From Flames to the Ocean: Biomass Burning Aerosols Are Associated With Changes in Prokaryotic Communities in the Mediterranean Sea

The Mediterranean basin faces enhanced wildfire risks associated with human-driven climate and land use changes. Wildfire-generated aerosols can reach the ocean, where they may subsequently impact marine prokaryotic communities, key drivers of global biogeochemical cycles. However, our understanding of the influence of wildfire airborne particles on the abundance and composition of marine microbes remains limited. We conducted experiments in which surface water from the northwestern Mediterranean Sea was incubated in 300 L minicosms amended with varying amounts of wildfire fine ash particles, previously collected during a Mediterranean wildfire. Wet deposition of wildfire fine ash particles had a short-term effect on prokaryotes by increasing their abundance and diversity, likely due to the release of both inorganic and organic substrates, alleviating nutrient limitations. Ash deposition could also indirectly affect prokaryotic communities via changes in the composition of phytoplankton populations. These mechanisms induced changes in prokaryotic community composition, reflecting a succession of taxa likely adapted to different substrate qualities. Ash had a negative effect on Cyanobiaceae but promoted the growth of Flavobacteriaceae, Rhodobacteraceae and SAR11 clade I among other taxa. Our findings demonstrate that wildfire ash can alter Mediterranean prokaryotic communities during oligotrophic periods, further exacerbating the impact of wildfires on marine ecosystems.