Resilience and response of marine microbes to the future ocean and a marine heatwave - insights from a mesocosm experiment.

Anthropogenic influences are reshaping ocean conditions, with rising sea surface temperatures, elevated CO2 concentrations, and shifting nutrient dynamics occurring alongside more frequent extreme weather events. While previous studies indicated that various planktonic taxa may be impacted by long-term environmental change, the response of the microbial community remains less explored, and we know particularly little about how short-term events such as marine heatwaves interact with long-term environmental changes. Here, we investigated the impact of a heatwave (+2°C for 5 days) on the microbial community in mesocosms mimicking ambient and future coastal conditions (1000 ppm CO2, +3°C, Nitrogen: Phosphate ratio of 25), according to IPCC predictions (high-emission scenario RCP 8.5) for 2100. We used 16S rRNA gene sequencing, fluorescence microscopy, and experimental approaches, to study the microbial community taxonomic composition, cell abundances, virus-like particle abundances, as well as cell division and mortality rates. Our results indicate that future coastal conditions may impact the microbial community composition, biodiversity, as well as the abundance of total bacterioplankton. In contrast, the marine heatwave we simulated had a smaller impact on the microbial community composition, and we did not observe any effect of the heatwave on virus-like particle counts and cell division rates. In conclusion, our findings suggest a certain resilience of the microbial community to short-term thermal disturbances.