mg/L, depending on different batches, attributable to industrial processing fluctuations (Table 1). Natural seawater was collected from the coastal area near the First Bathing Beach of Yantai and filtered through Whatman GF/C filters. The SCBB was diluted with seawater to prepare culture media. All culture media were autoclaved at 121 °C for 30 min before use.

2.2. Composition Analysis of SCBB

The nutritional analysis of SCBB mainly includes the analysis of organic acids, sugars, and amino acids. Organic acids are analyzed quantitatively by high-performance liquid chromatography (HPLC, L-20A, Shimadzu, Japan) equipped with a ZORBAX Eclipse Plus C18 column (4.6 × 250 mm, 5 μm). Carbohydrate profiling utilized ion chromatography (ThermoFisher ICS-5000, Waltham, MA, USA) with a Dionex CarboPac™ PA10 analytical column (4 × 250 mm). Amino acid analysis was performed using HPLC (Biochrom 30+, Holliston, MA, USA) equipped with a ZORBAX Eclipse Plus C18 column (4.6 × 150 mm, 3.5 μm), with retention time compared to standard samples, and quantified using a UV detector.

2.3. The Isolation and Characterization of PSB

Environmental samples were collected from the intertidal zone of First Bathing Beach of Yantai (37°32′15″ N, 121°25′2″ E). Surface debris (0–5 cm depth) was removed using sterile spatulas to access the anaerobic subsurface stratum. Approximately 200 g of black benthic slurry was collected vertically from 10 to 15 cm depth. Samples were transferred to autoclaved Erlenmeyer flasks and immediately transported to the laboratory. Five enrichment groups were established with gradient-diluted SCBB media (COD 6522–1304 mg/L), each overlaid with sterile liquid paraffin (2 mm thickness) and sealed with parafilm to establish microaerobic conditions. Cultures were incubated under controlled conditions (30 °C, 100 μmol photons/m2/s, 14:10 light-dark cycle). Enrichment success was confirmed by visible pigmentation development.
PSB strains were then isolated using a mixed plate method. Aliquots (100 μL) of the microbial consortium were inoculated into sterilized agar media (1.5% w/v), pre-cooled to 45 °C. After vortex homogenization, the mixtures were aseptically plated and overlaid with molten paraffin (50 °C) to establish microaerobic conditions, followed by sealing film application. Plates underwent controlled conditions (30 °C, 100 μmol photons/m2/s, 14:10 light-dark cycle) until discrete colony formation. Colonies were subjected to quadrant purification (≥4 successive passages) using the same protocol. Pure isolates were preserved in liquid media containing sterile liquid paraffin overlays.
For molecular characterization, late-exponential phase cultures (2 mL) were centrifuged (4500× g, 10 min, 4 °C), with pellets washed twice in TE buffer (10 mM Tris-HCl, 1 mM EDTA, pH 8.0) prior to genomic DNA extraction using the MiniBEST Universal Genomic DNA Extraction Kit (TaKaRa, Dalian, China). The 16S rRNA gene was amplified via PCR (27F/1492R primers) and sequenced by the China Center for Type Culture Collection. Resultant sequences were aligned against the NCBI RefSeq database using BLASTn, with phylogenetic reconstruction performed in MEGA10.0 via the neighbor-joining algorithm (1000 bootstrap replicates).

2.4. Optimization of the COD Loading Concentrations of SCBB

Mid-log phase cultures were aseptically transferred into serum bottles containing SCBB media with sequential COD gradients (6522 mg/L, 5217 mg/L, 3913 mg/L, 2609 mg/L, and 1304 mg/L). Cultivation occurred in a light incubator (MQDS-B1HG, Minquan, Shanghai, China) maintained at 30 ± 1 °C with continuous illumination (100 μmol photons/m2/s, LED array). Biological triplicates were established for each COD concentration. At 24-h intervals, 2 mL aliquots were sampled for spectrophotometric analysis at 660 nm (Yoke, Shanghai, China). Monitoring continued until reaching the growth plateau.

2.5. Optimization of Growth Conditions

Single-factor optimization was conducted to determine the optimal growth conditions for PSB. Each parameter (irradiance, temperature, pH) was independently tested while maintaining other variables at predetermined optimal levels. Biological triplicates were established for all experimental groups. The inoculation, incubation, and OD determination methods were as same as above. Mid-log phase inocula were transferred into SCBB medium containing the pre-optimized COD concentration (3913 mg/L, determined from prior experiments). For temperature optimization, cultures were incubated in the light incubators with incremental thermal gradients (20, 25, 30, 35, and 40 °C). For pH optimization, sterile SCBB medium was adjusted to target pH values (6.5, 7.0, 7.5, 8.0, and 8.5) using 0.1 N HCl/NaOH. For irradiance optimization, cultures were incubated in the light incubators under controlled irradiance levels (40, 60, 80, 100, and 120 μmol photons/m2/s) using LED arrays.

2.6. Treatment Effect of PSB on SCBB Under Batch Culture

Triplicate cultures were aseptically inoculated into pre-optimized SCBB medium using mid-log phase inoculum. Post 96-h incubation, biomass was separated by centrifugation (4000× g, 10 min). The supernatant was then analyzed for total phosphorus (TP), total nitrogen (TN), and chemical oxygen demand (COD) using the following methods: ammonium molybdate spectrophotometry [20], alkaline potassium persulfate digestion-ultraviolet spectrophotometry [21], and rapid digestion-spectrophotometry [22], respectively.

2.7. Treatment Effect of PSB on SCBB in Fed-Batch Fermenter

A 5 L glass-jacketed fermenter (BLBIO-5GJ, Bailun, Shanghai, China) was aseptically inoculated with mid-log phase cultures. The system maintained controlled parameters through integrated sensors: dissolved oxygen (0.2% via polarographic probe, VisiFerm DO Arc 225 H2, Hamilton, Switzerland), temperature (30.0 ± 0.5 °C, by circulating water bath, HWKT-0506, Ketai, Zhengzhou, China), and photosynthetic photon flux density (100 ± 5 μmol photons/m2/s, LED array). Beginning at 48 h, the medium was exchanged semi-continuously using pre-optimized SCBB, with 10% v/v replaced every 24 h. pH homeostasis was achieved through adding an appropriate amount of 0.1 N NaOH solution to adjust the pH to 7.0. OD660 values were measured every 12 h. After fermentation was completed, cultures were centrifuged at 4000× g for 10 min to collect the supernatant, and finally, TP, TN, and COD contents were measured to calculate the percentage of removal.

2.8. Data Processing and Analysis

Data analysis was performed using GraphPad Prism 9.5. Experimental results are expressed as the mean ± standard deviation (SD) of triplicate measurements. Data with parametric distribution were subjected to one-way analysis of variance (ANOVA), and mean values were compared using Tukey’s test. Statistical significance for all tests was defined as p < 0.05.

3. Results

3.1. Analysis of the SCBB Composition

Amino acid profiling analysis revealed 17 free amino acids in SCBB (Table 2), dominated by glutamate (0.322 mg/mL) and aspartate (0.235 mg/mL). The content of different amino acids varied significantly, with glutamate having the highest content (0.322 mg/mL), while some amino acids, such as cysteine and methionine, had lower content. The test results also contained various essential amino acids, indicating that SCBB had high nutritional value. The total amino acid content was 2.07 mg/mL, indicating that it had certain advantages in amino acid nutrition and was beneficial for providing nitrogen sources for the growth of PSB.
Six carboxylic acids, including oxalic acid (350 mg/L), tartaric acid (395 mg/L), malic acid (42.7 mg/L), lactic acid (1530 mg/L), acetic acid (571 mg/L), and citric acid (959 mg/L), were mainly detected (Table 2). However, formic acid, succinic acid, and propionic acid were not detected. The highest content of lactic acid may have a significant impact on the acidity and flavor characteristics of SCBB.
Eight monosaccharides, such as fucose, glucosamine, glucosamine, galactose, glucose, mannose, fructose, and ribose, were detected, while rhamnose, arabinose, xylose, etc., were not detected (Table 2). The content of different monosaccharides varied greatly, with fructose having the highest content (29.282 μg/mg), followed by mannose (17.688 μg/mg), and glucose at 3.725 μg/mg. The differences in the content of various monosaccharides reflected their proportion in sea cucumber polysaccharides, which may be related to their biological activity.

3.2. Identification of Photosynthetic Bacteria

The 16S rRNA gene sequence of strain DYYC01 has been deposited in the GenBase of the China National Center for Bioinformation (CNCB) with accession number GB0005789. The phylogenetic analysis based on the 16S rRNA gene sequence showed that the strain formed a different lineage within the Chromatiaceae family from the closest related species, Marichromatium purpuratum and M. gracile, with 16S rRNA gene sequence similarities of 99.8% and 99.7%, respectively (Figure 1). Therefore, the obtained strain was temporarily named as Marichromatium sp. DYYC01. Despite high sequence similarity, DYYC01 had formed an independent lineage in the phylogenetic tree.

3.3. COD Loading Concentration Optimization

As demonstrated in Figure 2, strain DYYC01 exhibited concentration-dependent growth kinetics in SCBB media across a COD gradient (1304–6522 mg/L). Suboptimal growth was observed at 1304 mg/L COD. Progressive enhancement occurred at elevated COD levels, peaking at 3913 mg/L COD (OD660 = 0.831). Subsequent COD increases (5217–6522 mg/L) induced growth inhibition (ΔOD660 = −0.150). Temporal analysis revealed consistent entry into the stationary phase at 96 h across all groups, indicating COD-independent transition to nutrient limitation. Based on growth optimization, the 3913 mg/L COD medium with 4-day incubation was selected as the optimal cultivation regime for subsequent experiments.

3.4. Growth Conditions Optimization

Growth of strain DYYC01 exhibited significant light density dependence (40–120 μmol photons/m2/s), as quantified in Figure 3. Under incremental irradiance (40–100 μmol photons/m2/s), growth demonstrated a positive correlation, reaching a maximum value at 100 μmol photons/m2/s. Notably, supraoptimal illumination (120 μmol photons/m2/s) induced photoinhibition, manifesting as a 2.9% decrease in final OD660 compared to optimal conditions. Therefore, 100 μmol photons/m2/s is the physiologically optimized irradiance for sustained photomixotrophic cultivation.
The thermal response of strain DYYC01 exhibited distinct phase transitions across the experimental temperature gradient (20–40 °C), as quantitatively demonstrated in Figure 4. Within the mesophilic range (20–30 °C), OD660 increased positively with temperature elevation, reaching a maximum value (0.858) at 30 °C. Notably, discontinuity emerged beyond 35 °C, correlating with a 23.4% OD660 decrease at 40 °C compared to optimal conditions. Therefore, 30 °C was established as the cultivation optimum for sustained photosynthetic activity.
The acid-base homeostasis analysis revealed that strain DYYC01 maintained functional viability within pH 6.5–8.5, though with significant metabolic divergence. As quantified in Figure 5, suboptimal growth occurred at pH 6.5, indicative of proton motive force dissipation. Physiological optimization emerged at circumneutral pH (7.0–7.5), peaking at pH 7.0 with maximal biomass yield (OD660 = 0.924). Alkaline stress (pH ≥ 8.0) induced progressive metabolic inhibition. Therefore, a pH value of 7.0 was ultimately chosen as the optimal pH condition for cultivating strain DYYC01.

3.5. Treatment Effect on SCBB Under Batch Culture

Under optimized cultivation parameters (30 °C, 100 μmol photons/m2/s, pH 7.0), strain DYYC01 demonstrated significant bioremediation efficacy in SCBB treatment. Figure 6 showed that after treatment with DYYC01, the TP, TN, and COD contents of SCBB were 71.37 mg/L, 312.7 mg/L, and 921.3 mg/L, respectively. Compared with raw SCBB, the corresponding percentages of removal were 56.67%, 55.82%, and 76.45%, respectively.

3.6. Treatment Effect on SCBB in Fed-Batch Fermenter

The growth kinetics of strain DYYC01 in the fermenter exhibited characteristic exponential-phase progression (0–96 h) followed by a stationary-phase transition, mirroring the growth pattern observed in batch cultivation (Figure 7). After 144 h of fed-batch operation, the final OD660 reached 1.2, representing a 29.8% enhancement compared to batch culture maxima (0.924), demonstrating the efficacy of controlled nutrient supplementation in overcoming substrate limitation and sustaining exponential-phase progression. Concurrently, progressive acidification (ΔpH = −0.38 over 48 h) was observed during cultivation. Because acidification demonstrated inhibitory effects on strain growth (Figure 5), maintenance of pH homeostasis at 7.0 through controlled neutralization was proved critical for sustaining optimal growth.
Under controlled fed-batch conditions (30 °C, 100 μmol photons/m2/s, pH 7.0), strain DYYC01 demonstrated enhanced bioremediation efficiency in semi-continuous SCBB treatment, achieving significant reductions in TP (43.97 mg/L, 73.07%), TN (199.0 ± 8.6 mg/L, 72.17%), and COD: (660.0 ± 25.3 mg/L, 83.13%) compared to pretreatment levels TP (163.5 mg/L), TN (715.2 mg/L), and COD (3913 mg/L; p < 0.05), respectively (Figure 8). It can be concluded that DYYC01 had a faster growth rate, higher yield, and higher percentage of removal of SCBB in the fermentation tank than in batch culture.

4. Discussion

Marichromatium species are photosynthetic γ-proteobacteria found in marine environments. In the phylogenetic tree, the genus Marichromatium was divided into three main branches: M. bheemlicum and M. fluminis each formed a branch, while the other branch comprised M. indicum, M. gracile, and M. purpuratum (Figure 1). The shared branch of M. indicum JA100 and M. gracile DSM 203 had a bootstrap support value of 99, and the shared branch of DYYC01 and M. purpuratum 984 had a support value of 82. DYYC01 formed a separate node, indicating significant differences in its 16S rRNA gene sequence compared to closely related species within the same genus, suggesting that it may represent a new species or subspecies [23].
DYYC01 is a highly efficient photosynthetic bacterium for treating high-salinity SCBB. The optimal conditions (30 °C, pH 7.0, 100 μmol photons/m2/s light intensity, and high salt tolerance) are highly compatible with the characteristics of the target wastewater. Compared to other bacteria in the Marichromatium genus, DYYC01 shares some commonalities in its core growth conditions but also exhibits significant differences. In terms of temperature, DYYC01 aligns with most mesophilic Marichromatium species (optimal temperature range of 25–37 °C), but differs from extremophiles such as halophiles, which can tolerate high temperatures of 45 °C or low temperatures of 10 °C [24,25]. The light requirements of DYYC01 fall within the typical range (50–200 μmol/m2/s), distinguishing it from deep-sea strains adapted to low light intensities (20–50 μmol/m2/s) and some strains with specific light preferences (e.g., infrared light) [26]. In terms of pH tolerance, strain DYYC01 can grow within the pH range of 6.5–8.5, with optimal growth observed at pH 6.8–7.2. Bacteria of the genus Marichromatium generally thrive best in neutral to weakly alkaline environments, with an optimal pH range of approximately 6.5–8.5. Some strains can maintain certain activity within a pH range of 6.0–9.0, but growth and metabolic activity significantly decline outside this range [27].
When treating SCBB, DYYC01 achieved a TN removal rate of 55.82% in batch culture, which increased to 72.17% in a fed-batch fermenter (Figure 6b and Figure 8b). In comparison, the TN removal percentage of other salt-tolerant photosynthetic bacteria are generally lower. For example, Ref. [28] reported that salt-tolerant photosynthetic bacteria isolated from mangroves achieved a TN removal rate of approximately 40–50% when treating aquaculture wastewater. Similarly, other studies have reported that purple sulfur bacteria (e.g., M. purpuratum) typically exhibit TN removal percentage below 50% under similar high-salinity conditions [29,30]. The significant advantage of DYYC01 may be attributed to its efficient nitrogen metabolism pathways, such as the activity of nitrogenase to convert organic nitrogen (e.g., glutamic acid and aspartic acid in SCBB) into bioavailable forms [31]. The TP removal percentage of DYYC01 was 56.67% in batch culture and 73.07% in a fed-batch fermenter (Figure 6b and Figure 8b). In contrast, other strains such as Rhodobacter sphaeroides typically achieve a TP removal rate of 50–60% when treating high-salinity wastewater [32]. Most photosynthetic bacteria absorb phosphorus through phosphate transport systems, but their efficiency is limited by environmental osmotic pressure [33]. The ability of DYYC01 to maintain high phosphorus uptake under high-salinity conditions may be attributed to its unique osmotic regulation mechanisms (such as the accumulation of compatible solutes like glycine betaine) and the adaptive expression of phosphorus transport proteins [34]. The COD removal rates of DYYC01 were 76.45% in batch culture and 83.13% in a fed-batch fermenter (Figure 6b and Figure 8b), significantly higher than those of other strains. For example, Rhodopseudomonas palustris achieved a COD removal rate of approximately 70–75% when treating similar high-concentration organic wastewater [35]. The efficient degradation capability of DYYC01 is closely related to its preference for specific carbon sources in SCBB. The boiling broth contains abundant lactic acid (1530 mg/L), acetic acid (571 mg/L), and citric acid (959 mg/L), which are organic acids preferentially utilized by photosynthetic bacteria. Additionally, monosaccharides such as fructose (29.282 μg/mg) and mannose (17.688 μg/mg) may also provide easily degradable substrates for DYYC01.
The efficient utilization of organic matter in SCBB by DYYC01 is closely related to its metabolic characteristics. Small-molecule organic acids such as lactic acid and acetic acid serve as primary carbon sources for photosynthetic bacteria, as they can directly enter the TCA cycle to generate energy. Literature indicates that the metabolic rate of lactic acid by purple non-sulfur bacteria is 2–3 times higher than that of glucose [36]. The high concentration of lactic acid (1530 mg/L) in SCBB provides DYYC01 with an abundant energy substrate. SCBB contains amino acids such as glutamic acid (0.322 mg/mL) and aspartic acid, which can be converted into NH₃ through deamination and subsequently assimilated into bacterial protein [37]. Generally, photosynthetic bacteria have high activity of glutamine synthetase (GS) and glutamate dehydrogenase (GDH), optimizing nitrogen metabolism efficiency [38,39]. In fed-batch cultivation in a fermenter, supplementing carbon sources is more effective than batch culture [40]. Carbon sources support anaerobic metabolism, as photosynthetic bacteria rely on organic matter as electron donors for anoxygenic photosynthesis under anaerobic conditions. Supplementing carbon sources can maintain the continuity of anaerobic metabolism, thereby preventing metabolic stagnation or a shift to competitive metabolic pathways (e.g., acid fermentation) due to insufficient electron donors. During the feeding process, supplementing nitrogen and phosphorus may introduce oxidative substances, indirectly affecting the anaerobic environment. In contrast, carbon sources (e.g., organic acids) are non-oxidative and more conducive to maintaining anaerobic conditions [41].

5. Conclusions

The marine phototrophic bacterium Marichromatium sp. DYYC01, isolated from Yantai intertidal sediments, represents a phylogenetically distinct lineage, suggesting novel taxonomic status. Optimal growth occurred at COD 3913 mg/L, 30 °C, 100 μmol photons/m2/s, and pH 7.0, achieving 83.1% COD, 72.2% TN, and 73.1% TP removal of SCBB in the fed-batch cultivation. While demonstrating superior organic load tolerance (≤6522 mg/L COD) and haloadaptation (≤5% salinity), its pH sensitivity (optimal 6.8–7.2) necessitates environmental controls.
This study has preliminarily validated the effectiveness of DYYC01 in treating SCBB in the fed-batch cultivation, though further optimization is required to enhance its stability and cost-effectiveness for large-scale applications. Future research should focus on investigating various fed-batch cultivation operational parameters, including dissolved oxygen control, feeding strategies, and agitation rates, to improve both biomass productivity and pollutant removal efficiency of DYYC01.
The strain DYYC01 demonstrates notable advantages in treating hypersaline and high-organic-load wastewater under anaerobic conditions; however, its growth requirements and bioremediation potential for exogenous compounds beyond SCBB remain underexplored. Future studies should comprehensively investigate its growth and metabolic characteristics under diverse environmental conditions while systematically evaluating its long-term operational stability and stress resistance in complex wastewater matrices, which will provide crucial theoretical foundations for practical wastewater treatment applications.
During the treatment of SCBB, DYYC01 demonstrates dual functionality by effectively degrading organic compounds while simultaneously accumulating high-value cellular biomass and metabolic byproducts. Future investigations should focus on exploring the strain’s resource recovery potential, particularly its development as aquafeed additives, biofertilizers, or bioenergy precursors. Further research directions should encompass the optimization of extraction and purification processes for its metabolic products to facilitate the production of high-value bioproducts, thereby enhancing the overall economic viability of this biotechnological application.
After all, the marine phototrophic bacterium DYYC01 represents a novel microbial resource with demonstrated potential for high-strength organic wastewater treatment.

Author Contributions

S.D.: Investigation, Data curation, Project administration, Formal analysis, Writing—original draft. Y.G.: Data curation, Visualization, Formal analysis, Writing—review & editing. J.J.: Project administration, Writing—review & editing. P.S. and N.M.: Investigation, Writing—review & editing. H.Q.: Conceptualization, Methodology, Formal analysis, Supervision, Writing—review & editing. J.C.: Methodology, Supervision, Writing—review & editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Talent Induction Program for Youth Innovation Teams in Colleges and Universities of Shandong Province (2022–2024), Shandong Provincial Natural Science Foundation of Shandong Province (ZR2024MC078), National Natural Science Foundation of China (grant No. 32072997), and Ludong University Major Applied Technology Innovation Project. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflict of interest.

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Fermentation 11 00284 g001
Figure 1. Phylogenetic tree analysis of the 16S rRNA gene of DYYC01.
Figure 1. Phylogenetic tree analysis of the 16S rRNA gene of DYYC01.
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Fermentation 11 00284 g002
Figure 2. Growth curves of strain DYYC01 in different COD concentrations of sea cucumber boiling broth.
Figure 2. Growth curves of strain DYYC01 in different COD concentrations of sea cucumber boiling broth.
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Fermentation 11 00284 g003
Figure 3. OD660 obtained for strain DYYC01 under different irradiance conditions. Different lowercase letters indicate significant differences between groups (p < 0.05).
Figure 3. OD660 obtained for strain DYYC01 under different irradiance conditions. Different lowercase letters indicate significant differences between groups (p < 0.05).
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Figure 4. OD660 obtained for strain DYYC01 at different temperatures. Different lowercase letters indicate significant differences between groups (p < 0.05).
Figure 4. OD660 obtained for strain DYYC01 at different temperatures. Different lowercase letters indicate significant differences between groups (p < 0.05).
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Fermentation 11 00284 g005
Figure 5. OD660 obtained for strain DYYC01 at different pH values. Different lowercase letters indicate significant differences between groups (p < 0.05).
Figure 5. OD660 obtained for strain DYYC01 at different pH values. Different lowercase letters indicate significant differences between groups (p < 0.05).
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Fermentation 11 00284 g006
Figure 6. The treatment efficiency of strain DYYC01 on sea cucumber boiling broth under optimal conditions. (a) Variations in the concentration of TP, TN, and COD; (b) percentage of removal.
Figure 6. The treatment efficiency of strain DYYC01 on sea cucumber boiling broth under optimal conditions. (a) Variations in the concentration of TP, TN, and COD; (b) percentage of removal.
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Figure 7. Growth curve and pH variation of DYYC01 in fermentation production.
Figure 7. Growth curve and pH variation of DYYC01 in fermentation production.
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Fermentation 11 00284 g008
Figure 8. The treatment efficiency of strain DYYC01 on sea cucumber boiling broth under optimal conditions in the fermentation tank. (a) Variations in the concentration of TP, TN, and COD; (b) percentage of removal.
Figure 8. The treatment efficiency of strain DYYC01 on sea cucumber boiling broth under optimal conditions in the fermentation tank. (a) Variations in the concentration of TP, TN, and COD; (b) percentage of removal.
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Table 1. Raw chemical composition of sea cucumber boiling broth.
Table 1. Raw chemical composition of sea cucumber boiling broth.
ParameterSalinitypHTNTPCOD
Concentration2–9%6.6–6.8278–1100 mg/L80–203 mg/L1420–15,600 mg/L
Table 2. Ingredients of sea cucumber boiling broth.
Table 2. Ingredients of sea cucumber boiling broth.
Amino AcidOrganic AcidMonosaccharide
ItemConcentration (mg/mL)ItemConcentration
(mg/mL)		ItemConcentration
(μg/mg)
aspartic acid0.235oxalic acid0.350fucose2.259
threonine0.129tartaric acid0.395rhamnoseN.D
serine0.085formic acidN.Dgalactosamine hydrochloride0.244
glutamic acid0.322malic acid0.427arabinoseN.D
glycine0.144lactic acid1.530glucosamine hydrochloride2.674
alanine0.129acetic acid0.571galactose0.770
cystine0.052citric acid0.959glucose3.725
valine0.128succinic acidN.Dmannose17.688
methionine0.057propionic acidN.DxyloseN.D
isoleucine0.095--fructose29.282
leucine0.146--ribose1.423
tyrosine0.061--galacturonic acidN.D
phenylalanine0.141--guluronic acidN.D
histidine0.048--glucuronic acidN.D
lysine0.145--mannoturonic acidN.D
arginine0.071----
proline0.082----
Note: N.D, not detected.
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MDPI and ACS Style

Dong, S.; Guo, Y.; Ji, J.; Song, P.; Ma, N.; Qiao, H.; Cai, J. A Photosynthetic Bacterium Suitable for Treating High-Salinity Sea Cucumber Boiling Broth. Fermentation 2025, 11, 284. https://doi.org/10.3390/fermentation11050284

AMA Style

Dong S, Guo Y, Ji J, Song P, Ma N, Qiao H, Cai J. A Photosynthetic Bacterium Suitable for Treating High-Salinity Sea Cucumber Boiling Broth. Fermentation. 2025; 11(5):284. https://doi.org/10.3390/fermentation11050284

Chicago/Turabian Style

Dong, Shaokun, Yusi Guo, Jinrui Ji, Pu Song, Ning Ma, Hongjin Qiao, and Jinling Cai. 2025. "A Photosynthetic Bacterium Suitable for Treating High-Salinity Sea Cucumber Boiling Broth" Fermentation 11, no. 5: 284. https://doi.org/10.3390/fermentation11050284

APA Style

Dong, S., Guo, Y., Ji, J., Song, P., Ma, N., Qiao, H., & Cai, J. (2025). A Photosynthetic Bacterium Suitable for Treating High-Salinity Sea Cucumber Boiling Broth. Fermentation, 11(5), 284. https://doi.org/10.3390/fermentation11050284

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