Abstract
The effect and fate of dry atmospheric deposition on nutrient-starved plankton in the Eastern Mediterranean Sea (EMS; Crete, 2012) by spiking oligotrophic surface seawater mesocosms (3 m3) with Saharan dust (SD; 1.6 g L-1; 23 nmol NOx mg-1; 2.4 nmol PO4 mg-1) or mixed aerosols (A; 1.0 g L-1; 54 nmol NOx mg-1; 3.0 nmol PO4 mg-1) collected from natural and anthropogenic sources. Using high resolution liquid chromatography-mass spectrometry, the concentrations of over 350 individual lipids were measured in suspended particles to track variations in the lipidome associated with dust fertilization. Bacterial and eukaryotic intact polar lipid (IPL) biomarkers were categorized into 15 lipid classes based on headgroup identity, including four novel IPL headgroups. Bulk IPL concentrations and archaeal tetraether lipids were uncoupled with the doubling of chlorophyll concentrations that defined the stimulation response of oligotrophic plankton to SD or A amendment. However, molecular level analysis revealed the dynamics of the IPL pool, with significant additions or losses of specific IPLs following dust spikes that were consistent among treatment mesocosms. Multivariate redundancy analysis further demonstrated that distribution of IPL headgroups and molecular modifications within their alkyl chains were strongly correlated with the temporal evolution of the plankton community and cycling of phosphate. IPLs with phosphatidylcholine, betaine, and an alkylamine-like headgroup increased in the post-stimulated period, when phosphate turnover time had decreased by an order of magnitude and phosphorus uptake was dominated by plankton > 2 µm. For most IPL classes, spiking with SD or A yielded significant increases in the length and unsaturation of alkyl chains. A lack of corresponding shifts in the plankton community suggests that the biosynthesis of nitrogenous and phosphatidyl lipids may respond to physiological controls during episodic additions of dust to the EMS. Furthermore, alkyl chain distributions of IPLs containing N, P, and S invoked a bacterial source, suggesting that bacterioplankton are able to modulate these lipids in response to nutrient stress.
Highlights
In the ultra-oligotrophic Eastern Mediterranean Sea (EMS), biological production islimited by both nitrogen (N) and phosphorus (P) (e.g., Krom et al, 1991; Thingstad et al, 2005a) and is primarily driven by picoplankton and recycling within the microbial loop
Chlorophyll a (Chl-a) concentrations had doubled in mesocosms on days 2 and 3 following the addition of Saharan dust (SD) or A, which were significantly higher compared to the control; this increase coincided with significant increases in Synechococcus and autotrophic picoeukaryotes (Guo et al, 2016)
The significant increases in both chlorophyll a (Chl-a) and cell counts were indicative that dust and aerosol additions had positive effects on the plankton community, hereafter referred to as “stimulated plankton,” even though Chl-a concentrations remained relatively low in these oligotrophic mesocosms in comparison to nutrient replete oceanographic regions
Summary
In the ultra-oligotrophic Eastern Mediterranean Sea (EMS), biological production is (co)limited by both nitrogen (N) and phosphorus (P) (e.g., Krom et al, 1991; Thingstad et al, 2005a) and is primarily driven by picoplankton and recycling within the microbial loop. Responses of the planktonic community to episodic dust pulses should play an increasing role in determining food web structure and the carbon budget of the oligotrophic ocean To further investigate this phenomenon, the MESOAQUA experiment (May 2012; Heraklion, Crete) simulated intense dry atmospheric deposition in the ultraoligotrophic EMS by spiking surface seawater mesocosms (Cretan Sea) with two different natural aerosols collected in the Levantine Basin: Saharan dust (SD) or mixed aerosols (A; Herut et al, 2016). Biogeochemical responses to the addition of SD or A, including plankton community composition and production, pigment concentration, and cycling of macro-nutrients and trace elements, were compared in triplicate to unamended control mesocosms over a period of 8 days (Guo et al, 2016; Herut et al, 2016; Rahav et al, 2016; Tsiola et al, 2016; Tsagaraki et al, under revision, this SI). Sophisticated molecular networks have been employed to facilitate the interpretation of the large data sets obtained by such analyses (Kharbush et al, 2016)
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