Mesopelagic prokaryotes (archaea and bacteria), which are transported together with nutrient-rich intermediate-water to the surface layer by deep convection in the oceans (e.g., winter mixing, ...upwelling systems), can interact with surface microbial populations. This interaction can potentially affect production rates and biomass of surface microbial populations, and thus play an important role in the marine carbon cycle and oceanic carbon sequestration. The Eastern Mediterranean Sea (EMS) is one of the most oligotrophic and warm systems in the world’s oceans, with usually very shallow winter mixing (<200 m) and lack of large-size spring algal blooms. In this study, we collected seawater (0-1500 m) in 9 different cruises at the open EMS during both the stratified and the mixed seasons. We show that the EMS is a highly oligotrophic regime, resulting in low autotrophic biomass and primary productivity and relatively high heterotrophic prokaryotic biomass and production. Further, we simulated deep water mixing in on-board microcosms using Levantine surface (LSW, ~0.5 m) and intermediate (LIW, ~400 m) waters at a 9:1 ratio, respectively and examined the responses of the microbial populations to such a scenario. We hypothesized that the LIW, being nutrient-rich (e.g., N, P) and a ‘hot-spot’ for microbial activity (due to the warm conditions that prevail in these depths), may supply the LSW with not only key-limiting nutrients but also with viable and active heterotrophic prokaryotes that can interact with the ambient surface microbial population. Indeed, we show that LIW heterotrophic prokaryotes negatively affected the surface phytoplankton populations, resulting in lower chlorophyll-a levels and primary production rates. This may be due to out-competition of phytoplankton by LIW populations for resources and/or by a phytoplankton cell lysis via viral infection. Our results suggest that phytoplankton in the EMS may not likely form blooms, even after exceptionally deep winter mixing, and therefore have a very small overall effect on the vertical flux of organic matter to the deep sea.
Abstract
The temporal distributions of microphytoplankton were studied monthly over 2 years in the shelf and the offshore waters of the “low nutrients low chlorophyll” (LNLC) southeastern (SE) ...Mediterranean Sea. The microphytoplankton were comprised mostly of diatoms (0–542 cells L−1, ~4 × 106–44 × 106 cells m−2) of the genera Rhizosolenia, Thalassionema, Chaetoceros, Leptocylindrus and Pseudo-nitzschia, and dinoflagellates (1–89 cells L−1, ~1 × 106–10 × 106 cells m−2) of the genera Tripos (formally known as Ceratium), Ornithocercus, Protoperidinium, Ceratocorys and Dinophysis. During the winter mixing, microphytoplankton abundance was ~2-fold higher than during the stratified summer months, in accordance with the higher inorganic nutrient levels. Diatoms were mostly found in the upper illuminated layers (0–100 m), while dinoflagellate distribution was patchy and spread from surface down to 200 m. We also calculated that diatoms and dinoflagellates contribute negligibly (~4%) to the total vertical carbon flux in the offshore water, suggesting fast recycling of organic matter at the photic layer. Our results provide a baseline to better understand carbon and biogenic silica fluxes at the LNLC SE Mediterranean Sea.
This study explores the potential impacts of microbes deposited into the surface seawater of the southeastern Mediterranean Sea (SEMS) along with atmospheric particles on marine autotrophic and ...heterotrophic production. We compared in situ changes in autotrophic and heterotrophic microbial abundance and production rates before and during an intense dust storm event in early September 2015. Additionally, we measured the activity of microbes associated with atmospheric dry deposition (also referred to as airborne microbes) in sterile SEMS water using the same particles collected during the dust storm. A high diversity of prokaryotes and a low diversity of autotrophic eukaryotic algae were delivered to surface SEMS waters by the storm. Autotrophic airborne microbial abundance and activity were low, contributing ~1% of natural abundance in SEMS water and accounting for 1-4% to primary production. Airborne heterotrophic bacteria comprised 30-50% of the cells and accounted for 13-42% of bacterial production. Our results demonstrate that atmospheric dry deposition may supply not only chemical constitutes but also microbes that can affect ambient microbial populations and their activity in the surface ocean. Airborne microbes may play a greater role in ocean biogeochemistry in the future in light of the expected enhancement of dust storm durations and frequencies due to climate change and desertification processes.
Seawater reverse osmosis (SWRO) desalination discharge brine-waste as part of the process with immediate (short-term) and chronic (long-term) impacts to marine coastal environment. Brine-waste is ...often denser than the receiving environment, therefore sinks and flows as a saline plume in adjacency to the sea floor. We suggest that the saline flow over the bottom may impose an osmotic stress, and alter benthic heterotrophic microbial diversity, activity and growth. In this study, we examined the short-term (48h) effects of different salinities on benthic heterotrophic bacteria attached to the sediment at the eastern Mediterranean coast. To this end, 12 cylinders were filled with sediment and incubated in full darkness with rising salinities ranging from 2% to 20% over the ambient levels. During the summer experiments, heterotrophic bacterial abundance has reduced by 60% at salinity levels >5% above the ambient concentration. Further, bacterial cell specific activity significantly increased following high salinity scenarios. Our experimental results provide the first scientifically-based data on the immediate effects of SWRO brine over benthic heterotrophic bacteria. However, we stress that long-term studies are imperative at the outfall of operating desalination facilities to determine the chronic effects of brine on benthic bacteria.
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•Brine (salinity) discharge of 5% over the ambient reduces benthic bacteria abundance.•Short-term exposure to brine did not affect the diversity of benthic bacteria.•The metabolic response per cell was enhanced due short term exposure to SWRO brine.•The impact of a brine plume on the benthic bacteria may extend up to few hundreds of m2.
Submarine groundwater discharge (SGD) has been shown to be an important source of nutrients in coastal environments, especially nitrogen and silica, and thereby relive nutrient limitation to ...phytoplankton. Here, we followed autotrophic microbial biomass, activity, and community composition at a site strongly influenced by SGD and a nearby nutrients-poor reference site at the oligotrophic Israeli shallow rocky coast southeastern Mediterranean Sea (SEMS) between 2011 and 2019. The surface water at the SGD-affected area had significantly higher NO
3
+ NO
2
(∼10-fold) and Si(OH)
4
(∼2-fold) levels compared to the reference site, while no significant differences were observed for PO
4
or NH
4
. This resulted in a significant increase in algae biomass (∼3.5-fold), which was attributed to elevated
Synechococcus
(∼3.5-fold) and picoeukaryotes (∼2-fold) at the SGD-affected site, and in elevated primary production rates (∼2.5-fold). Contrary to most SGD-affected coastal areas, diatoms biomass remained unchanged between sites, despite the elevated N and Si, suggesting the dominance of picophytoplankton over microphytoplankton at the SEMS. DNA sequencing of the 16S and 18S rDNA supported these findings. These results highlight the influence of SGD on shallow-water microbial populations. Our observations are consistent with recent studies showing that phytoplankton along the Israeli coast are likely nitrogen + silica limited, and may have important ecological and regulatory implications for environmental policy and management of coastal aquifers.
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.
Recent studies have indicated that heterotrophic diazotrophs are highly diverse and fix N
in aquatic environments with
adverse conditions for diazotrophy, such as oxic and rich in total nitrogen. In ...this study, we compared the activity and diversity of heterotrophic diazotrophs associated with aggregates (>12 μm) to free-living cells in the eutrophic Qishon River during the winter and summer seasons. Overall, measured heterotrophic N
fixation rates in the Qishon River ranged between 2.6-3.5 nmol N L
d
. Heterotrophic N
fixation was mainly associated with aggregates in the summer samples (74 ± 24%), whereas during the winter the bulk diazotrophic activity was mostly ascribed to the free-living fraction (90 ± 6%). In addition, immunolabeled micrographs indicated the presence of aggregate-associated heterotrophic diazotrophs in both seasons, while phototrophic diazotrophs were also captured during the winter. The richness of free-living and aggregate-associated heterotrophic diazotrophs were overall similar, yet the evenness of the later was significantly smaller, suggesting that few of the species gained advantage from particle lifestyle. The differences in the activity, micro-localization and diversity of the diazotrophic community were mostly attributed to spatiotemporal changes in the ambient
:
ratios (total organic carbon, TOC: total nitrogen) and the TOC concentrations. Taken together, our results shed new light on the contribution of heterotrophic diazotroph associated with aggregates to total heterotrophic N
fixation in oxic, highly eutrophic aquatic environments.
Unicellular diazotrophs are recognized as important contributors to the aquatic nitrogen cycle. Yet, quantifying the number of aquatic diazotrophs, especially the unicellular fraction, is highly ...challenging and relies mostly on microscopical and molecular approaches. In this study, a new flow cytometry-based method was developed to quantify unicellular diazotrophs by immunolabeling the nitrogenase enzyme. The quantification method was initially developed using a diazotrophic monoculture (
Vibrio natriegens
) and verified by different controls as well as various auxiliary approaches such as N
2
fixation and confocal laser scanning microscopy. It was evident that only 15–20% of the total number of
V. natriegens
cells synthesized the nitrogenase enzyme, even though the media was N-limited and anaerobic. This approach was further tested on field samples collected from marine and freshwater environments. Overall, the number of unicellular diazotrophs measured in various aquatic environments ranged from 1 to 6 × 10
7
cells L
−1
. However, the fraction of these diazotrophs compared to total bacteria was significantly lower in the Eastern Mediterranean Sea (0.1%) than that collected along the Jordan River (4.7%). Nevertheless, N
2
fixation rates per cell were highest in the Mediterranean Sea (~ 90 attomole N cell
−1
d
−1
), while lower in the lake and the river measured herein (1.4 to 0.3 attomole N cell
−1
d
−1
, respectively). Following the above, this direct quantification approach can provide new insights on the number and specific contribution of unicellular diazotrophs to total N
2
fixation in marine and freshwater environments.
The atmospheric deposition of gases and particulates from the Sahara Desert and European landmass is an important source of nutrients for the Mediterranean Sea. In this study, we investigated how ...such atmospheric input might affect bacterial metabolic activities and community dynamics in the ultra-oligotrophic Eastern Mediterranean Sea. Thus a mesocosm simulation experiment was conducted using “pure” Saharan dust (SD) and mixed aerosols (A, polluted and desert origin). The cell specific bacterial production (BP) was stimulated soon after the addition of SD and A, with a higher degree of stimulation being observed in the activity of Alphaproteobacteria than in Gammaproteobacteria, and this lead to significant increases in community BP. Subsequently, a shift between these two dominating classes was observed (such that the proportion of Gammaproteobacteria increased while that of Alphaproteobacteria decreased), along with significant increases in bacterial abundance and chlorophyll a concentration. After a few days, although the abundance of bacteria was still significantly higher in the SD- or A-treated groups, differences in the active community composition between the treatment and control groups were reduced. The altered activity of the two dominating Proteobacteria classes observed, might reflect their different strategies in responding to external nutrient input: with Alphaproteobacteria being more responsive to the direct dust input, whereas Gammaproteobacteria seemed to benefit more from the increase in phytoplankton biomass. In addition, the input of A had a stronger immediate effect and longer lasting influence on changing the active bacterial community composition than did that of SD. Our findings show that episodic atmospheric deposition events might affect the microbial community with regards to their abundance, activity and composition over a short period of time, and thus regulate the function of the microbial community and carbon cycling in oligotrophic waters.
Seawater desalination facilities continuously discharge hyper-saline brine into the coastal environment which often flows as a concentrated plume over the seafloor, hence possibly impacting benthic ...microorganisms. Yet, the effects of brine discharge from desalination plants on benthic bacteria, key players in biodegradation of organic material and nutrient recycling is unknown. In this study, we tested the chronic (years) effects of brine discharge from three large-scale desalination facilities on the abundance, metabolic activity and community composition of benthic bacteria. To this end, four sampling campaigns were carried at the outfall areas of the Ashkelon, Sorek and Hadera desalination facilities. The effects of the brine were compared to corresponding reference stations which were not influenced by the brine (i.e., water temperature and salinity). Our sampling data indicate that bacterial abundance and activity that includes bacterial growth efficiency were 1.3-2.6-fold higher at the outfall area than the reference station. Concomitant analysis pointed out that the bacterial community structure at the brine discharge area was also different than the reference station, yet varied between each desalination facility. Our results demonstrate that the impact of brine effluent from desalination facilities on benthic bacteria are site-specific and localized (<1.4 Km
) around the discharge point. Namely, that the effects on benthic bacteria are prominent at the brine mixing zone and change according to the discharge method used to disperse the brine as well as local stressors (e.g., eutrophication and elevated water temperature). Our results contribute new insights on the effects of desalination-brine to benthic microbes, while providing scientifically-based aspects on the ecological impacts of brine dispersion for decision makers.
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