Emerging Technologies for Enhancing Microalgae Biofuel Production: Recent Progress, Barriers, and Limitations

Round 1

Reviewer 1 Report

This manuscript discusses about role of microalgae in biofuel production. Author has discussed various technologies and challenges related to biofuel production. Manuscript need following changes to improve it further.

Comments:

Line 37: Furthermore, fossil fuels have a high environmental impact……add more information on fossil fuel impact.

Line 39: Rewrite the statement as biofuels are not extracted from biomass but used as raw material and various processes are used to convert them into biofuels like pyrolysis, dark fermentation, anaerobic digestion etc. Authors are suggested to discuss these recent articles in support.

An Overview on Co-Pyrolysis of Biodegradable and Non-Biodegradable Wastes

Biorefinery of anaerobic digestate in a circular bioeconomy: Opportunities, challenges and perspectives

Fermentative Production of 2, 3-Butanediol using Bread Waste–A Green Approach for Sustainable Management of Food Waste.

Line 47: Even though second-generation feedstocks…you have not discussed lignocellulosic biomass. Discuss about this and challenges associated with it. Discuss Recent developments in pretreatment technologies on lignocellulosic biomass: effect of key parameters, technological improvements, and challenges.

Line 50: In both third and fourth generations….not only the extracted lipids is used for biofuel but residual biomass is also used for biofuel production using it feedstock for fermentation and direct pyrolysis.

Line 69: . The microalgae species are mainly catego rized into rhodophytes, also known as red algae….this is wrong statement. You are discussing microalgae not macroalgae.

Line 73: Microalgae species can produce various essential biochemical….mention few products.

Line 87: For environmental applications…..discuss Valorization of wastewater resources into biofuel and value-added products using microalgal system.

Line 112: the application of microalgae for wastewatr treatment…discuss Progress in microalgal mediated bioremediation systems for the removal of antibiotics and pharmaceuticals from wastewater.

Table 1. Write name of all microbes in italics and mention full name when used first time.

Section 2.2: Rearrange the section as 1) catalysts for oil extraction and 2) catalysts for biodiesel production.

Section 2.2.3: Add more information in this section. Biochar is also use as a catalyst add some information.

Section 2.3: Section 2.3 is not making any sense as a separate section. Move relevant information to respective section as 1) catalysts for oil extraction and 2) catalysts for biodiesel production as suggested to prepare new sections.

Your manuscript title mention biofuel but in your manuscript, you have discussed about biodiesel only. You are suggested to include more examples and tables to represent other biofuels like biohydrogen, syngas, bio oil etc. with role of nano catalysts.

Author Response

File attached

Author Response File: Author Response.docx

Reviewer 2 Report

Abstract

-  It is necessary to include the most relevant aspects highlighted in the manuscript and include a conclusion.

Introduction

- Lines 63-64: The main components for microalgae growth are sunlight, CO2, and water [20].

Comments:  In this sentence should be included the nutrients that are essential elements.

-        Lines 75-77: Microalgae applications can mainly be categorized into industrial, food, environmental, and pharmaceutical applications.

Comments: Please, include a reference to support this text.

-        Lines 86-87: Biomass residual is high in proteins but low in lignin; thus, it can be utilized for 86 the production of various commercial products [27–29].

Comments: Include a description of what types of products can be obtained.

-        Lines 91-92: There is an abrupt change between paragraphs; it is necessary to include a connecting text.

-        Lines 106 to 110: Please, include a reference to support this text.

-        Line 126. Avoid use etc.

Comments: avoid the use of etc. please detail the examples.

-        Line 158. Sheehan et al. came …  missing a year for Sheehan citation and is not included in the references section. At the end of the paragraph describing this text, there is a reference (52), but it is not from Sheehan et al.

-        Line 163 to 166. In two consecutive paragraphs reference 49 is used; you can omit the use of the first on line 164.

-        Table1, include the full name of the genus of each species used in the table.

-        Line 261, In a study carried out by [72],  … must be changed by In a study carried out by Duraiarasan et al. [72], …

-        Line 264, avoid using apostrophes

-        Line 314, Table 2. There is no history of use of marine species; it would be important if there were this data to include in the table

-        Line 340, the use of electricity to produce microalgae cultures generates an increase in production costs; this point is not discussed by the authors and should be considered.

-        Line 361, Table 3. Is it possible to include some economic cost data in this table?

-        Line 408, To avoid the damage caused by introducing microalgae species, the use of native species can be favored; please include a text related to this topic.

-        Line 414, avoid using apostrophes; please revise the manuscript.

Conclusion

 Line 429, need to improve the conclusion.

General comments

From my point of view, some works related to transcriptomics and gene synthesis pathways associated with the production of metabolites of interest should be included.

Author Response

File attached

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript highlights some important aspects that could unlock the future potential of microalgae-based fuels. There are major points that need to be addressed prior to my reconsideration. Overall, the authors need to be more critical. They go over and over basic and common knowledge points and do not present arguments good enough to support their statements. They do not provide critical hypotheses on their own on how they can also make some predictions about the future of this field.

My main comments, apart from this, are

1) Methodology - how did you collect the articles to write this review? Describe this in the manuscript.

2) Abstract - Be more direct in the abstract concerning microalgae-based biofuels. You should mention some critical indications, rather than simply generic terms such as nanomaterials and genetic engineering.

3) The title is way too broad for the topic of the paper.

4) Section 2.1 - this should be an introduction to the section. I was expecting more critical results described here. 

5) Line 180 - Recent studies have concentrated on catalytic improving of oil obtained from algae - improving oil???

6) No tables concerning catalysts for biodiesel production? This is way too shallow for a review article. The authors simply resemble a series of sentences that do not connect to each other without a proper goal in this section.

Overall, the authors need to go through a very thorough revision process to make sure this is not simply another review on top of the many excellent review papers on this field.

Author Response

File attached

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The author has not mentioned line numbers in the response file where corrections and changes have been incorporated. It's hard to trace changes, and suggested literature is also not discussed and listed in the reference list.  Add references in the table. 1 as data collected from different articles.

Author Response

Please see the attached file.

Author Response File: Author Response.docx

Reviewer 2 Report

General comments

The manuscript “Biofuel from microalgae: Recent progress, Barriers, and Limitation” is acceptable for publication due to needing minor revisions to be reconsidered.

Introduction

- Line 49, Correct CO2 subscript

- Lines 119-122, change the text from bold to standard text

Methodology

-      Lines 184 to 203, avoid using capital letters such as "AND" and "OR" in the course of the paragraph.

-      Lines 206-207, modify the wording to avoid "Incremental increase ...”

-      Table 1, Species names center left for easy reading.

-      Line 424, do not use letters in bold

-      Table 2, do not use italics to describe the species abbreviation "sp." In Nannochloropsis sp.

-Line 665, avoid use of ;; modify by ;

General comments

- Extend the description of the text that describes the use of biorefineries in the production of biofuels with microalgae.

- In the section on cultivation technology, it is recommended to consider the following work:

Stephens-Camacho, B.E., Sauceda-Carvajal, D., Sánchez-Saavedra M.P 2021. Growth, proximate composition, and photosynthesis of Chlorella vulgaris cultures between a photobioreactor based in a compound parabolic concentrator and a plain tubular system for a biorefinery. BioEnergy Research 15(1): 230-241.

Author Response

Please see the attached file.

Author Response File: Author Response.docx

Reviewer 3 Report

The authors have answered all my queries

Author Response

Please see the attached file.

Author Response File: Author Response.docx