Theory of microbial ecology: Applications in constructing a recirculating aquaculture system

Concepts from microbial ecology have been applied to constructing a bioreactor in RAS for the production of tilapia (Oreochromis niloticus). The bioreactor consists of covered and uncovered tanks, with and without polyethylene net, separating the participation of photoautotrophic organisms from other microorganisms capable of absorbing nutrients. Absorption of nutrients by macrophytes, planktonic algae and bacteria was evaluated by the increase in macrophyte biomass, algae density and number of bacterial cells respectively. Density of bacteria adhering to the net and roots of the macrophytes was quantified. All the bacterial quantifications were made using Fluorescent in situ hybridisation. Temperature and pH were measured throughout the experiment. Concentrations of ammoniacal nitrogen, nitrite, nitrate and phosphate were measured at the beginning and final stages. Increase in the nitrite/nitrate concentration and abundance of ammonia-oxidising and nitrite-oxidising bacteria, as well as the pH decrease are an evidence of the nitrification. Algae/macrophytes growth in the bioreactor tanks appears to assist in phosphorous immobilisation. There was an increase in the mean abundance of bacteria in the RAS tanks, while an increase was noted in the abundance of Nitrobacter and AOB-beta-proteobacteria in the bioreactor entry tank compared with the water exit tank. Such observations enable us to state that nitrifying bacteria adapt to the conditions of the bioreactor tanks, acting to maintain ammoniacal nitrogen levels in the system. Our results show that the arrangement of the constructed RAS kept the water quality in suitable levels to the tilapia cultivation.