Microbial-derived nutrients have been drawing attention because some fishmeal alternatives, such as plant meal, may not be entirely suitable for certain aquatic species. The presence of anti-nutritional factors and amino acid restrictions, for example, can also limit the use of plant meal or lead to additional processing and costs.
"Microbial-derived nutrients refer to the biomass that has been produced under a range of production systems from microalgae, bacteria and yeast, and further processed to yield a specific final product," explained Gamboa-Delgado who, together with his colleague Julia Mariana Márquez-Reyes, published a review paper in 2016 on the potential of microbial-derived nutrients in aquaculture nutrition.
"This product might be represented by bulk microalgal biomass such as paste or dry powder, or specific, isolated high-value products."
A related term is Single Cell Protein, or SCP, the processed biomass or extracted protein from cultures of microorganisms that can be used as ingredients for food or animal feed. Known to grow rapidly and produce a high yield, microorganisms have high protein content and can convert various substrates efficiently. They can also be classified according to the types of carbon and energy sources that they use, and have been produced through various energy sources such as solar or artificial light, and different sources of carbon. Some microalgae species have the physiological ability to grow under heterotrophic conditions and are currently cultured in confined bioreactors (e.g. Schizochytrium sp). In addition, the diversity of metabolisms found in bacteria has led to a huge culture using unconventional substrates such as methane and wastes from the food industry.
"We began this particular research line because, in the past, we evaluated alternative ingredients in shrimp diets using various nutritional techniques, and became interested in evaluating different sources of microbial biomass as fish meal replacements," said Gamboa-Delgado.
"Different microbe species offer nutritional characteristics that are just as good as, or even better than, plant-derived products. Thanks to novel production methods, it's also possible to predict high production of single cell proteins, and a concomitant reduction of retail prices."
Extracted products from yeast, bacteria and microalgae can elicit important growth and immunostimulatory effects in larvae and juveniles of aquatic organisms. Microalgal proteins, for example, present high digestibility and can be compared to conventional vegetable proteins.
Yeasts such as Saccharomyces cerevisiae have been tested in a range of aquatic animals and also display high digestibility. Tests have suggested that dietary yeast levels can be substantially increased to replace fishmeal as long as the diets are well balanced. Bacterial biomass obtained from species such as Methylococcus capsulatus is known to perform well as an alternative protein source in feed formulations for species like Atlantic salmon, while yeast biomass has been successfully used to replace high levels of fishmeal in shrimp diets.
Isolated microbial products can also be used as feeding attractants (nucleotides), pigmentation agents (carotenoids) and immunostimulants (beta-glucans). Because of the nutritional benefits of microorganisms and intensive ongoing research, plant-derived products that are currently used in aquaculture diets may eventually be significantly replaced by microbial-derived ones. Additional reasons are the variable price of plant ingredients, the presence of anti-nutritional factors and a conflicting demand of plant meals for other uses.
"Only in the last few years has there been more interest in the use of different forms of microbial biomass in aquaculture nutrition," Gamboa-Delgado explained.
"An increasing number of companies are now producing higher volumes of microalgae, yeast and bacteria especially for the aquaculture industry for things such as fish meal replacements, dietary additives, palatability enhancers and immunostimulants."
In view of results obtained in the laboratory and the fact that several species of microorganisms with a good nutritional profile have now been pinpointed, Gamboa Delgado believes that through his research, it will be possible to simultaneously compare the nutritional effects of several microbial products on different species of crustacean and fish. There are also other possibilities.
"We have highlighted various nutritional techniques that have been applied to evaluate the role of microbial biomass as a nutritional supplement and fishmeal replacement," he said. "For researchers, students and technical personnel undertaking nutritional studies with microorganisms, our paper is a source of updated information on the physiological ability of aquatic organisms to utilize these alternative ingredients. It also forecasts a sharp increment in the production of different kinds of microbial biomass that are specifically aimed at the animal nutrition sector."
In future bioassays, Gamboa-Delgado intends to test diets formulated with different types of microbial biomass offering specific nutritional advantages. For example, some microalgae species, such as Schyzochytrium, are excellent sources of lipids, while some yeasts and bacterial species like Candida utilis and Methylococcus sp, are important protein sources. Their biomass can be combined in experimental diets to achieve near-ideal nutrient profiles for specific aquatic organisms. It is also hoped that better production techniques and methods to enhance the nutritional value of microbial biomass will lead to increased production and high-quality end products.
"The production of microbial biomass is expected to rise in future," Gamboa-Delgado explained. "This may happen not only because of improved production techniques but also due to higher demand and diversified new applications in animal nutrition. In the meantime, we have been approached by several companies that are interested in developing growth trials to explore the nutritional contributions of microbial biomass through isotopic methodologies. We are confident that results will show additional nutritional benefits for farmed aquatic animals."
To access the original research paper by Dr Gamboa-Delgado and his team, please click here.