Microalgae-bacteria interactions: a key for improving water quality in recirculating aquaculture systems?

The roles of algae in improving aquaculture water quality are well-established. However, the integration of algae in a recirculating aquaculture systems (RAS) is less popular mainly due to the large area required for photosynthesis. As science progresses, a growing number of reports are available on the benefits of algae to water quality and fish health. This motivated the author to investigate the effects of algae on a RAS stability, by measuring the water quality and the effects on bacterial community composition in a RAS. A review was conducted on nitrogen removal by algae and the operation of an algae reactor in a RAS. This showed that a RAS configuration influence algae performance by affecting nitrogen loading and nitrogen species (ammonium versus nitrate), cultivation methods (suspended versus attached) and environmental conditions (light, temperature, pH, oxygen, and carbon dioxide). Next, a periphytic microalga, Stigeoclonium nanum was cultured in suspension or immobilized. The growth and nitrogen uptake of S. nanum was higher when immobilized than when cultured in suspension. S. nanum preferred ammonia rather than nitrate as nitrogen species. Further effects of S. nanum on the RAS water quality (total ammonia nitrogen (TAN), nitrite, nitrate, and phosphate) were also investigated. No difference of TAN between the RAS with algae (RAS+A) and the RAS without algae (RAS-A) was observed. However, nitrite, nitrate and phosphate were significantly lower in the RAS+A than in the RAS-A. When the RAS systems were perturbed by an acute pH drop (from pH 7 to 4 over three hours), no significant difference was observed between the RAS+A and the RAS-A on the resistance towards the stressor. This was shown by an increase in the TAN and the nitrite concentration in both treatments after the perturbation. However, the algae helped the RAS+A to regain a low nitrite level faster than the RAS-A. The diversity of bacterial community between the RAS+A and the RAS-A was not different, while the composition of bacterial community was significantly different between the RAS+A and the RAS-A, thus influencing the functioning of the RAS.