Effects of marine heatwaves on the dynamics of marine coastal microbial communities

Climate change is projected to intensify and prolong marine heatwaves, characterized by abnormally high sea surface temperatures. These events can profoundly alter ecosystem composition and functioning, sometimes triggering mass mortality events. The Mediterranean Sea, due to its semi-enclosed nature, is particularly susceptible to warming, with future climate scenarios predicting a temperature increase of up to 3.8 °C and at least one persistent heatwave annually by 2100. Despite this vulnerability, the effects of marine heatwaves on seawater microbial and viral communities remain poorly understood.

Using microcosm experiments, we examined microbial and viral dynamics under control conditions (20 °C) and two simulated marine heatwaves (MHWs) (23 °C and 25 °C). By the end of the experiment, microbial assemblages in all three conditions were dominated by metagenome-assembled genomes (MAGs) that were not detected in the initial natural sample, indicating the competitive success of rare biosphere taxa over initially abundant species. Virulence factors and antibiotic resistance genes increased in relative abundance throughout the incubation, but such increase was amplified under warming conditions. Temperature also shaped viral strategies, with heatwaves showing a higher percentage of integrated lysogenic viruses compared to control samples. This trend was consistent with observations from natural samples, where lysogenic viruses peaked during warmer months.

The shift toward lysogeny observed under elevated temperatures may enhance horizontal gene transfer, accelerating the spread of virulence and antibiotic resistance genes. In fact, we observed an increased abundance of these genes in samples under heat stress. These processes could weaken ecosystem resilience, disrupt microbial-driven biogeochemical cycles, and amplify risks to marine and human health. Our study underscores the need to integrate microbial and viral responses into predictions of ocean functioning in a rapidly warming world.