Bio-aerosols negatively affect Prochlorococcus in oligotrophic aerosol-rich marine regions

The marine cyanobacterium Prochlorococcus is a dominant photoautotroph in many oligotrophic Low-Nutrients-Low-Chlorophyll (LNLC) regions. While the chemical impact of aerosols upon interaction with surface seawater was documented in numerous studies, we show that Prochlorococcus cells are affected also by bio-aerosols (potentially biological agents in the dust/aerosols such as membrane-bound extracellular vesicles, small-size bacteria and/or viruses), resulting in lower surface seawater abundances in the oligotrophic Mediterranean Sea. We conducted experimental amendments of 'live' aerosol/dust particles and aerosol filtrates (<0.22-μm) to surface Southeastern Mediterranean seawater or to pure Prochlorococcus cultures (MED4). Results show a significant decline in cell biomass (<90%), while UV-sterilized aerosols elicited a much weaker and non-significant response (~10%). We suggest that the difference is due to a negative effect of bio-aerosols specific to Prochlorococcus. Accordingly, the dominance of Synechococcus over Prochlorococcus throughout the surface Mediterranean Sea (observed mainly in spring when atmospheric aerosol levels are relatively high) and the lack of spatial westward gradient in Prochlorococcus biomass as typically observed for chlorophyll-a or other cyanobacteria may be attributed, at least to some extent, to the impact of bio-aerosol deposition across the basin. Predictions for enhanced desertification and increased dust emissions may intensify the transport and potential impact of bio-aerosols in LNLC marine systems.