Bacterial Growth and Mortality after Deposition of Saharan Dust and Mixed Aerosols in the Eastern Mediterranean Sea: A Mesocosm Experiment

Anastasia Tsiola,Nebil Yücel,Nikolaos Nikolioudakis,Antonia Giannakourou,Paraskevi Pitta,Barak Herut,Tatiana M Tsagaraki

doi:10.3389/fmars.2016.00281

Abstract

The impact of viral lysis and grazing by flagellates on bacterioplankton production was assessed during a mesocosm experiment in the Eastern Mediterranean Sea, in response to Saharan dust (SD) vs. mixed aerosols (A) addition. The results highlight a positive effect on bacterial abundance, production and growth rate (~1.2, ~2.4 and ~1.9 –fold higher than the controls) in both SD and A, which was also confirmed by the increased portion of high DNA content bacteria (up to 48% of the bacterial community). Lytic viral production and the portion of bacterial production lost due to viral lysis were lower in SD and A after dust addition than in the controls (0.33 ± 0.17 x106 virus-like particles mL-1 h-1 and 6 ± 4%, respectively). Potential ingestion rate of bacteria by flagellates increased upon dust enrichment, but did not differ between mesocosms. Larger predators possibly down regulated flagellate abundance, and the calculated portion of bacterial production lost due to flagellate grazing was probably an artifact. Higher frequency of lysogenic cells in A compared to SD and the controls four days after dust addition may reflect faster phosphorus limitation in A, due to receiving less dissolved inorganic phosphorus and more dissolved inorganic nitrogen than SD.

Highlights

The Eastern Mediterranean Sea (EMS) is characterized by ultra—oligotrophic conditions, with low nutrient concentrations in surface layers (Krom et al, 1991) and low primary productivity and phytoplankton biomass (Siokou-Frangou et al, 2010)
Phosphate concentration decreased upon aerosol addition in both A and Saharan dust (SD), while it remained stable in the controls
Aeolian dust from the Saharan desert and southern Europe is deposited in the EMS, supplying the surface waters with nutrients, trace metals and organic matter and influencing the structure and function of the planktonic food web

Summary

Introduction

The Eastern Mediterranean Sea (EMS) is characterized by ultra—oligotrophic conditions, with low nutrient concentrations in surface layers (Krom et al, 1991) and low primary productivity and phytoplankton biomass (Siokou-Frangou et al, 2010). The high fluxes of atmospheric deposition during that period introduce material of Saharan desert and southern European origin into the marine system (Volpe et al, 2009). Atmospheric chemical processes, such as acidification, are mainly driven by human processes and air pollution (Seinfeld and Pandis, 1998) and significantly affect the final amount and chemical properties of the leaching procedure (Shi et al, 2015)

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