From flames to oceans: biomass burning aerosols shape microbial communities in the Mediterranean Sea

Over recent decades, the Mediterranean basin has faced enhanced wildfire risks due to longer dry seasons and higher temperatures associated with global changes. Wildfire-generated aerosols could travel long distances and ultimately end in the ocean, where they may subsequently impact the marine microbial communities that are the key drivers of global geochemical cycles. However, the current knowledge about the influence of wildfire ash on the abundance and composition of marine microbes remains limited. We conducted experiments where surface seawater from the Northwestern Mediterranean Sea was incubated (up to 10 days) in (300L) minicosms amended with different concentrations of ashes, which were previously collected during a real wildfire in the Mediterranean region. Ash additions had a direct effect by increasing microbial abundance and diversity, likely due to the release of both inorganic and organic substrates that alleviated nutrient limitations. At a later stage, ash additions also indirectly affected microbial biomass and diversity via the stimulation and ulterior decline of phytoplankton communities. These mechanisms induced changes in prokaryotic community composition, reflecting a succession of different taxa adapted to different nutrient qualities and quantities. Ashes had a negative effect on Cyanobiaceae, but promoted the growth of Flavobacteriaceae, Rhodobacteraceae, and SAR11 clade I among other taxa. Our findings suggest that wildfire ash can alter Mediterranean prokaryotic communities over time during oligotrophic periods, with broad implications for the Mediterranean Sea biogeochemical cycles.