Differential impacts of temperature increase on prokaryotes across temperature regimes in subtropical coastal waters: insights from field experiments

Prokaryotic communities play a dominant role in driving biogeochemical cycling in marine ecosystems. How short‐term temperature increase impacts prokaryotes in subtropical coastal waters is still largely unknown. Here, 14 field experiments were conducted to investigate the response of prokaryotes in subtropical coastal waters to temperature increases of 3°C and 6°C, encompassing a range of ambient temperatures from 17°C to 31°C. We found that responses of prokaryotic growth, grazing pressure, community, and transcriptomes to increased temperatures were largely affected by ambient temperatures. Increased temperatures enhanced the growth rate and grazing pressure of heterotrophic prokaryotes when ambient temperatures were below 26–28°C. The increased temperatures had greater negative effects on the grazing rate compared to the growth rate; therefore, the abundance of heterotrophic prokaryotes generally increased after temperature increase across all temperature regimes. Metatranscriptomics analysis showed that at an ambient temperature of 30°C, genes involved in the adenosine triphosphate synthase were significantly downregulated by the increased temperature. This could be a major factor contributing to the decreased prokaryotic growth rate. In comparison, autotrophic prokaryotes (Synechococcus) exhibited better performance in response to elevated temperatures, thriving up to 35°C, beyond which their growth rate experienced a dramatic decline. When exposing to extremely high temperatures, genes involved in photosynthesis significantly decreased. These findings highlight the differential ecological impacts of temperature increase on prokaryotic communities, varying across different ambient temperatures and taxa in subtropical coastal waters.