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Bacterial response to dust pulses in the western Mediterranean: Implications for carbon cycling in the oligotrophic ocean

The bacterial response to dust pulses was investigated in the Mediterranean Sea through a combined field and experimental study. During the stratification period, characterized by a nutrient‐starved mixed layer isolated from the depth, a Saharan dust event (2.6 g m−2) induced a 1.5‐fold increase in bacterial abundance (BA) and a 2‐fold increase in bacterial respiration (BR). Experimental dust additions (equivalent to fluxes of 5 and 20 g m−2) to bacteria natural assemblages also stimulated BA (between 2‐ and 4‐fold increases) and BR (between 1.5‐ and 3‐fold increases). Pooling the in situ and experimental data, linear relationships were obtained between dust concentration and BA (r2 = 0.86; p < 0.01) and BR (r2 = 0.89; p < 0.001). The dust‐induced bacterial bloom resulted in a C mineralization of 0.5 g m−2, which may represent up to 70% of bioavailable DOC annually exported to the depth in the Mediterranean. These results demonstrate that heterotrophic bacteria may play a much larger role in the connections between dust and the ocean carbon cycle than previously recognized and highlight the need for a more accurate understanding of how dust pulses may affect C export in the oligotrophic ocean.