Experimental warming effects on microbial community growth and mortality during the cold season in coastal waters of Taiwan and Japan

Abstract

We conducted three in situ microcosm experiments in Taiwan and Japan during the winter of 2023 to investigate microbial plankton community responses to warming in Pacific coastal waters. Monitoring and analyzing microbial communities, including viruses, bacteria, and picophytoplankton (Synechococcus spp., Prochlorococcus spp., and picoeukaryotes), were performed by flow cytometry over seven consecutive days. Control microcosms were maintained at ambient coastal water temperature, while experimental microcosms were warmed by + 2–3°C. The modified dilution method estimated picoplankton (heterotrophic bacteria and picophytoplankton) growth, grazing, and viral lysis rates on days two and five. Our time-series incubation experiments revealed that warming did not increase bacterial abundance, but viral abundance significantly increased with temperature, indicating a strong lytic impact on bacteria. Moreover, the relative increase in viral abundance was related to a rise in Synechococcus spp. abundance under warming conditions across all study sites, relative to ambient conditions. Similar trends were observed in nanoflagellate abundance between the two treatments across all stations. In modified dilution experiments, viral lysis accounted for up to 51% of picoplankton mortality, compared to total mortality, indicating that viral lysis was the primary driver of picoplankton mortality under warmed conditions. These findings highlight the critical role of viruses in cold marine environments and suggest the potential for modeling viral functions to predict the effects of global warming on microbial dynamics.