Algae-Based Protective Coatings for Sustainable Infrastructure: A Novel Framework Linking Material Chemistry, Techno-Economics, and Environmental Functionality

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript «Algae-Based Protective Coatings for Sustainable Infrastructure: A Novel Framework Linking Material Chemistry, Techno-Economics, and Environmental Functionality» provides a comprehensive, timely, and innovative overview of the development and potential applications of algae-based protective coatings. The authors successfully integrate disparate fields of knowledge, including materials science, chemistry, engineering, economics, and ecology, into a coherent conceptual framework. This work significantly contributes to the field of sustainable materials by offering not just a replacement for existing products but also a new paradigm: a transition from passive protective coatings to active, multifunctional systems that protect infrastructure and improve the environment simultaneously. The manuscript scientific value is high, laying the foundation for future research and industrial implementation. The issues discussing in the manuscript matches perfectly the aims and scope of Phycology.

Despite its high quality, the article could be improved by addressing the following points. The title of the article is about algae-based coatings, but the text provides data related to cyanobacteria as well as algae. Cyanobacteria are bacteria, so referring to them as algae is incorrect. Secondly, modern requirements for reviews necessitate the use of advanced methods of analyzing and visualizing the collected data.

The article should include another new section on what factors affect the degradation of surfaces. This section should be the first point, because without assessing the degree of influence of factors on surface degradation, it is not possible to make recommendations. This section requires a detailed explanation of which abiotic and biotic factors contribute to material degradation and how these factors affect the surfaces of structures.It is important to address this topic because very little attention has been paid to biocorrosion in this review. Biocorrosion can be divided into bacterial and mycological types. The most common and corrosive microorganisms include three groups of bacteria: aerobic, anaerobic, and silicate bacteria.Fungi are characterized by the presence of all currently known enzyme groups, enabling them to destroy almost all organic substances that come into contact with them. The rate of corrosion increases with the accumulation of water and pollution and with the number and activity of microorganisms. Due to their high adaptability to environmental conditions and species diversity, the volume of materials damaged by fungi significantly exceeds that damaged by bacteria. After studying this issue, the authors should conclude whether the proposed materials are effective against biocorrosion. I would also like to know the extent of the damage caused by various factors, as this information will help you decide what you need to protect the surface of structures from in the first place.

I can recommend authors to add a summary table or diagram that visually compares the key performance indicators of algae-based coatings and traditional ones (polyurethane, epoxy resins). This may include ranges of values for adhesion, UV resistance, corrosion rate, estimated cost, etc. This will make it much easier to quickly compare and understand the current status of the technology.

It is also recommended to add a schematic illustration depicting the conceptual scheme proposed by the authors, which will become a visualization of the "new paradigm" described in the text. It could be a flowchart linking "Cultivation - Extraction - Application - Multifunctional productivity (Protection + Sequestration of Co₂ + Cleaning), etc.".

Table 1: Despite its value, the table is very lengthy. It is recommended that it be divided into several thematic sections, such as "1. Algae – Components" and "2. Components – Properties" or that it be placed in an appendix of the final version of the article. In the main text, an abbreviated version or key conclusions could be included. The table will also be easier to understand if the first and second columns are switched so that the starting point is a biological resource, i.e., an alga from which a chemical component will be obtained.

In section 5.1, it would be useful to present a more structured qualitative sensitivity analysis. What factors (for example, the cost of energy, the yield of biomass, the price of carbon credits) have the greatest impact on the final cost of the product? What are the targets (for example, $/kg of biomass) needed to achieve price competitiveness with petrochemical analogues?

The conclusions need to be shortened. More specific, structured information on the study's points is necessary, as well as a list of three to five of the highest priorities for future research resulting from the analysis.

I recommend reviewing literature on biocorrosion, such as the following:

Videla H.A. Microbiologically influenced corrosion: looking to the future / H.A. Videla, L.K. Herrera // International Microbiol. — 2005. — Vol. 8(3). — Р. 169–180..

Improvement of functional performance of concrete in livestock buildings through the use of complex admixtures / O. Shkromada, A. Paliy, O. Nechyporenko et al. // Eastern-European Journal of Enterprise Technologies. — 2019. — Vol. 1(7(97)). — Р. 41–49.

Latin names throughout the manuscript are not italicised; please italicise them. In addition, you can add the authors of the species. For some species, outdated names are given, for example Spirulina platensis.

Author Response

General Comment: Reviewer: “The manuscript... provides a comprehensive, timely, and innovative overview... The authors successfully integrate disparate fields of knowledge... This work significantly contributes to the field of sustainable materials... The manuscript scientific value is high.”

Response: We are sincerely grateful to the reviewer for such a detailed evaluation and for the encouraging comments regarding the manuscript’s scientific value and innovative approach. The constructive criticism provided has significantly strengthened the paper, particularly regarding the mechanisms of surface degradation and the techno-economic analysis. We have addressed all points raised, as detailed below.

Point 1: Taxonomy (Cyanobacteria vs. Algae) Reviewer: “The title of the article is about algae-based coatings, but the text provides data related to cyanobacteria as well as algae. Cyanobacteria are bacteria, so referring to them as algae is incorrect.”

Response: We agree with the reviewer’s observation. To address this:

1. We have corrected the taxonomy throughout the manuscript. Specifically, we have replaced the outdated name Spirulina platensis with the scientifically correct Arthrospira platensis (a cyanobacterium).

2. We have added a clarifying statement in the Introduction acknowledging that while cyanobacteria are prokaryotic, they are often grouped with microalgae in industrial biotechnology contexts due to functional similarities. However, we have ensured the distinction is clear in the scientific descriptions.

3. The Abstract has been updated to explicitly mention "microalgae and cyanobacteria."

Point 2: New Section on Surface Degradation Reviewer: “The article should include another new section on what factors affect the degradation of surfaces. This section should be the first point... It is important to address this topic because very little attention has been paid to biocorrosion in this review.”

Response: We have accepted this suggestion and made it a central part of the revision. We have inserted a new Section 2: Surface Degradation Mechanisms in Infrastructure Materials.

• This section now precedes the discussion on coating formulation, establishing the "problem" before the "solution."

• We have detailed both Abiotic Factors (UV, thermal, chemical) and Biotic Factors (Microbiologically Influenced Corrosion - MIC).

• As requested, we specifically discuss the roles of aerobic, anaerobic, and silicate bacteria, as well as the severe impact of fungal degradation.

Point 3: Biocorrosion Literature & Effectiveness Reviewer: “I recommend reviewing literature on biocorrosion, such as Videla H.A. (2005) and Shkromada O. et al. (2019)... After studying this issue, the authors should conclude whether the proposed materials are effective against biocorrosion.”

Response: We have reviewed the recommended literature and incorporated it into the new Section 2.

• We cited Videla & Herrera (2005) regarding the distinction between bacterial and fungal corrosion mechanisms.

• We cited Shkromada et al. (2019) regarding functional performance in concrete structures.

• In Section 5.2 (Protective Properties), we explicitly discuss the effectiveness of algae-based coatings against these biocorrosion vectors. We highlight that unlike passive coatings, algae-based formulations containing antifouling compounds (e.g., phenols, thymol) can actively inhibit the biofilm formation that precedes biocorrosion.

Point 4: Comparison Table/Diagram Reviewer: “I can recommend authors to add a summary table or diagram that visually compares the key performance indicators of algae-based coatings and traditional ones (polyurethane, epoxy resins).”

Response: We have added Table 2: Comparison of Performance Characteristics, which directly contrasts Algae-Based Coatings against Traditional Coatings (Epoxy/Polyurethane). This table compares specific metrics including adhesion strength, UV resistance, corrosion rate, and CO₂ sequestration potential, allowing for a quick assessment of the technology's current status vs. industry standards.

Point 5: Schematic Illustration Reviewer: “It is also recommended to add a schematic illustration depicting the conceptual scheme proposed by the authors... linking Cultivation - Extraction - Application...”

Response: We have included Figure 1, a schematic flowchart that visualizes the entire lifecycle of the coating. This figure illustrates the "new paradigm" by linking the production stages (Cultivation, Extraction) to the multifunctional outputs (Surface Protection + Carbon Sequestration + Air Purification), visually summarizing the circular economy framework described in the text.

Point 6: Table 1 Restructuring Reviewer: “Table 1: Despite its value, the table is very lengthy... recommended that it be placed in an appendix... easier to understand if the first and second columns are switched so that the starting point is a biological resource.”

Response: We fully agree that the table was disrupting the flow of the text.

1. We have moved the detailed data to Appendix 1.

2. We have restructured the table as requested: The Algae Species is now the first column, followed by the Chemical Component. This makes it much easier to identify the biological source of the functional materials.

Point 7: Sensitivity Analysis Reviewer: “In section 5.1, it would be useful to present a more structured qualitative sensitivity analysis. What factors... have the greatest impact on the final cost?”

Response: We have expanded the economic analysis significantly. We added a new subsection 6.2 Sensitivity Analysis of Key Economic Drivers and a corresponding Table 4.

• This analysis identifies Biomass Productivity and Extraction Efficiency as the highest-impact factors.

• We discuss specific targets, noting that volumetric productivity needs to exceed 0.6 g·L⁻¹·day⁻¹ to achieve price competitiveness with petrochemical analogues.

Point 8: Conclusion Reviewer: “The conclusions need to be shortened. More specific, structured information... as well as a list of three to five of the highest priorities for future research.”

Response: We have completely rewritten Section 7 (Conclusion).

• It has been condensed to focus on the core findings.

• We have explicitly listed four high-priority areas for future research: (1) Biomass Standardization, (2) Advanced Hybrid Formulations, (3) Extraction Process Intensification, and (4) Regulatory Framework Development.

Point 9: Latin Names & Formatting Reviewer: “Latin names throughout the manuscript are not italicised; please italicise them... For some species, outdated names are given, for example Spirulina platensis.”

Response: We have carefully proofread the manuscript.

• All scientific names (e.g., Chlorella vulgaris, Haematococcus pluvialis) are now italicized.

• We have updated the nomenclature, replacing Spirulina platensis with Arthrospira platensis throughout the text and tables.

Reviewer 2 Report

Comments and Suggestions for Authors

This article compares algae-based protective coatings with conventional petroleum-based coating systems with respect to their functional performance, techno-economic feasibility, and industrial scalability within a circular economy context. The authors also review algae cultivation systems and the biochemical composition of the obtained biomass in relation with its used as protective coating systems. Due to the importance of algae-based protective coating systems and the importance of replacement of petroleum-based products with their sustainable counterparts, this study will attract the attention of researchers from a wide range of fields and it will make a significant contribution to the field.
However, there are several minor issues that prevent the publication of the work in its current form:
1. The authors repeat themselves in many places and touch upon the same subject more than once. For example, algae cultivation systems based on open ponds and photobioreactors were compared at Lines 327-347 and at Lines 740-749 ones more. Even the acronym “Photobioreactor (PBR)” has been explained many times (Lines 326, 338, 458, 740, 817, 910). If authors shorten these repetitions, the article will be easier to read.

2. The authors compare algae-based protective coatings with only conventional petroleum-based coating systems. Comparison with other bio-based coating systems would be very beneficial.

Author Response

General Comment: Reviewer: “This study will attract the attention of researchers from a wide range of fields and it will make a significant contribution to the field... However, there are several minor issues that prevent the publication of the work in its current form.”

Response: We wish to express our sincere gratitude to the reviewer for the positive assessment of our work and for recognizing its potential contribution to the field of sustainable materials. We have carefully addressed the minor issues raised, specifically regarding textual repetition and the scope of the comparative analysis.

Point 1: Repetition of Topics and Acronyms Reviewer: “The authors repeat themselves in many places... algae cultivation systems based on open ponds and photobioreactors were compared at Lines 327-347 and at Lines 740-749 ones more. Even the acronym “Photobioreactor (PBR)” has been explained many times... If authors shorten these repetitions, the article will be easier to read.”

Response: We agree that the redundancy in describing cultivation systems and defining acronyms disrupted the flow of the manuscript. We have made the following revisions to improve readability:

1. Consolidation of Cultivation Descriptions: We have retained the technical description of Open Ponds and Photobioreactors (PBRs) in Section 4.1 (Cultivation Systems). In the economic analysis (Section 6.1), we have removed the descriptive text explaining how these systems work and now focus strictly on their cost implications (CAPEX/OPEX), referencing back to Section 4.1 where necessary.

2. Acronym Standardization: We have performed a thorough check of the manuscript to ensure that "Photobioreactor (PBR)" and other acronyms are defined only at their first appearance. Subsequent mentions now consistently use the abbreviated form.

Point 2: Comparison with Other Bio-Based Systems Reviewer: “The authors compare algae-based protective coatings with only conventional petroleum-based coating systems. Comparison with other bio-based coating systems would be very beneficial.”

Response: We appreciate this valuable suggestion, as it helps position algae-based coatings within the broader context of the bioeconomy. We have added a new subsection, Section 3.4: Comparison with Other Bio-Based Coating Systems.

• This section critically compares algae-based coatings against Lignin, Nanocellulose, Chitosan, Shellac, and Bacterial Biopolymers.

• We highlight that while materials like Nanocellulose excel in barrier properties and Lignin in UV stability, algae-based coatings offer a unique multifunctional profile that combines intrinsic UV protection with active carbon sequestration.

• We have also added Table 3, which summarizes these comparisons across key metrics such as adhesion, biodegradability, cost, and scalability.

Reviewer 3 Report

Comments and Suggestions for Authors

Overall, this is a well-written and comprehensive review article. The topic is timely and relevant, and the authors successfully outline the environmental limitations of traditional polymer-based coatings, the potential of bio-based alternatives, and the emerging interest in algae-derived coating materials. The manuscript is generally clear, and the structure is coherent.

However, some aspects could be improved to strengthen the contribution and better support some key claims. In particular, if the authors aim to demonstrate that microalgae represent a truly promising and scalable alternative to conventional or other bio-based coatings, this must be more clearly substantiated throughout the text. Additionally, the manuscript would benefit from more visual synthesis (figures, tables, comparative schematics) to enhance readability and provide a clearer comparison between traditional coatings, oil-based biocoatings, and algae-based coatings. Minor formatting issues are also present, such as the unexpected use of bold text.

Author Response

General Comment: Reviewer: “Overall, this is a well-written and comprehensive review article... The manuscript is generally clear, and the structure is coherent.”

Response: We thank the reviewer for their positive assessment of the manuscript’s structure and relevance. We appreciate the constructive feedback regarding the need for stronger substantiation of our claims and better visual synthesis. We have addressed these points as follows.

Point 1: Substantiating Scalability and Promise vs. Other Systems Reviewer: “If the authors aim to demonstrate that microalgae represent a truly promising and scalable alternative... this must be more clearly substantiated throughout the text.”

Response: We have significantly strengthened the comparative analysis to better substantiate the distinct advantages of algae-based systems:

1. New Section 3.4 (Comparison with Other Bio-Based Coating Systems): We added a dedicated subsection comparing algae-based coatings not just to petroleum products, but to other bio-based competitors including Lignin, Nanocellulose, Chitosan, Shellac, and Vegetable Oils.

2. Comparative Tables: We introduced Table 3, which explicitly contrasts these materials across nine criteria (including Sustainability, Barrier Properties, and Cost). This analysis substantiates our claim that while other bio-based materials excel in specific areas (e.g., Lignin in UV stability), algae-based coatings offer a unique multifunctional advantage via active carbon sequestration and antimicrobial activity.

3. Scalability Analysis: We expanded the economic assessment in Section 6.2 (Sensitivity Analysis) to provide a realistic view of the scalability challenges (specifically biomass productivity targets) required to make these systems commercially viable.

Point 2: Visual Synthesis (Figures and Comparative Schematics) Reviewer: “The manuscript would benefit from more visual synthesis (figures, tables, comparative schematics) to enhance readability and provide a clearer comparison...”

Response: We have integrated additional visual and tabular elements to synthesize the complex data:

1. Figure 1 (Schematic): We added a conceptual flowchart illustrating the "Circular Coating Paradigm," visualizing the link between Cultivation, Extraction, Formulation, and Multifunctional Application.

2. Table 2 (Algae vs. Traditional): A direct comparison of performance metrics (Adhesion, UV Resistance, Cost) between algae-based and conventional Epoxy/Polyurethane coatings.

3. Table 3 (Algae vs. Bio-Based): A comprehensive matrix comparing algae against oil-based and biopolymer-based alternatives, as requested.

Point 3: Formatting (Bold Text) Reviewer: “Minor formatting issues are also present, such as the unexpected use of bold text.”

Response: We have reviewed the manuscript for formatting consistency. Specifically, we have removed the bold paragraph headers in Sections 4.1 and 4.2 to ensure a standard academic presentation style. All scientific names have also been checked for proper italicization.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have done a great work to improve the manuscript. All my comments have been taken into account, and I am glad that they have helped to improve the manuscript so much. The added tables and figures also make a significant contribution. It's good that the authors corrected the taxonomy. Now, in my opinion, cyanobacteria can be added to the keywords.

Author Response

Reviewer Comment - The authors have done a great work to improve the manuscript. All my comments have been taken into account, and I am glad that they have helped to improve the manuscript so much. The added tables and figures also make a significant contribution. It's good that the authors corrected the taxonomy. Now, in my opinion, cyanobacteria can be added to the keywords.

 

Author Response - We sincerely thank the reviewer for the positive feedback and helpful suggestion. We have updated the manuscript keywords to include “Cyanobacteria” as recommended. We greatly appreciate your valuable comments which have improved the manuscript.