Summary
Metacaspases are cysteine specific proteases implicated in cell‐signalling, stress acclimation and programmed cell death (PCD) pathways in plants, fungi, protozoa, bacteria and algae. We ...investigated metacaspase‐like gene expression and biochemical activity in the bloom‐forming, N2‐fixing, marine cyanobacterium Trichodesmium, which undergoes PCD under low iron and high‐light stress. We examined these patterns with respect to in‐silico analyses of protein domain architectures that revealed a diverse array of regulatory domains within Trichodesmium metacaspases‐like (TeMC) proteins. Experimental manipulations of laboratory cultures and oceanic surface blooms of Trichodesmium from the South Pacific Ocean triggered PCD under Fe‐limitation and high light along with enhanced TeMC activity and upregulated expression of diverse TeMC representatives containing different regulatory domains. Furthermore, TeMC activity was significantly and positively correlated with caspase‐like activity, which has been routinely observed to increase with PCD induction in Trichodesmium. Although both TeMC and caspase‐like activities were stimulated upon PCD induction, inhibitor treatments of these proteolytic activities provided further evidence of largely distinct substrate specificities, even though some inhibitory crossover was observed. Our findings are the first results linking metacaspase expression and activity in PCD induced mortality in Trichodesmium. Yet, the role/s and specific activities of these different proteins remain to be elucidated.
Dinitrogen (N2) fixation was investigated at a pelagic station in the oligotrophic waters of the northern Gulf of Aqaba, Red Sea, between February 2016 and December 2018. In situ 15N2 and 13C ...incubations were used to evaluate photic and aphotic N2 fixation rates and diazotrophic contribution to water column productivity. N2 fixation rates were typically low (below detection to <0.5 nmol N L−1 d−1). Maximal rates of 3.1 nmol L−1 d−1, measured at 100 m when conspicuous slicks of the cyanobacterium Trichodesmium appeared on the surface water. Amplicon sequencing of nifH demonstrated that non‐cyanobacterial diazotrophs, mostly α‐ and γ‐proteobacteria comprised the majority (82–100%) of amplicon sequence variants retrieved from photic and aphotic depths when low N2 fixation rates were measured, while amplicons representing cyanobacteria were nearly absent, but appeared in low abundance (~ 2%) when maximal rates were measured. During the stratified summer‐period, water‐column N2 fixation rates (10–75 μmol m−2 d−1) comprised ~ 1–40% of new production (NP). During the winter mixing period N2 fixation rates were considerably higher (11–242 μmol m−2 d−1) but made up only <1% of NP. This is because, during this period, nitrate supplied to the photic zone by the vertical mixing becomes the major N source for NP. We conclude that on an annual average, diazotrophy plays a minor role in the NP of the Gulf. The major “new” nitrogen sources are cross‐thermocline turbulent diffusion of nitrate during summer stratification and vertical mixing during the fall–winter.
Estimates of primary productivity have traditionally disregarded dark inorganic carbon fixation by marine microorganisms. Currently, only limited data are available from different systems on this ...potentially ecologically important process. We present monthly dark inorganic carbon fixation and photosynthetic rates from the euphotic layer of the northern Gulf of Aqaba collected over a decade between 2010 and 2020. Averaged dark inorganic carbon fixation rates from surface to 100 m depth, ranged from 99 to 173 mg C m−2 d−1, which corresponds to ~ 43% of the annual primary productivity at this location. The lowest dark inorganic carbon fixation rates were found during winter, contributing ~ 7.5% of the integrated primary productivity. During the oligotrophic summer, dark inorganic carbon fixation comprised a larger fraction of the integrated primary productivity estimated as ~ 12%. In accordance, dark inorganic carbon fixation contributed ~ 6% to the particulate organic carbon flux during the winter and ~ 30% during summertime. Complimentary nutrient‐enrichment bioassays of seawater from 5 m show that dissolved organic nutrient enrichment (P and C based) significantly elevates dark inorganic carbon fixation, whereas addition of dissolved inorganic nutrients (PO43+, NO3−, or both) significantly increased photosynthesis but to a lesser extent dark inorganic carbon fixation. These results suggest that dark inorganic carbon fixation may be an important biochemical process throughout the euphotic zone of oligotrophic seas, and thus should be incorporated into oceanic carbon production estimates.
Unicellular photoautotrophic diazotrophs such as Crocosphaera spp. are ubiquitous in many oligotrophic and N‐limited oceans, as they can reduce N2 into bioavailable ammonia. The Mediterranean Sea is ...potentially an ideal environment for photoautotrophic diazotrophic activity, and yet N2‐fixation rates measured in the last two decades are typically very low and no diazotrophic blooms have been recorded in its offshore waters. Previous studies suggest that diazotrophs, as well as nondiazotrophic phytoplankton and heterotrophic bacteria, may be P‐limited, hence their low biomass and activity. Here, we amended surface seawater from six stations across a nutrient gradient in the Mediterranean Sea (east to west transect) with dissolved inorganic phosphorus (DIP), and with seawater‐acclimated inocula of Crocosphaera watsonii, a unicellular cyanobacterial diazotroph, to examine if DIP can stimulate diazotrophy. Our results demonstrate that C. watsonii are poor competitors for DIP relative to native nondiazotrophic heterotrophic microbial populations, especially in the ultraoligotrophic eastern Mediterranean basin, resulting in low N2‐fixation rates. Moreover, the results indicate that when the ambient DIP concentrations are < 35 nmol L−1, unicellular photoautotrophic diazotrophs such as C. watsonii will likely be outcompeted for this macronutrient in the Mediterranean Sea, whereas above 35 nmol L−1 diazotrophy can be stimulated. Our findings support the “bypass theory” stating that photoautotrophs may be outcompeted by heterotrophic bacteria for DIP in nutrient‐poor regions such as the Mediterranean Sea.
Nitrogen is essential for life but is often a major limiting nutrient for growth in the ocean. Biological dinitrogen fixation is a major source of new nitrogen to surface waters and promotes marine ...productivity. Yet the fate of diazotroph-derived nitrogen (DDN) in marine ecosystems has been poorly studied, and its transfer to auto- and heterotrophic plankton has not been measured. Here, we use high-resolution nanometer scale secondary ion mass spectrometry (nanoSIMS) coupled with 15N₂ isotopic labelling and flow cytometry cell sorting to examine the DDN transfer to specific groups of natural phytoplankton and bacteria during three diazotroph blooms dominated by the cyanobacterium Trichodesmium spp. in the South West Pacific. During these experiments, 13%±2% to 48%±5% of the fixed 15N₂ was released into the dissolved pool and 6%±1% to 8%±2% of this DDN was transferred to non-diazotrophic plankton after 48 h. The primary beneficiaries of this DDN were diatoms (45%±4% to 61%±38%) and bacteria (22%±27% to 38%±12%), followed by pico-phytoplankton (3%±1% to 21%±14%). The DDN was quickly converted to non-diazotrophic plankton biomass, in particular that of diatoms, which increased in abundance by a factor of 1.4–15 over the course of the three experiments. The single-cell approach we used enabled quantification of the actual transfer of DDN to specific groups of autotrophic and heterotrophic plankton in the surface ocean, revealing a previously unseen level of complexity in the pathways that occur between N₂ fixation and the eventual export of DDN from the photic zone.
Transparent exopolymer particles (TEPs) are planktonic, organic microgels that are ubiquitous in aqueous environments. Increasing evidence indicates that TEPs play an active role in the process of ...aquatic biofilm formation. Frequently, TEPs are intensely colonized by bacteria and other microorganisms, thus serving as hot spots of intense microbial activity. We introduce the term "protobiofilm" to refer to TEPs with extensive microbial outgrowth and colonization. Such particles display most of the characteristics of developing biofilm, with the exception of being attached to a surface. In this study, coastal seawater was passed through custom-designed flow cells that enabled direct observation of TEPs and protobiofilm in the feedwater stream by bright-field and epifluorescence microscopy. Additionally, we could follow biofilm development on immersed surfaces inside the flow cells. Within minutes, we observed TEP and protobiofilm patches adhering to these surfaces. By 30 min, confocal laser-scanning microscopy (CLSM) revealed numerous patches of Con A and SYTO 9 staining structures covering the surfaces. Atomic force microscopy showed details of a thin, highly sticky, organic conditioning layer between these patches. Bright-field and epifluorescence microscopy and CLSM showed that biofilm development (observed until 24 h) was profoundly inhibited in flow cells with seawater prefiltered to remove most large TEPs and protobiofilm. We propose a revised paradigm for aquatic biofilm development that emphasizes the critical role of microgel particles such as TEPs and protobiofilm in facilitating this process. Recognition of the role of planktonic microgels in aquatic biofilm formation can have applied importance for the water industry.
Global warming may exacerbate inorganic nutrient limitation, including phosphorus (P), in the surface‐waters of tropical oceans that are home to extensive blooms of the marine diazotrophic ...cyanobacterium, Trichodesmium. We examined the combined effects of P limitation and pCO₂, forecast under ocean acidification scenarios, on Trichodesmium erythraeum IMS101 cultures. We measured nitrogen acquisition, glutamine synthetase activity, C uptake rates, intracellular Adenosine Triphosphate (ATP) concentration and the pool sizes of related key proteins. Here, we present data supporting the idea that cellular energy re‐allocation enables the higher growth and N₂ fixation rates detected in Trichodesmium cultured under high pCO₂. This is reflected in altered protein abundance and metabolic pools. Also modified are particulate organic carbon and nitrogen production rates, enzymatic activities, and cellular ATP concentrations. We suggest that adjusting these cellular pathways to changing environmental conditions enables Trichodesmium to compensate for low P availability and to thrive in acidified oceans. Moreover, elevated pCO₂ could provide Trichodesmium with a competitive dominance that would extend its niche, particularly in P‐limited regions of the tropical and subtropical oceans.
Summary
Seawater desalination plants increase local coastal salinities by discharging concentrated brine back to the sea with ∼50% higher than ambient salinities. The impacts of high salinities on ...microbial coastal populations of the eastern Mediterranean Sea (EMS) were examined in two mesocosm experiments; first, during the mixed‐spring and second, during the stratified‐summer periods with average salinity of ∼39. Ambient salinities were increased by 5% and 15%. Higher salinity (15%) mesocosms induced rapid (within 2 h) declines in both primary productivity (PP) and algal biomass parallel to an increase in bacterial productivity. Subsequently, for the duration of the experiments (11–12 days), both Chlorophyll a and PP rates increased (2 to 5 and 1.5 to 2.5–fold, respectively) relative to unamended controls. The initial assemblages of the ambient microbial populations and intensity of salinity enrichments influenced the community responses. During the mixed‐spring experiment, the composition of prokaryotic and eukaryotic populations shifted only slightly, suggesting high functional plasticity of the initial populations. While during the stratified‐summer experiment, high salinity changed the composition and reduced the biodiversity of the microbial communities. In an ultra‐oligotrophic environment such as the EMS, salinity induced declines in microbial diversity may provide a tipping point destabilizing the local aquatic food web.
Sediment trap data set and 234Th profiles (deep water excesses and deficits) reveal that particulate organic carbon (POC) export at the highly oligotrophic Levantine Sea is dominated by lateral ...transport from the nearby margin. These intermediate nepheloid layers (INL) operate at multi‐depth, with the silt‐to‐clay size particulate matter (PM) fraction transported at water depths of about 100–500 m, while finer fraction arrives also at deeper depths. The shallow NIL is triggered by winter storms, manipulated by coastal flash floods and shelf resuspension and assisted by cross‐shore currents, which allow the arrival of PM at a distance of 50 km within about 10 days. The deeper INL could be related to sediments initially driven to depth by density currents. Our data show that inter‐annual differences in sediment trap fluxes were related to changes in both the intensity of coastal floods and current velocity. The frequent observation of deep‐water 234Th excesses during a (relatively) low export winter (2018) is related to lessened cleansing of the water column, that is, reduced removal of fine‐grained PM by sinking coarser‐grained material. These observations highlight the importance of winter storm intensity in the POC budget of marginal seas like the Levantine Basin (LB) even in areas with limited river discharge. This further suggests that the anticipated increase in extreme weather events due to the on‐going climate change should have an impact on this coastal‐deep sea conveyor and on POC export in the LB.
Plain Language Summary
We present sediment traps and radioisotope results from the DeepLev marine station, the first deep water mooring to be deployed in the highly fragile ecosystem of the Levantine Basin (eastern Mediterranean Sea). Unlike the open ocean, particulate organic carbon export from surface to deep water is controlled by the transport of particulate matter from the nearby coast/continental shelf. We show that this land‐sea conveyor is nurtured by flash floods and shelf sediment resuspension, and is further manipulated by cross‐shore currents. The conveyor operates at multiple depths, with silt and clay transported sub‐horizontally from the shelf, arriving at DeepLev within ca. 10 days at 100–500 m depth (shallow intermediate nepheloid layer, INL), while colloids being carried through the whole water column (deep INL). The latter is probably related to particle‐laden density currents, which flow down submarine canyons, cutting into the nearby continental slope. Inter‐annual changes in winter events and cross‐shore current velocity result in significant changes of POC export intensity. This further implies that the predicted increase in the occurrence of extreme meteorological events could result in an enhanced transport of particular carbon, with important implications to the POC export in this and other marginal basins.
Key Points
POC export in the Levantine Basin is controlled by lateral transport of multi‐depths intermediate nepheloid layers (INL)
The shallow INL carries silt and clay from the coast and shelf, while the deep INL hauls colloids related to transport through submarine canyons
Inter‐annual variability in winter event intensity results in lateral transport and vertical export variability
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