Temperature-Mediated Microbial Community Transitions in the Temperate Coastal Ocean

Microbial communities exhibited repeated annual changes in composition driven largely by seasonal cycles in environmental variables. In examining the microbial community composition over three years of weekly samples from a temperate, coastal time series (Pivers Island Coastal Observatory: PICO), we similarly observed strong seasonal changes between summer and winter communities with temperature showing the greatest explanatory power of the measured environmental variables. Remarkably, however, temporal changes in microbial community composition are not continuous, rather transitions between summer and winter-associated taxa occur over several weeks. These rapid changes suggest the existence of either a biological temperature threshold across the community or a critical temperature-dependent keystone organism that structures the microbial community. In order to test the potential for temperature (rather than other highly-correlated environmental variables) to alter the community composition, we incubated seawater poised at this transition point (19 °C) at ambient and artificially warmed (24 °C) conditions for five days. In both the warmed and ambient mesocosms the summer taxa increased in relative abundance and the winter taxa remained at the same relative abundance or declined, supporting the idea of a narrow temperature threshold mediating seasonal transitions in this system. These data suggest that temperature may be a critical environmental variable responsible for much of the observed seasonal transitions in microbial community composition.