Abstract
In oligotrophic regions, such as the Mediterranean Sea, atmospheric deposition has the potential to stimulate heterotrophic prokaryote growth and production in surface waters, especially during the summer stratification period. Previous studies focused on the role of leaching nutrients from mineral particles of Saharan (S) origin, and were restricted to single locations at given times of the year. In this study, we evaluate the effect of atmospheric particles from diverse sources and with a markedly different chemical composition [S dust and anthropogenic (A) aerosols] on marine planktonic communities from three locations of the northwestern Mediterranean with contrasted anthropogenic footprint. Experiments were also carried out at different times of the year, considering diverse initial conditions. We followed the dynamics of the heterotrophic community and a range of biogeochemical and physiological parameters in six experiments. While the effect of aerosols on bacterial abundance was overall low, bacterial heterotrophic production was up to 3.3 and 2.1 times higher in the samples amended with A and S aerosols, respectively, than in the controls. Extracellular enzymatic activities [leu-aminopeptidase (AMA) and β-glucosidase (β-Gl)] were also enhanced with aerosols, especially from A origin. AMA and β-Gl increased up to 7.1 in the samples amended with A aerosols, and up to 1.7 and 2.1 times, respectively, with S dust. The larger stimulation observed with A aerosols might be attributed to their higher content in nitrate. However, the response was variable depending the initial status of the seawater. In addition, we found that both A and S aerosols stimulated bacterial abundance and metabolism significantly more in the absence of competitors and predators.
Highlights
Marine heterotrophic prokaryotes play fundamental roles in the oceanic part of the carbon cycle (Jiao et al, 2010)
Inorganic nutrients were overall at higher concentration in BCN than in the other locations, the lowest concentrations found in BLA during SU and in the offshore Balearic Islands (OFF) station
Nitrate (NO−3 ), NH+4, and total inorganic phosphorous (TIP) at the beginning of the WI experiment were at lower concentrations than expected at that time of the year, what we attribute to a post-bloom situation, since chlorophyll (Chl) concentration was quite higher than expected (Marín et al, 2017)
Summary
Marine heterotrophic prokaryotes (hereafter referred to as “bacteria”) play fundamental roles in the oceanic part of the carbon cycle (Jiao et al, 2010) In oligotrophic regions, such as the Mediterranean Sea, most of the organic carbon (OC) is recycled in the surface layer and rapidly re-exchanged with the atmosphere (Moutin and Raimbault, 2002). Once the most labile fraction of OC is exhausted, bacteria have to synthetize extracellular enzymes in order to be able to hydrolyze polymeric substrates and take up the monomers produced (Chróst, 1990). These processes are dependent on the availability of inorganic nutrients to bacteria inhabiting the surface waters. Instead, during periods of winter mixing – when inorganic nutrients are more abundant – bacterioplankton growth and production has been found to be limited in OC (Thingstad et al, 1997; Pinhassi et al, 2006)
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