Microcosm sludge bioreactors under varying frequencies of double organic loading disturbance, Metagenomes and 16S rRNA gene v3-v4 amplicon Metabarcodes

Diversity is frequently linked to the functional stability of ecological communities. However, its association with assembly mechanisms remains largely unknown, particularly under fluctuating disturbances. Here, we tested the general hypothesis that intermediate disturbance frequencies would favor stochastic assembly processes resulting in higher alpha-diversity, namely, the intermediate stochasticity hypothesis (ISH). We subjected complex bacterial communities in bioreactor microcosms to different frequencies of organic loading shocks, tracking temporal dynamics in their assembly, structure and function. Phylogenetic null modelling revealed a stronger role of stochastic assembly processes at intermediate disturbance frequencies, which preceded the formation of a peak in alpha-diversity. Communities at extreme ends of the disturbance range had the lowest alpha-diversity and highest similarity in terms of beta-diversity, with stronger deterministic assembly. Stochasticity prevailed during the initial successional stages, coinciding with better specialized function (nitrogen removal) at intermediate disturbance frequencies. General functions (carbon removal and microbial aggregate settleability) performed better with stronger deterministic processes. Together, our results support the ISH, advancing our understanding of the assembly processes underlying disturbance-induced changes in community structure and function.