Can microalgae improve the intestinal health of gilthead sea bream?

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By Milthon Lujan

Gilthead sea bream. Source: CSIC
Gilthead sea bream. Source: CSIC

Fish farming of carnivorous species like gilthead sea bream (Sparus aurata) heavily relies on fish meal and fish oil, putting pressure on wild fish populations and raising sustainability concerns. Finding alternative sources of proteins and lipids is crucial.

Microalgae are promising sustainable alternatives, rich in nutrients and easy to cultivate. But how do they affect the intestinal health of these fish, which plays a vital role in their overall well-being?

Researchers from the University of Thessaly and the University Research and Innovation Centre “IASON” evaluated the effects of dietary microalgae blends as substitutes for fish oil on the bacterial microbiota of the midgut in gilthead sea bream.

Microalgae: Tiny powerhouses for sustainable aquaculture

Microalgae are microscopic aquatic organisms packed with nutrients such as proteins, fatty acids, and vitamins. They can be sustainably cultivated, reducing dependence on wild fish and offering a more environmentally friendly option for aquaculture feed. The study specifically focuses on four species of microalgae:

  • Phaeodactylum tricornutum: Rich in eicosapentaenoic acid (EPA), crucial for fish health.
  • Michrochloropsis gaditana: High in EPA and contains docosahexaenoic acid (DHA), another crucial fatty acid.
  • Schizochytrium sp.: Particularly rich in DHA.
  • Tisochrysis sp.: Known for its high DHA content.

Phaeodactylum tricornutum and Michrochloropsis gaditana have been studied before. P. tricornutum showed mixed results, sometimes reducing intestinal microbial diversity but also providing immune-stimulating effects. M. gaditana seemed to have minimal impact on intestinal health or even increased richness.

These microalgae seem suitable for replacing EPA and DHA derived from fish oil, but their combined effects on intestinal health require further research.

Intestinal microbiome: The unsung hero of fish health

Like humans, fish have a complex intestinal microbiome, a community of microorganisms that plays a vital role in their health. Fish intestine microbiomes are composed of thousands of bacterial species interacting with each other.

The fish diet plays a significant role in shaping this microbiome, influencing factors such as:

  • Development and maturation of the immune system: A healthy microbiome is crucial for a strong immune system.
  • Function and development of the nervous system: Emerging research suggests that intestinal microbes may even influence brain function.
  • Protection against pathogens: Beneficial intestinal bacteria can help fight harmful pathogens.

Research on the impact of microalgae on the intestinal microbiome

The research aimed to understand how the four species of microalgae, used in combination, affect the intestinal microbiome of gilthead sea bream, a commercially important fish. The researchers hypothesized that microalgae would promote an intestinal microbiota specifically adapted to metabolize them.

“The control diet (FO) contained only fish oil as a source of lipids, EPA, and DHA, while three experimental diets were used, where fish oil was replaced by 67% with one of the following microalgae biomass mixtures: Michrochloropsis gaditana and Isochrysis sp. (Tisochrysis lutea) (MI), Phaeodactylum tricornutum and Isochrysis sp. (PI), and Schizochytrium sp. and P. tricornutum (SP),” report the scientists.

The key findings of the study were:

  • Microalgae diets significantly altered the composition of the intestinal microbiome compared to the control group.
  • Specific bacterial pathways related to the metabolism of unique sugars in microalgae were more active in fish fed with microalgae.
  • This suggests that the fish intestinal microbiome adapted efficiently to the new food source.
  • Interestingly, one of the microalgae blends (MI) even appeared to promote beneficial bacteria with potential probiotic properties.

Importance for gilthead sea bream farming

Understanding the intestinal microbiome’s response to different diet components is crucial for optimizing fish health and promoting sustainable aquaculture practices. This study adds valuable information to ongoing research, paving the way for developing effective algae-based diets for healthier and more sustainable fish farming.

Potential benefits:

  • Reduced dependence on fish meal and fish oil: A more sustainable aquaculture industry.
  • Improved fish health and growth: A healthy intestinal microbiome leads to better nutrient absorption and disease resistance.
  • Enhanced nutritional value of farmed fish: Microalgae can enrich fish with beneficial nutrients like omega-3 fatty acids.


“The MI diet (Michrochloropsis gaditana and Isochrysis sp.) appears to promote various beneficial bacteria with potential probiotic capabilities in the fish intestine,” conclude the scientists.

This research highlights the potential of microalgae to transform aquaculture into a more sustainable and responsible industry. By understanding the impact of these tiny organisms on fish health and the intestinal microbiome, we can develop improved aquaculture feeds that benefit both fish and the environment.

Konstantinos A. Kormas
Department of Agriculture Ichthyology & Aquatic Environment, University of Thessaly, Volos, Magnesia, Greece
Email: kkormas@uth.gr

Reference (open access)
Katsoulis-Dimitriou, S., Nikouli, E., Gkalogianni, E. Z., Karapanagiotidis, I., & Kormas, K. (2024). The effect of dietary fish oil replacement by microalgae on the gilthead sea bream midgut bacterial microbiota. bioRxiv, 2024-01.