Tilapia recirculating aquaculture systems as a source of plant growth promoting bacteria

A recirculating aquaculture system with farmed tilapia is the most popular combination in aquaponics, an integration of aquaculture and hydroponics. Despite nutrient-rich fish-rearing water being regarded as a valuable resource for aquaponics, the quality and value of inhabitant microorganisms are certainly understudied. Our present research illustrates the feasibility of the tilapia-rearing water as a valuable source of beneficial microorganisms called plant growth promoting bacteria (PGPB). Microbial communities were examined with a combination of culture-independent high-throughput 16S rRNA gene sequencing and cultivation methods. Microbial communities determined using high-throughput sequencing indicated the usefulness of Bacteroidetes and Alphaproteobacteria as beneficial microbial indicators to assess the health condition of recirculating aquaculture systems. Siderophore production, ammonia production and phosphate solubilization assays were used for screening and 41% of isolates were identified as plant growth promoting bacteria. These bacteria were classified as Actinobacteria (eight strains [32% in total], Dietzia, Gordonia, Microbacterium, Mycobacterium and Rhodococcus), Bacilli (six strains [24%], Bacillus and Paenibacillus), Flavobacteriia (one strain [4%], Myroides), Betaproteobacteria (two strains [8%], Acidovorax and Chromobacterium) and Gammaproteobacteria (eight strains [32%], Aeromonas, Plesiomonas and Pseudomonas). We found that the tilapia-rearing water naturally contained various lineages of PGPB and could be esteemed as a worthy seed bank of PGPB. Because aquaponics is a difficult system to use pesticides and herbicides, the role of PGPB to prevent plant pathogens and maintain healthy root system may be more important than traditional agricultural settings.