Abstract:

The current conventional food production systems are neither ecologically sound, nor do they address the issues of the circular economy of decreasing waste. Aquaponics, a technology that integrates aquatic organisms and plants, provides part of the solution. As microbes are vital to the functioning of aquaponic systems, the integration of probiotics in aquaponics is a possible strategy for promoting sustainable agriculture. Prior to the commencement of this research, there was no published data on the effects of addition of probiotics on microbial diversity in aquaponics. The aim of this research was toinvestigate the effects of the addition of a commercial probiotic mixture of Bacillus subtilis and B. licheniformis to a coupled aquaponic system on plant and fish growth, water quality, and bacterial diversity. Results from plant growth trials indicated that vegetative growth of lettuce was significantly higher in the Z?acz7/zM'-treated systems than the untreated control systems. The concentrations of phosphorus, potassium, and zinc in the lettuce leaves were significantly higher in Bacillus-treated than control systems. There was a significant increase in the concentration of nitrate and phosphate in the 2tacz7/iw-treated systems which may have contributed to improved lettuce growth. Although Proteobacteria was the most dominant bacterial phylum associated with the lettuce root and leaf surfaces, 16S rRNA gene sequence analysis showed that the addition of Bacillus species significantly increased species richness and species evenness of the lettuce roots as compared to leaves. The alpha diversity indices showed that root-associated bacterial communities were significantly more diverse than those associated with leaves when a probiotic was added to aquaponic water. Analysis of lettuce epiphytic bacterial communities revealed that fresh lettuce leaf surfaces were inhabited by food-borne pathogens belonging to Shigella and Aeromonas.