Volcanic eruptions are considered to be some of the most important events affecting local and global ecosystems and climate. An increasing body of research has been concerned with discerning the influence of volcanic ash on marine ecosystems. However, studies on the responses of autotrophic and heterotrophic microorganisms to volcanic ash are rare. In this study we conducted two microcosm experiments in the low-nutrient and low-chlorophyll western Pacific Ocean and found that volcanic ash first stimulated the abundance of heterotrophic bacterioplankton, followed by phytoplankton bloom that included both picoeukaryotes and larger eukaryotes such as diatoms. Using terminal restriction fragment length polymorphism and clone library analyses, we observed an altered bacterial diversity and community structure with volcanic ash addition. Our study showed that volcanic ash affects the community composition of both heterotrophic bacterioplankton and phytoplankton in the surface ocean. These results elucidate the overlooked impacts of natural volcanic eruption events on microbial communities, which play important ecological and biogeochemical roles in the marine ecosystem.

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