Response of the Calanoid Copepod Clausocalanus furcatus, to Atmospheric Deposition Events: Outcomes from a Mesocosm Study

Atmospheric deposition is assumed to stimulate heterotrophic processes in highly oligotrophic marine systems, controlling the dynamics and trophic efficiency of planktonic food webs, and is expected to be influenced by climate change. In the course of an 8-day mesocosm experiment, we examined the channeling, of the Saharan dust (SD) and mixed aerosols (A) effects on microplankton up to the copepod trophic level, in the highly oligotrophic Eastern Mediterranean Sea. Based on mesocosms with SD and A treatments, we evaluated the feeding response of the dominant copepod Clausocalanus furcatus every other day. We hypothesized that increased food availability under atmospheric deposition would result in increased copepod ingestion rates, selectivity and production. Overall, no robust pattern of food selection was documented, and daily rations on the prey assemblage of all mesocosms were very low indicating severe food limitation of C. furcatus. Although increased food availability was not true, after few days ingestion of ciliates was maximized, followed by egg production, in both the SD and A treatments, indicating their importance in the diet of this copepod as well as a response of C. furcatus feeding performance. Our results help in understanding the trophic efficiency of marine food webs in ultra-oligotrophic environments under atmospheric deposition. We suggest that future mesocosm research in oligotrophic waters should consider more than one copepod speciesVersión del edito

The potential impact of Saharan dust and polluted aerosols on microbial populations in the East Mediterranean Sea, an overview of a mesocosm experimental approach.

Recent estimates of nutrient budgets for the Eastern Mediterranean Sea (EMS) indicate that atmospheric aerosols play a significant role as suppliers of macro- and micro- nutrients to its Low Nutrient Low Chlorophyll water. Here we present the first mesocosm experimental study that examines the overall response of the oligotrophic EMS surface mixed layer (Cretan Sea, May 2012) to two different types of natural aerosol additions, “pure” Saharan dust (SD, 1.6 mg l-1) and mixed aerosols (A - polluted and desert origin, 1 mg l-1). We describe the rationale, the experimental set-up, the chemical characteristics of the ambient water and aerosols and the relative maximal biological impacts that resulted from the added aerosols. The two treatments, run in triplicates (3 m3 each), were compared to control-unamended runs. Leaching of approximately 2.1-2.8 and 2.2-3.7 nmol PO4 and 20-26 and 53-55 nmol NOx was measured per each milligram of SD and A, respectively, representing an addition of approximately 30% of the ambient phosphate concentrations. The nitrate/phosphate ratios added in the A treatment were twice than those added in the SD treatment. Both types of dry aerosols triggered a positive change (25-600% normalized per 1 mg l-1 addition) in most of the rate and state variables that were measured: bacterial abundance (BA), bacterial production (BP), Synechococcus (Syn) abundance, chlorophyll-a (chl-a), primary production (PP) and dinitrogen fixation (N2-fix), with relative changes among them following the sequence BP>PP≈N2-fix>chl-a≈BA≈Syn. Our results show that the ‘polluted’ aerosols triggered a relatively larger biological change compared to the SD amendments (per a similar amount of mass addition), especially regarding BP and PP. We speculate that despite the co-limitation of P and N in the EMS, the additional N released by the A treatment may have triggered the relatively larger response in most of the rate and state variables as compared to SD. An implication of our study is that a warmer atmosphere in the future may increase dust emissions and influence the intensity and length of the already well stratified water column in the EMS and hence the impact of the aerosols as a significant external source of new nutrients

Experiment design and bacterial abundance control extracellular H2O2 concentrations during four series of mesocosm experiments

The extracellular concentration of H2O2 in surface aquatic environments is controlled by a balance between photochemical production and the microbial synthesis of catalase and peroxidase enzymes to remove H2O2 from solution. In any kind of incubation experiment, the formation rates and equilibrium concentrations of reactive oxygen species (ROSs) such as H2O2 may be sensitive to both the experiment design, particularly to the regulation of incident light, and the abundance of different microbial groups, as both cellular H2O2 production and catalase–peroxidase enzyme production rates differ between species. Whilst there are extensive measurements of photochemical H2O2 formation rates and the distribution of H2O2 in the marine environment, it is poorly constrained how different microbial groups affect extracellular H2O2 concentrations, how comparable extracellular H2O2 concentrations within large-scale incubation experiments are to those observed in the surface-mixed layer, and to what extent a mismatch with environmentally relevant concentrations of ROS in incubations could influence biological processes differently to what would be observed in nature. Here we show that both experiment design and bacterial abundance consistently exert control on extracellular H2O2 concentrations across a range of incubation experiments in diverse marine environments. During four large-scale (>1000 L) mesocosm experiments (in Gran Canaria, the Mediterranean, Patagonia and Svalbard) most experimental factors appeared to exert only minor, or no, direct effect on H2O2 concentrations. For example, in three of four experiments where pH was manipulated to 0.4–0.5 below ambient pH, no significant change was evident in extracellular H2O2 concentrations relative to controls. An influence was sometimes inferred from zooplankton density, but not consistently between different incubation experiments, and no change in H2O2 was evident in controlled experiments using different densities of the copepod Calanus finmarchicus grazing on the diatom Skeletonema costatum (<1 % change in [H2O2] comparing copepod densities from 1 to 10 L−1). Instead, the changes in H2O2 concentration contrasting high- and low-zooplankton incubations appeared to arise from the resulting changes in bacterial activity. The correlation between bacterial abundance and extracellular H2O2 was stronger in some incubations than others (R2 range 0.09 to 0.55), yet high bacterial densities were consistently associated with low H2O2. Nonetheless, the main control on H2O2 concentrations during incubation experiments relative to those in ambient, unenclosed waters was the regulation of incident light. In an open (lidless) mesocosm experiment in Gran Canaria, H2O2 was persistently elevated (2–6-fold) above ambient concentrations; whereas using closed high-density polyethylene mesocosms in Crete, Svalbard and Patagonia H2O2 within incubations was always reduced (median 10 %–90 %) relative to ambient waters

Fe(II) stability in coastal seawater during experiments in Patagonia, Svalbard, and Gran Canaria

The speciation of dissolved iron (DFe) in the ocean is widely assumed to consist almost exclusively of Fe(III)-ligand complexes. Yet in most aqueous environments a poorly defined fraction of DFe also exists as Fe(II), the speciation of which is uncertain. Here we deploy flow injection analysis to measure in situ Fe(II) concentrations during a series of mesocosm/microcosm/multistressor experiments in coastal environments in addition to the decay rate of this Fe(II) when moved into the dark. During five mesocosm/microcosm/multistressor experiments in Svalbard and Patagonia, where dissolved (0.2 µm) Fe and Fe(II) were quantified simultaneously, Fe(II) constituted 24 %–65 % of DFe, suggesting that Fe(II) was a large fraction of the DFe pool. When this Fe(II) was allowed to decay in the dark, the vast majority of measured oxidation rate constants were less than calculated constants derived from ambient temperature, salinity, pH, and dissolved O2. The oxidation rates of Fe(II) spikes added to Atlantic seawater more closely matched calculated rate constants. The difference between observed and theoretical decay rates in Svalbard and Patagonia was most pronounced at Fe(II) concentrations <2 nM, suggesting that the effect may have arisen from organic Fe(II) ligands. This apparent enhancement of Fe(II) stability under post-bloom conditions and the existence of such a high fraction of DFe as Fe(II) challenge the assumption that DFe speciation in coastal seawater is dominated by ligand bound-Fe(III) species

Impact of Aerosols (Saharan Dust and Mixed) on the East Mediterranean Oligotrophic Ecosystem; Results from Experimental Studies

In oligotrophic environments, dust and nutrient inputs via atmospheric routes are considered important sources of macro-nutrients and micro-trace metals fuelling primary and secondary production. Yet, the impact of these dust inputs on the microbial populations is not fully investigated in the Eastern Mediterranean Sea (EMS). The response of oligotrophic systems to dust inputs, whether as positive or negative feedbacks to autotrophic and heterotrophic production and thus to biogeochemical cycling, is important to examine further. Experimental studies have explored nutrient additions in various combinations to determine the limiting resource to productivity or N2 fixation. Recent experimental studies have applied dust enrichments to bottle or mesocosm incubations of seawater from different oceanic regions. This research topic presents two Eastern Mediterranean dust addition mesocosm experiments using, for the first time, real aerosol additions, pure Saharan dust and mixed aerosols (a natural mixture of desert dust and polluted European particles), as well as other EMS aerosol experimental studies. The Topic includes manuscripts introducing results on: a) the impact of Saharan dust vs mixed aerosols on the autotrophic and heterotrophic surface microbial populations in the EMS, b) the impact of single vs multi-pulses of Saharan dust introduction into the pelagic environment of the EMS and c) other experimental studies of aerosol impacts on the EMS ecosystem

Responses of Free-Living Planktonic Bacterial Communities to Experimental Acidification and Warming

Climate change driven by human activities encompasses the increase in atmospheric CO2 concentration and sea-surface temperature. Little is known regarding the synergistic effects of these phenomena on bacterial communities in oligotrophic marine ecosystems that are expected to be particularly vulnerable. Here, we studied bacterial community composition changes based on 16S rRNA sequencing at two fractions (0.1–0.2 and >0.2 μm) during a 10- day fully factorial mesocosm experiment in the eastern Mediterranean where the pH decreased by ~0.3 units and temperature increased by ~3 °C to project possible future changes in surface waters. The bacterial community experienced significant taxonomic differences driven by the combined effect of time and treatment; a community shift one day after the manipulations was noticed, followed by a similar state between all mesocosms at the third day, and mild shifts later on, which were remarkable mainly under sole acidification. The abundance of Synechococcus increased in response to warming, while the SAR11 clade immediately benefited from the combined acidification and warming. The effect of the acidification itself had a more persistent impact on community composition. This study highlights the importance of studying climate change consequences on ecosystem functioning both separately and simultaneously, considering the ambient environmental parameters

Evaluating the impact of atmospheric depositions on springtime dinitrogen fixation in the Cretan Sea (Eastern Mediterranean) - A mesocosm approach

Large amounts of dust and atmospheric aerosols, originating from surrounding desert areas (e.g., Sahara and Middle East) are deposited annually on the surface of the Eastern Mediterranean Sea. These depositions can provide high amounts of micro (such as Fe, Zn, Co) and macro nutrients (such as P and N) to supplement nutrient-poor surface waters- that typically limit primary productivity and also dinitrogen (N2) fixation in many marine environments. Here, we studied the impact of the atmospheric deposition of dust and aerosols on N2 fixation in the Cretan Sea (Eastern Mediterranean Sea). Mixed polluted aerosols (hereafter A) and Saharan dust (hereafter SD) were added to nine mesocosms (3-m3 each) containing surface mixed layer seawater (~10 m), and N2 fixation was evaluated for 6 days during May 2012 (springtime). The addition of SD triggered a rapid (30 h) and robust (2-4 fold) increase in N2 fixation rates that remained high for 6 days and contributed 3-8% of the primary productivity. The A addition also resulted in higher N2 fixation rates compared to the unamended control mesocosms, although the responses were less profound (1.5-2 fold) and accounted for only 2-4% of the primary productivity. The microbial community responded differently to the two additions. Heterotrophic bacterial N2 fixers dominated the diazotroph community in A and the control mesocosms, while the non-filamentous cyanobacterial group Trichodesmium prevailed in the SD treatment (68% of all the operational taxonomic units, verified by qPCR analyses). Our results indicate that the aerosol source, its route prior to deposition, and its specific chemical composition, can alter the diazotrophic diversity and activity in the Eastern Mediterranean Sea and may thus impact both the N and C dynamics in this impoverished environment

Planktonic Lipidome Responses to Aeolian Dust Input in Low-Biomass Oligotrophic Marine Mesocosms

The effect and fate of dry atmospheric deposition on nutrient-starved plankton in the Eastern Mediterranean Sea (EMS; Crete, 2012) was tested 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 the 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 &gt;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