The role of sedimentation in the structuring of microbial communities in biofloc-dominated aquaculture tanks

In no water exchange aquaculture operations, sedimentation has been employed to control the level of microbial flocs to decrease oxygen consumption and to prevent clogging of gills in cultured organisms. Nevertheless, the impact of this practice on the culture microbial community was not documented. The current study evaluated the impact of adding different concentrations of sedimentation from an intensive shrimp culture tank on the microbial communities in an experimental tank system operated with no water exchange. The trial evaluated three concentrations (mg L-1) of total suspended solids levels: < 200, 680, and 1030. The analysis showed that settleable microbial community has a different set of species compared with the unsettled biomass, which was also less diverse. We argue that the difference in species composition is probably related to the biological flocculation, i.e., the ability exhibited by microorganisms to spontaneously form large, dense and quick settling aggregates. The results suggest that settling tanks act as a selective force by reducing the abundance of microorganisms strongly associated with the settled flocs, whilst favoring planktonic lifestyle bacteria or microorganisms associated with small and open flocs. These effects, especially loss of diversity, need to be considered in aquaculture management practices since it can increase the risk for invasion of opportunistic pathogenic species.