Fish is one of the most popular foods in Germany. In order to meet the high demand and avoid overfishing, fish such as salmon and trout are often bred in aquacultures. However, enormous quantities of wild stocks are processed into fishmeal and fish oil and added to the feed. In this way, the farmed fish are supplied with vital and polyunsaturated omega-3 fatty acids such as docosahexaenoic acid (DHA), which are essential for a healthy diet.

Sustainable food for aquaculture fish

Bioprocess engineers at the Technical University of Berlin have now shown that the valuable omega-3 fatty acids can also be obtained from plant residues with the help of bacteria and algae. With this sustainable diet for aquaculture fish, the Berlin research team wants to prevent farmed fish from continuing to be ‘fattened’ with wild fish.

Stefan Junne and his team are focusing on so-called heterotrophic microalgae, which are able to synthesise DHA from organic material. ‘In order for the algae to produce DHA, they have to be fed with a substrate. This can be sugar, for example from cereals such as maize,’ explains Junne. ‘However, as we follow the philosophy here at the department of not using food in biotechnological processes wherever possible, we are experimenting with barn straw, among other things. However, food waste and other biological residues such as leaves or green waste are also possible,’ emphasises the researcher.

Microbes convert straw into fatty acid and fertiliser

As Junne's team reports, promising results have already been achieved with straw. The plant residues were broken down into short-chain carboxylic acids such as acetic acid in a 200-litre Plexiglas bioreactor with the help of bacteria and then fed to the microalgae so that they could metabolise it into DHA and accumulate it in their cells.

‘As we are working with residual materials, we have to switch the microbial hydrolysis, i.e. the decomposition of the straw by bacteria, upstream of the actual synthesis of DHA in the algae. We are therefore coupling two bioprocesses,’ explains Stefan Junne. According to the researchers, the microbial hydrolysis produced not only liquid acetic acid but also a solid that can be used as a fertiliser.

Fatty acid content in fish feed substitutes comparable to commercial mixtures

As part of the ‘FENA - Fish meal and oil replacer for sustainable aquaculture’ project, the team says it has already been able to produce almost four tonnes of algae-based fish feed replacer. ‘The substitute had a concentration of around 20 % DHA. The composition of the omega-3 fatty acids was therefore similar to that of commercial fish oil and fishmeal mixtures,’ reports Junne. Initial feeding trials were also successful and show that fatty acids from plant residues and algae can replace conventional fishmeal and fish oil and make aquaculture sustainable.

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