During the process of converting macroalgae into biofuel, significant biomass waste occurs. By employing enzymatic saccharification (breaking sucrose down into glucose and fructose), the theory goes, this waste could be converted into a single-cell detritus product - and potentially fed to farmed bivalves, such as oysters.
With macroalgae for biofuels still in the early stages of development, the possibility of using biorefinery by-products is still some time off. Nevertheless, the idea does present an interesting direction for the development of microalgal feed – and potentially an additional source of revenue for biorefineries.
To test how well this biorefinery by-product can perform as a feed, Dr Stefano Carboni (University of Stirling) and fellow scientists put together six different diets, which they fed to juvenile oysters (Crassostrea gigas).
Alongside the single-cell detritus product (the biorefinery by-product feed), the team put together a natural detritus-based diet consisting of farmed sea-urchin digesta, and two microalgae-based diets, one mixed with 50 per cent of the urchin-based detritus, and the other with 50 per cent of the single-cell detritus.
They also tested a live microalgal diet and an algal paste diet, both of which are already commonly used as feed in the aquaculture industry.
Oysters were separated into low volume static tanks where they were fed one of the six diets for four weeks. The performance of the diets produced some similar results, with the oyster’s survival rates, and lipid and carbohydrate contents showing very little difference from one diet to the next.
Although this indicates that the oysters were able to feed efficiently on all of diets, there were some notable differences in their growth rates.
Those fed with the biorefinery by-product diet (the single-cell detritus), and the sea-urchin detritus diet saw the lowest growth rates, though mixing these detritus diets with live microalgae did produce better growth rates. The best diet of all was the live microalgal-sea urchin mix, which produced individual oysters with mean weights significantly higher than those found in any of the other diets.
Whilst the findings of the study show some promise for the use of biorefinery by-products as a feed supplement, the scientists note that the growth rates achieved in commercial oyster operations which use upwelling systems, are much better than the scientists saw in the small, static rearing tanks (including those fed on the industry-standard diets).
The next step in determining if biorefinery by-products could act as a feed (or feed supplement), is to test the diets in a real-world commercial set-up. The scientists also recommend exploring the relative success of the live microalgal-sea urchin diet itself. Despite having a similar nutritional profile to the live microalgal-biorefinery diet, the microalgal-sea urchin diet did perform better.
Investigating the reasons for this by, for example, examining ingestion and assimilation rates, could help in the development of more efficient oyster feed.
Given that approximately 90 per cent of industrial finfish and shellfish aquaculturalists rear animals that have larval or juvenile stages which feed on microplankton, there is a lot of demand of unicellular algal-based feed.
Generating sufficient levels of microalgae can be challenging for the industry, in part because of the costs, and in part because it is a highly technical, labour-intensive process. Additional challenges come where algae is raised in outdoor systems, where fluctuations in sunlight and temperature have impact the rate of microalgae production.
Given these challenges, if future research into the biorefinery by-products for feed does continue to show promise- even as a supplement to feed rather than a feed in itself, it could help alleviate the risk of microalgal-based feed bottlenecks occurring in the industry.
