Producing Omega-3 Polyunsaturated Fatty Acids: A Review of Sustainable Sources and Future Trends for the EPA and DHA Market
, Thomas Dietrich 1, Izaskun Marañón 1, Maria Carmen Villarán 1 and Ramón J. Barrio 2Round 1
Reviewer 1 Report
I have reviewed the manuscript by Oliver and colleagues, entitled “producing Omega-3 Polyunsaturated fatty acids: a review of sustainable sources and future trends for DHA and EPA market."
Major comments
- The level of English is deficient, the manuscript needs a full English grammar revision.
For a start, in the Title, why is the definite article “the” not used? It is “…future trends for the DHA and EPA market”, not “..for DHA and EPA market”.
- The structure of the review is not good. In many sections, paragraphs are merely bullet points of one or two sentences. Please create paragraphs of 5 to 6 sentences, with each paragraph describing a particular aspect of your argument. Take a good quality review from the literature, and follow that structure. For example from the journal Resources itself (https://www.mdpi.com/journal/resources/most_cited ).
- The order of the review is chaotic and not logical. For example, The Introduction starts with a general description of the importance of omega-3s for health. But in the second paragraph, the authors immediately jump to mentioning recent studies of omega-3s in covid-19. Why? That is a very new and still niche area of research on omega-3s and unrelated to the focus of this review. Just focus on the main benefits of omega-3s that have been shown and intensively studied in the past decades.
There is no need to be so extensive on the health benefits of omega-3s if the focus of this review is on the sustainable production of omega-3 LCPUFA. If you desire to have a section on health benefits, please mention the most important health benefits in section 2. Section 6 also discusses health benefits again, for example on colon cancer. Just integrate that with section 2, or just leave it out since it does not contribute to substantiate the point of your review. I rather see the text in lines 142 – 155 should figure much earlier in this review, if not constituting the main part of the introduction.
- Section 2 reviews the enzymes involved in EPA and DHA biosynthesis and those that form bioactive lipid mediators derived from EPA and DHA. However, the authors mix up these enzyme systems. For example, line 69 states that Omega-3 fatty acids compete with precursors for omega-6 fatty acids. Please be clear – which omega-3 fatty acids? And with which precursors? Competition by which specific fatty acids for which enzymes? Now this is followed by a statements that competition occurs for the same enzymes to advance in their metabolic pathways – you mean the biosynthetic enzymes that convert precursors alpha-linolenic acid and linoleic acid to EPA/DHA and arachidonic acid, respectively, or the enzymes that convert the LCPUFA to bioactive mediators? Cyclooxygenases and lipoxygenases are not involved in the biosynthesis and do not contribute to omega-3 LCPUFA generation, so lines 69-72 are extremely confusing. Please clarify and rewrite. I think section 2 could almost entirely be deleted since it does not contribute to the focus of this review, and the authors are not able to clearly explain the metabolic pathways either.
- Overall, I think believe most of the text on health benefits, and section 2 in particular, are not needed in this review. According to the title, this review is about sustainable sources of EPA and DHA and trends in the EPA and DHA market. These two topics should be enhanced and health benefits can be summarized in one short paragraph and several good reviews of health benefits cited.
- The most interesting part of this review, and also mentioned in the abstract, is the potential use of volatile fatty acids as carbon source for microalgae growth and EPA/DHA production. However, the section where this is described, lines 276-283, is extremely short. This could be the main, or a major, topic of the review since it is noteworthy and novel to most future readers of your review. But then, the authors do not explain what these volatile fatty acids are. Are these short chain fatty acids with low boiling points? Are these volatile secondary oxidation products derived from fatty acids? Why not include a graph with their chemical structures, and provide an overview how these VFAs are metabolized and converted into the carbon substrates that microalgae use for EPA/DHA biosynthesis. It would also be interesting to know what levels of such VFAs are produced in fermentation effluents, and by what means and how much of these VFAs can be transferred into microalgae cultures. What are “dark fermentation effluents” – the authors are familiar with this terminology but the readers of your review will not be, so please expand on this. Are the gases of the dark fermentation effluents directly bubbled into the microalgae cultures (with requirements for co-localization), or are the VFAs first concentrated somehow and then provided to the microalgae? And why would this constitute a more economical and scalable carbon source than sugar? At what stage of development are such applications, and can you give real-life examples in the review? Please explain all this to substantiate why you consider VFAs a “better approach” (line 288) and make this review more interesting.
- Section 6 is about the market for EPA and DHA, but there are no numbers of the actual market for these ingredients. Please describe the volumes of oil and market values for these ingredients going into the various market segments like infant formula, dietary supplementation, functional foods, and pharmaceutical ingredients. Is there really a case to be made for cheaper carbon sources, and how would implementing VFAs use grow the availability of EPA and DHA omega-3s and contribute to sustainability?
- Section 6 contains a quite extensive discussion of demographics and the ageing population. But the link to the importance of omega-3 consumption is not given. Why write so much about demographics if there is no clear case argumented for increased intake of omega-3s in the elderly? Please clarify. Overall it would be good to weigh the amount of text given to topics that contribute to substantiating to focus of your review (as given in the title), versus seemingly less relevant areas. Right now, the review is very unbalanced and poorly focused.
Minor comments
Title – Producers of omega-3s typically mention EPA first, and then DHA. Feel free to amend your title.
Abstract – line 20 – You indicate that “…microalgae cultivated under heterotrophic conditions is a promising alternative source to supply omega-3 PUFA”. Please note that this technology has been used for 3 decades now to produce DHA, mainly as ingredient for infant formula. So, there is nothing “promising” about heterotrophic microalgae production. Rather use the wording “…microalgae cultivated under heterotrophic conditions is increasingly recognized as a suitable technology for the production of the omega-3 LCPUFA EPA and DHA”. (or similarly worded).
Line 57 – Please write that some specific microalgae species produce EPA or DHA. Not all microalgae have that biosynthetic capacity, which is what you are suggesting currently.
Line 58 – Why do you give the double bond locations for EPA and not for DHA?
Line 67 – In biology there is no “neutral” activity for bioactive mediators. Please rewrite. Also after the word “anti-inflammatory” there should be a noun, eg “activity”. Also the metabolic pathways themselves do not have a biological activity, the bioactive lipid mediators formed from EPA and DHA do. Please rewrite the sentence.
Line 67 – If you use the wording “In contrast,..” you suggest that omega-6 fatty acids have an opposite biological activity from that of the lipid mediators formed from omega-3 LCPUFA. However, anti-inflammatory and inflammation resolving mediators are also formed from arachidonic acid, for example lipoxin A4 and prostaglandin E2. It is easy to paint a picture that mediators derived from omega-3s are anti-inflammatory and omega-6 are pro-inflammatory but that is an over-simplification. Please rewrite.
Line 73 – Add the word “acid” to alpha-linolenic and linoleic. If you want to abbreviate linoleic acid, please use LA and not AL. It is not really necessary because in the rest of the review, linoleic acid is not mentioned again.
Line 82 – Please write a correct sentence, e.g. “ It is estimated that only 8-20% of a dose of ingested ALA is converted into EPA, and 0.5-9% into DHA”. Please check the literature because to my understanding a large portion of people are not able to convert ALA to DHA at all, for example men (check references here link ).
Line 89 – Please include a reference by William Lands, for example link (Lands, Progress Lipid Res 55, 17-29).
Line 90 – Contexts (plural)
Line 96 – It has been shown (not proven)
Line 100 – The authors again mention the Greenland Inuit study already mentioned in line 37. Please avoid repetitions
Line 105 – The authors explain fish species with significant omega-3 EPA/DHA levels here – this section should be connected with line 45, to make the review not jump around.
Line 108 – provide a reference to which organization suggests a weekly consumption of 2-3 servings of fatty fish.
Line 108 – I don’t think the author’s recommendation to make serving size advice dependent on fish species and the many variables that determine EPA/DHA content will be a practical one. This suggestion is off topic for this review.
Line 111 – If the authors suggest that due to overfishing the quantity of wild fish is constant, please provide references to support your rather unconventional point of view. Rather the authors may want to explain that the fishing of most fish species used for the production of fish meal and fish oil are tightly regulated by catch quota systems, usually at national level, with the most relevant example being that of Peruvian anchoveta. Also, overfishing is not a major concern with regard to fish use for fish oil and fish meal production today, please have a look at the work of MSC (https://www.msc.org/en-us/standards-and-certification/fisheries-standard). Rather the authors could argue that beyond the allowable catch volumes, alternative sources of EPA/DHA need to be further expanded, and significantly.
It is also not clear what the authors mean by saying that “Wild captured fish should be considered as reference”. As a reference for what?
Line 142-147 – If you start this paragraph with the last sentence (line 146-147, the paragraph makes more sense. Please provide a reference to that sentence also. Please explain better that the main species cultivated for human consumption (not for fish oil production for feeds) is Atlantic salmon. The sentence starting with “Normally” does not have a verb, it makes no sense, what do you mean to say?
Line 148 – Please check the grammar, this sentence makes no sense. Something must be leading the price increase.
Line 155 – please add the word “aquaculture” between healthy and seafood.
Line 170 – a ) is missing
Line 170 – This phrase is a direct copy of the title of the cited study. Please rewrite your interpretation of that publication in your own words.
Line 173 – Please add a reference.
Line 174 – The authors state that the temperature effect was confirmed, whereas the cited study merely makes a prediction. Please check.
Line 180 – There "is" not enough
Line 181 – It is not clear who is the user and the provider, the traditional sources or the human requirements? Please rewrite
Line 185 – The sentence starting with “(The) First step of..." has no verb. What is the meaning of this?
Line 186 – Pagrus major should be in italics
Line 192 – The sentence has not started correctly. Impossible to understand what the authors mean
Line 197. Please add references to support "Recent advances have been made..". Why not highlight some of these advances, in particular those related to EPA and DHA biosynthesis in microalgae that can be cultivated under heterotrophic conditions?
Line 201 – The authors make a general statement that phototrophic microalgae make only low levels of omega-3 PUFAs. First of all a reference is needed to substantiate this. Also, to which omega-3 PUFAs do the authors refer to? Only EPA or DHA or also other omega-3 fatty acids such as alpha-linolenic acid? The latter is ubiquitous in algae and plants. There are several companies that succesfully produce EPA-rich algae under phototrophic conditions at a scale sufficient for human consumption (XiaoZao Tech, China; ArizonaAlgae, USA, Simris Alg, Sweden).
Line 210 – Use the abbreviation EPA
Line 211 – commercial potential
Lines 212-214 – this general statement better suits the Introduction section
Line 217 – the verb is “to grow” (correct throughout the manuscript)
Line 218 – please change “could be” to “can be” or “are” – microalgae have long been cultivated under heterotrophic conditions by multiple producers
Line 225 – Why are these two families not mentioned in the section of lines 189-184?
Line 228 – Please write the species name correctly: Crypthecodinium cohnii, here and throughout the manuscript and in italics
Line 235 – a reference is needed
Line 236 – …, such as glucose.
Line 240 – The assumption of the authors is incorrect. Currently, heterotrophic culture of microalgae for EPA and DHA production is being carried out at commercial scale by several companies. Some examples: Fermentalg, France; DSM, The Netherlands; XiaoZao Tech, China. For sure, and that is the contribution of this review to point out, it can be carried out in cheaper ways following the identification of cheaper carbon sources. Please rephrase.
Line 261 – The point that the authors make about the two-stage cultivation and dependence on nitrogen consumption is interesting but without further explanation the reader is left in the dark why this is relevant. For example, what are those other substances in cane molasses that interfere with the fermentability of its sugars, and why does nitrogen first need to be consumed before carbon utilization occurs? Please expand, these are the interesting parts of your review.
Line 263 – Remove the word “nevertheless”. And use the plural "with different carbon sources"
Line 264 – have (plural)
Line 268 and 269 – cohnii. “was also growth” is not good English
Line 270 – “It” refers to galacturonic acid, to exhausted olive pomace, or C. cohnii? Also, what is exhausted olive pomace versus olive pomace?
Line 272 – “a combination” (in Spanish you also use the article, right? why not use it in English or at least check how to write English correctly)
Line 276 – What do you mean with “Among all,”? – among all the known microalgae species?
Line 276 – What do you mean with “to get PUFAs”? To produce PUFAs in an economic manner under heterotrophic conditions, or to isolate the EPA/DHA-rich oil from the microalgae?
Table 1:
- Please correct to Schizochytrium where needed
- All listed algae species have the family name indicated, except C. pyrenoidosa. Why? Please be consistent
- Only DHA-producing microalgae are listed. Don’t examples for EPA producing microalgae species exist?
Line 298 – “human health”, not “the human health”
Line 303 – Please add the industry association GOED (https://www.goedomega3.com/ ) of which most producers of algal oils (and other sources such as fish oils) are a member, and which also gives recommendations for daily intake of EPA and DHA (https://www.goedomega3.com/intake-recommendations ).
You can also see on ISSFAL’s website a link to a summary of intake recommendations by various international organizations: https://www.issfal.org/goed-recommendations-for-epa-dha
Line 332 – This paragraph highlights a future trend related to climate change. If the authors decide to stress the possibility that a higher impact of sunshine is a consequence of climate change, it is best to back this is up with a some good references, otherwise this sounds as speculation. I would remove this section or just mention in section 2 the importance of DHA for vision.
Line 250 – The review has not provided information about omega-3 PUFA production in general, for example how EPA and DHA oils are produced from fish, the refining and concentration technologies that are employed or how the market has developed. Neither have the authors addressed photobiotrophic omega-3 production. Hence, this general statement, and the Conclusion section overall, should be rephrased to something more related to EPA and DHA production in heterotrophic microalgae specifically.
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The article “Producing Omega-3 Polyunsaturated fatty acids: a review of sustainable sources and future trends for DHA and EPA market “ focuses to sustainable producing of omega-3 PUFAs for human nutrition and human health, and gives an accurate systematic review of the scientific literature in the field as well as a forthcoming scenario in the next future on this field. The article adds journalistic traits to a precise scientific rigor, resulting in an interesting and flowing, relatively easy reading of the text. Because the topic is of great importance, a strong diffusion of the article is expected, with indisputable merit of the authors and of the Journal.
There are minimal mistakes of points at lines 215 and 216, and of underscoring in 259 line.
Author Response
Please, see the attachment
Author Response File: Author Response.docx
Reviewer 3 Report
The manuscript by Oliver et al on “Producing Omega-3 Polyunsaturated fatty acids: a review of sustainable sources and future trends for DHA and EPA market” describes the in general current and future requirements of omega-3 PUFA and possibilities for its production.
The manuscript discusses microalgae as a of EPA and DHA, either directly or as feed for farmed fish.
It is explained in the text, but from my point of view a clear distinction should be made between omega-3 polyunsaturated fatty acids (PUFA) and long chain polyunsaturated fatty acids (LC-PUFA). Of relevance in this context are LC-PUFA, thus excluding alpha linolenic acid here.
Specific points
69-72: This is confusing, as there are competitions between omega-6 and omega-3 fatty acids not only for the enzymes converting fatty acids to eicosanoids, but also for converting essential fatty acids to LC-PUFA (delta-6 and delta 5 desaturases, elongases).
82: the sentence is unclear
86: references for the gives ratios should be provided
192: the sentence seems incomplete
233: grow instead of growth
233-235: As it stands this paragraph seems not very helpful, could there some quantitative information be available?
296: this context
Author Response
Please see the attachment
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
This manuscript has been significantly revised. It is now an understandable review with a logical organization, which clearly reviews the topics that the authors aimed to address, and more clearly addresses their focus on volatile fatty acids as economic carbon and energy substrates for microalgal heterotrophic growth and EPA/DHA omega-3 oil production. I am pleased to support this manuscript for publication. Nevertheless I still think an English grammar revision would certainly the quality of the review.
