| Summary: | The marine cyanobacterium <i>Prochlorococcus</i> 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 <i>Prochlorococcus</i> 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 <i>Prochlorococcus</i> 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 <i>Prochlorococcus.</i> Accordingly, the dominance of <i>Synechococcus</i> over <i>Prochlorococcus</i> throughout the surface Mediterranean Sea (observed mainly in spring when atmospheric aerosol levels are relatively high) and the lack of spatial westward gradient in <i>Prochlorococcus</i> biomass as typically observed for chlorophyll-<i>a</i> 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.
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