Disturbance History Shapes Microbial Community Assembly and Predictability in a Continuous Culture System.

Legacy effects that arise when past environmental conditions influence present community responses are increasingly recognized as important drivers of microbial dynamics. However, how repeated disturbances and the intervals between them shape community assembly, and whether they lead to predictable trajectories or more contingent outcomes, remains poorly understood. Here, we used a controlled continuous culture experiment to examine how two alternative histories of repeated minor salinity pulses shaped bacterial community responses to a subsequent major salinity pulse. Using a high-resolution time series of 26 sampling points over 50 days, we detected systematic treatment effects and increasing divergence among within-treatment replicates over time. Community responses to the second salinity pulse depended on disturbance history: Previous exposure altered both the magnitude and direction of richness change, and even promoted richness when recovery time was brief. Moreover, exposure to the subsequent salinity disturbance maintained elevated levels of inferred stochasticity in communities with disturbance histories following the major pulse. Together, these findings demonstrate that disturbance sequence and recovery interval critically shape microbial stability and influence the balance between stochastic and deterministic assembly processes.