The promise that is heterotrophic microalgae cultivation
Tank-based chemosynthesis cultures could solve issues with scale
While research into replacing commercial fish oil and flour within feed with microalgae has been ongoing, a paper from 2010 might provide essential answers to large-scale microalgae farming under light deprivation. (1)
What are microalgae and why are they so beneficial?
Mircoalgae are microscopic single cell organisms that are exist as hundreds of different species all over the world. The beneficial properties of microalgae have been long known and are more and more becoming common knowledge. Their rich nutritional profile (among others, they are the main Omega 3 source of the world) has lead to the commercial growing of microalgae in translucent tanks in the form of towers or shallow ponds in order to utilise their nutritional properties for the production of food supplements, various vitamins, additives for cosmetic and pharmaceutical production and biofuel. What differentiates them largely from macroalgae and plants is their nutritional content, which is largely made up of fats and protein and often to a lesser part carbohydrates. (2) For instance, the microalgae "Tetraselmis" contains 55% protein, 18% carbohydrates and 15% fats, with the remaining 12% made up of mainly water and various dissolved vitamins, salts and acids. However, the biodiversity within microalgae is enourmous. An estimation sets the total number of different species at about 200 000 - 800 000, with only 50 000 described, which have yielded 15 000 novel chemical compounds that have found useful application.
Making sense of fatty acids - What is Omega 3 and Omega 6?
While there are thousands of applications for many different types of microalgae, their nutritional profile is what makes them so relevant for fish farming. This is especially true for the Omega 3 they produce. Fish are often advertised as being rich in Omega 3, which in turn has been proven to carry numerous health benefits. However, only fish that have consumed substantial amounts of microalgae through their diet (direct or indirect) will be rich in Omega 3 because no fish in the world can produce it itself. And what is Omega 3 and why is it so relevant? I will attempt to clear up the vagueness of this topic as best as I can:
There are two main types of fatty acids, saturated and unsaturated. The saturated type lacks a double bond between its individual carbon atoms while unsaturated fatty acids have at least one of these bonds. This leads to one type (saturated) being (somewhat) solid at room temperature (e.g. Butter at 21 degrees Celsius) and the other (unsaturated) being liquid (Olive Oil at the same conditions) unless they are bound in solid foods like salmon or bacon.
Medical research has shown that there is a significant relationship between the consumption of saturated fats and the accumulation of blood cholesterol, increasing the risk for heart disease. Therefore, unsaturated fats should outweigh saturated ones in a balanced diet.
However, there is a classification within unsaturated fats that makes a very important difference:
Polyunsaturated Fatty Acids (PUFAs) and Monounsaturated Fatty Acids (MUFAs).
What beautiful acronyms. First of all, those fats are so called "essential fatty acids" for our body, meaning we cannot produce them ourselves and have to consume them. A trait we share with fish. Within the group of the unsaturated fatty acids, we find the Omega-Fats that are so much talked about. They are grouped into:
Omega 3 (PUFA)
Omega 6 (PUFA)
Omega 9 (MUFA)
Still with me? Good! But why the numbers? The number simply tells us, where in the carbon chain of the fatty acid the double bond between two carbon atoms that was mentioned above occurs.
The issue with our western diet is that it leans strongly in favour of Omega 6 while a diet considered optimal for the average human requires a weighting placing more emphasis on Omega 3. The average North American diet is an impressive 12 to 25 units of Omega 6 for each unit of Omega 3, somewhere between 12 - 26:1, whereas the optimal ratio lies somewhere between 4:1 and 1:1, which some nations like the Japanese and Inuit achieve due to their traditionally algae and seafood-rich diet. This type of diet also correlates highly with higher and healthier life expectancy, expressed in e.g. much lower cancer rates, significantly less cases of heart disease, stronger immune systems and lower rates of brain disease like ALS and Alzheimer. A large part of this is the fact that Omega 6 is associated with supporting inflammation in the body, while Omega 3 is not.
Perfect Smoothie material
There is a large bulk of science behind every single one of those results, please do keep in mind however that this is expressed in correlation, not necessarily causality. Diet is only one of many parts of a healthy life and one should always try to see the whole picture. This is in no way trying to evangelise a certain type of lifestyle. It is however trying to evangelise the right nutrition for fish.
The last step down the rabbit hole is the structure of Omega 3 itself. There are three types of fatty acids that are considered Omega 3.
DHA - Docosahexaenoic acid
EPA - Eicosapentaenoic acid
Those two long chain fatty acids mostly occur in animal fats (dependent on what they eat) and microalgae (where they largely originate) and are connected to a vast amount of benefits like anti-inflammatory impact, improved nervous system functionality, improved memory and neural function, better sleep, better muscle functionality, higher insulin sensitivity and the list goes on. The point is clear I believe.
ALA - Alpha-linolenic acid
This is a short chain fatty acid our body has to convert to DHA to make use of it. While it then still carries all the benefits named above, we can only gain about 8% of DHA out of 100% ALA, making it a much less efficient source of Omega 3. ALA is mainly found in plants and contained in products like chia seeds and flax oil. It is actually the main source of Omega 3 for many omnivores and herbivores like mice, which achieve a much higher conversion rate from ALA to DHA and therefore can use it much better to balance their diet efficiently. I doubt they know that, though.
A very basic overview
Regarding sources and deeper reading into fatty acids and their impact, have a look below the article at sources (3) - (6) If you are interested in a comprehensive overview, especially look at (3). I am neither qualified nor interested in giving medical advice, this is about fish food.
Now, back to the algae at hand. The 55% protein contained in our example algae Tetraselmis mentioned at the beginning of this article is mainly made up of ALA and EPA. Directly consumed, this carries many benefits, however a certain inefficiency due to a high ALA content and also, it does not taste very good.
However, fish do just great with ALA and are very efficient in converting it into DHA, which does wonders for the human body (and also does wonders for the fish). Current feed practices in aquaculture contain feed from plants and farmed animal leftovers to such a high degree that many fish are nearly fed the same type of diet we are eating on land, which they
are unable to metabolise properly (some literally burst open) and are completely unable to accumulate the much sought-after Omega 3 we praise and price them for. It comes from microalgae. Let's feed them microalgae. Their internal machine is perfect at making them thrive on it and us therefore thrive on them. (7)
Now in this light, consider microalgae as food supplements as an efficient source of Omega 3 and now microalgae consumed and metabolised by fish as the same. Where do we land?
Their nutritional value make them a real contester for industrial fish feed, presenting a realistic alternative to wild catch-based feed from fish flour and oil. While there are some obstacles to overcome like the digestibility of the algae due to very thick cell walls and the high loss of nutrition after drying, first proven products are already on the market (BioMar) and showing at least the same degree of success as wild catch-based feed. A large enough production of appropriate microalgae species could eventually provide a fully sustainable and very scalable alternative to wild feed, taking significant pressure off the ocean. However, this is only to be expected if the price is equal or less than existing options, all the while supplying at least equal or superior quality. This will require significant scale and testing.
Limitations in culturing microalgae and their potential solution
However the growth of many species is dependent on photosynthesis, requiring sunlight to produce chemical energy. This reality set a natural barrier for scale, since using large and thick tanks or deep ponds would mean that only a small part of the algae would receive enough sunlight to grow, keeping the yield low and therefore the price per kg of product very high. But photosynthesis happens to have a literal dark brother called chemosynthesis, which some microalgae are able to perform. This means utilising chemicals instead of sunlight as their energy source, absorbing them directly from the water and turning them into cellular energy (basically glucose). Those chemicals and nutrients microalgae require to thrive are mainly carbon and nitrogen, building blocks of life. (8) We even know how to control the relationship in concentration between those two in order to steer the nutritional content of the algae we are producing (e.g. starve for one, get more lipid, starve for the other, get more carbs). This makes it possible to engineer the same species towards different nutrient profiles in order to get the perfect raw material for the intended end product.
The independence from light makes much larger containers possible that require no translucent material, making a large scale production in hundreds of simple steel tanks possible, enabling yields that could provide us with an entire fully sustainable fish feed and fuel industry. While refining the raw material into fish food would still require a large amount of work, the abundance of the raw material would significantly decrease price and complexity in process due to larger test batches. Furthermore, there is a strong case for biofuel from microalgae, all we lack is scale (9). Let's work on that and make it happen, for healthy, tasty fish and clean fuel.
(1) Heterotrophic cultures of microalgae: Metabolism and potential products
(2) How marine algae could help feed the world
(3) Facts on fats: The basics
(4) The importance of omega-3 and omega-6 fatty acids
(5) Omega-3 and omega-6 polyunsaturated fatty acids: Dietary sources, metabolism, and significance - A review.
(6) Algae: our original omega-3 source
(7) Fatty acid metabolism in European sea bass (Dicentrarchus labrax): Effects of n-6 PUFA and MUFA in fish oil replaced diets
(8) Photosynthesis and chemosynthesis
(9) Microalgae biofuels as an alternative to fossil fuel for power generation