The Impact of Dry Atmospheric Deposition on the Sea-Surface Microlayer in the SE Mediterranean Sea: An Experimental Approach

Abstract

The oligotrophic southeastern Mediterranean Sea (SEMS) is frequently exposed to desert-dust deposition which supplies nutrients, trace metals and a wide array of viable airborne microorganisms. In this study, we experimentally examined the impact of aerosol addition, collected during an intense dust storm event in early September 2015, on the biomass and activity of pico-phytoplankton and heterotrophic bacterial populations at the sea-surface micro layer (SML) relative to the sub surface layer (SSL). Aerosol (1.5 mg L −1 ) was added to SML and SSL water samples in microcosms (4.5 L) and the water was frequently sampled over a period of 48 h. While the aerosol amendment triggered a moderate 1.5-2-fold increase in primary production in both the SML and the SSL, bacterial production increased by ∼3 and ∼7-folds in the SSL and SML, respectively. Concurrently, the abundance and flow-cytometric characteristics (green fluorescence and side scatter signals) of high nucleic acid (HNA) and low nucleic acid (LNA) bacterial cells showed a significant increase in the %HNA, in both SML and SSL samples following aerosol amendment. This shift in nucleic acid content took place at a much faster rate in the SML, suggesting a more active heterotrophic community. These changes were likely a result of higher rates of carbon utilizations in the SML following the dust addition, as assessed by a selected hydrocarbons and saccharides analysis. Additionally, a high absorption rate of hydrocarbons by the aerosol particles was measured following the additions, leaving less than 10% of these molecules available for potential heterotrophic microbial utilization. Our results suggest that the heterotrophic microbial community inhabiting the SML is more efficient in utilizing aerosol associated constituents than the community in the SSL.

Phytoplankton and Bacterial Response to Desert Dust Deposition in the Coastal Waters of the Southeastern Mediterranean Sea: A Four-Year In Situ Survey

“…It was assessed that during dust events, about 5 µmol PO 4 3+ and 300 µmol NO 3 − may be released per gram of deposited dust [2]. These nutrient additions can induce a fertilization effect and result in increased bacterial/phytoplankton biomass and activity, as observed in bioassay microcosm experiments using reagent-based nutrient additions [23] and/or dust/aerosol additions [13,18,61].…”

“…[7,8,10,11]. Numerous studies show that dust deposition in seawater can alter bacterial and phytoplankton biomass and activity [12][13][14][15][16][17][18]. Dust is therefore recognized as an important factor that affects the ecology of many aquatic settings, especially low-nutrient, low-chlorophyll (LNLC) environments [19].…”

“…Indeed, it has been demonstrated that atmospheric deposition is one of the most important external nutrient sources for the SEMS (i.e., NO 3 + NO 2 , PO 4 ) [31][32][33][34]. Recent microcosm bioassay studies show, for example, that dust added to the surface microlayer (upper 60 µm layer) resulted in up to fivefold and twofold increases in BP and PP, respectively [18,35]. However, such microcosm studies represent conditions during distinct and limited time points and may have some experimental limitations (i.e., bottle effect) [36].…”

Organic Matter in the Surface Microlayer: Insights From a Wind Wave Channel Experiment

“…Bacteria in the field bacteria may be enriched (Stolle et al, 2009) or depleted in the SML (Engel and Galgani, 2016;, or their growth and hence consumption rates of amino acids are reduced (Stolle et al, 2009;Santos et al, 2012). Possible controls of bacterioneuston activity include the prevailing meteorological conditions, UV light (Agogué et al, 2005), incident OM availability (Carlucci et al, 1985(Carlucci et al, , 1992, wind speed (Stolle et al, 2011;Rahlff et al, 2017) and aerosol deposition (Astrahan et al, 2016). While the importance of solar and UV irradiance for controlling neuston activity in the field is debated (Bailey et al, 1983;Carlucci et al, 1985;Santos et al, 2012), the high bacterial enrichment in the SML during the first days of this study suggest, that accumulation of labile OM and at the absence of light potentially favors bacterial growth at the air sea interface.…”

“…Organic components in the SML are furthermore expected to chemically interact with compounds entering the ocean via the atmosphere such as nutrients and dust (Astrahan et al, 2016). Organic macromolecules in the SML are known to form complexes with trace metals and may thus bind metal ions from atmospheric depositions, e.g., iron, before these enter deeper into the water column (Hardy et al, 1985).…”