Frequent pulse disturbances influence resistance and resilience in tropical marine microbial communities
The Johor Strait separates the island of Singapore from Peninsular Malaysia. A 1-kilometer causeway built in the early 1920s in the middle of the Strait effectively blocks water flowing to/from either side, resulting in low water turnover rates and build-up of nutrients in the inner Strait. We have previously shown that short-term rather than seasonal environmental changes influence microbial community composition in the Johor Strait. Here, we present a temporally-intensive study that uncovers the drivers keeping the microbial populations in check. We sampled the surface water at four sites in the inner Eastern Johor Strait every other day for two months, measured various water quality parameters, and analysed 16S amplicon sequences and flow-cytometry counts. We discovered that microbial community succession revolves around a common stable state resulting from frequent pulse disturbances. Among these, sporadic riverine freshwater input and regular tidal currents influence bottom-up controls including the availability of the limiting nutrient nitrogen and its biological release in readily available forms. From the top-down, marine viruses and predator bacteria (e.g. Saprospiraceae family) limit the proliferation of microbes in the water. Harmful algal blooms, which have been observed historically in these waters, may occur when there are gaps in the top-down and bottom-up controls. This study uncovers the complex interaction between these factors contributing to a low-resistance but high-resilience microbial community and speculate about the rare events that could lead to the occurrence of an algal bloom.