Effects of warming on microbial communities in the coastal waters of temperate and subtropical zones in the Northern Hemisphere, with a focus on Gammaproteobacteria

We examined the effect of warming on microbes in coastal waters in temperate and subtropical zones, Suruga Bay in Japan and Ha Long Bay in Viet Nam, respectively. Incubation of intact communities (500 mL each) at temperatures three or five degrees higher than the in-situ temperatures enhanced both the abundance and growth rate of prokaryotes. We also conducted size fractionation and dilution experiments to elucidate whether this behavior was the result of an increased rate of reproduction (“bottom-up” effect) or reduced mortality (“top-down” effect). For two of the five cases analyzed, enhancement of the growth rate could be ascribed to the bottom-up effect of the warming. For two other cases it could be ascribed to the top-down effect, irrespective of whether or not warming enhanced the bottom-up effect. The effect of warming was not clearly apparent for prokaryotes of subtropical coastal waters, where the in-situ temperature was nearly 30 °C. Among the prokaryotes, Gammaproteobacteria tended to predominate as a result of warming. These results suggest that warming may affect microbial growth even in subtropical coastal waters in some seasons and that the response of Gammaproteobacteria to warming of temperate and subtropical coastal waters is among the most rapid.