Combined Deterministic and Stochastic Processes Control Microbial Succession in Replicate Granular Biofilm Reactors

Granular sludge is an efficient and
compact biofilm process for
wastewater treatment. However, the ecological factors involved in
microbial community assembly during the granular biofilm formation
are poorly understood, and little is known about the reproducibility
of the process. Here, three replicate bioreactors were used to investigate
microbial succession during the formation of granular biofilms. We
identified three successional phases. During the initial phase, the
successional turnover was high and α-diversity decreased as
a result of the selection of taxa adapted to grow on acetate and form
aggregates. Despite these dynamic changes, the microbial communities
in the replicate reactors were similar. The second successional phase
occurred when the settling time was rapidly decreased to selectively
retain granules in the reactors. The influence of stochasticity on
succession increased and new niches were created as granules emerged,
resulting in temporarily increased α-diversity. The third successional
phase occurred when the settling time was kept stable and granules
dominated the biomass. Turnover was low, and selection resulted in
the same abundant taxa in the reactors, but drift, which mostly affected
low-abundant community members, caused the community in one reactor
to diverge from the other two. Even so, performance was stable and
similar between reactors.