Biogeochemical Gradients Research Articles

Sedimentary biogeochemical gradients across the Sunda Shelf in the South China Sea and correlations with satellite observations

Sedimentary biogeochemical gradients across the Sunda Shelf in the South China Sea and correlations with satellite observations

The Arctic summer microbiome across Fram Strait: Depth, longitude, and substrate concentrations structure microbial diversity in the euphotic zone.

The long-term dynamics of microbial communities across geographic, hydrographic, and biogeochemical gradients in the Arctic Ocean are largely unknown. To address this, we annually sampled polar, mixed, and Atlantic water masses of the Fram Strait (2015-2019; 5-100 m depth) to assess microbiome composition, substrate concentrations, and oceanographic parameters. Longitude and water depth were the major determinants (~30%) of microbial community variability. Bacterial alpha diversity was highest in lower-photic polar waters. Community composition shifted from west to east, with the prevalence of, for example, Dadabacteriales and Thiotrichales in Arctic- and Atlantic-influenced waters, respectively. Concentrations of dissolved organic carbon peaked in the western, compared to carbohydrates in the chlorophyll-maximum of eastern Fram Strait. Interannual differences due to the time of sampling, which varied between early (June 2016/2018) and late (September 2019) phytoplankton bloom stages, illustrated that phytoplankton composition and resulting availability of labile substrates influence bacterial dynamics. We identified 10 species clusters with stable environmental correlations, representing signature populations of distinct ecosystem states. In context with published metagenomic evidence, our microbial-biogeochemical inventory of a key Arctic region establishes a benchmark to assess ecosystem dynamics and the imprint of climate change.

Persistent hot spots of CO2 and CH4 in coastal nearshore environments

AbstractNearshore environments are typically supersaturated with the potent greenhouse gases methane and carbon dioxide, due to intense remineralization of the elevated supply of organic carbon in these systems. These environments are characterized by overlapping biogeochemical gradients and heterogeneous morphology, and the overall spatial variability in nearshore greenhouse gas concentrations remains unclear. We measured surface water partial pressures of carbon dioxide and methane synoptically with water quality parameters in the coastal Baltic Sea, covering two ice‐free seasons. The high‐frequency flow‐through data revealed sites with recurring very high partial pressures of carbon dioxide and methane (i.e., hot spots) scattered around the 50 km × 40 km study area, exceeding overall partial pressure averages by 455 μatm (CH4) and 2396 μatm (CO2). High partial pressures were linked with elevated inputs of allochthonous and autochthonous organic matter, underpinning the major role of organic enrichment of coastal environments in global carbon cycling.

Highly diverse diazotrophs drive high N2 fixation rates in a shallow submarine hydrothermal system

Dinitrogen (N2) fixation provides nitrogen sources supporting active chemoautotrophic processes in submarine hydrothermal systems. Here, the diazotrophic phylogenetic diversity and N2 fixation profile were investigated in depth along a sharp biogeochemical gradient from the hydrothermal vents to ambient seawater in a shallow submarine hydrothermal system (SHS) off Kueishantao Islet, Taiwan. Diazotrophic community compositions in the SHS covered all four known nifH clusters (I, II, III, and IV) and contained both characteristics of diazotrophic communities in deep-sea hydrothermal systems and terrestrial geothermal systems. The seawater around the vents was primarily dominated by nifH phylotypes affiliated with chemolithotrophic sulfur-oxidizing and iron-reducing bacteria. In contrast, in the seawater directly above the hydrothermal vents, the nifH community markedly contained diverse heterotrophic bacteria. The analysis, which focused on amino acid composition, protein spatial structure, and the possible co-occurrence of alternative nitrogenases in the presence of molybdenum-nitrogenase, further revealed that diazotrophic nitrogenase found around the hydrothermal vents exhibits potential molecular adaptations to high temperature, low pH, and sulfur-rich conditions. The relatively low N2 fixation rates (NFRs) were observed around the vents, where high concentrations of inorganic nitrogen compounds were present. However, notably, high NFRs were detected in seawater directly above the hydrothermal vents, which were 2–5 times higher than those at the reference site. The abundant iron, along with the depletion of nitrogen nutrients potentially resulting from nitrogen assimilation and loss processes, may have led to a high iron:nitrogen ratio, inducing the observed high NFRs. The co-occurrence of diverse nifH-containing heterotrophic bacteria and high NFRs suggests that these heterotrophs may play a major role in N2 fixation in this environment. This study suggests that the SHSs may be potential hotspots for marine N2 fixation, contributing significantly to the global ocean's N2 fixation budget and warranting further investigation.

Spatial patterns in host-associated and free-living bacterial communities across six temperate estuaries.

A major goal of microbial ecology is to establish the importance of spatial and environmental factors in driving community variation. Their relative importance likely varies across spatial scales, but focus has primarily been on free-living communities within well-connected aquatic environments rather than less connected island-like habitats such as estuaries, and key host-associated communities within these systems. Here we sampled both free-living (seawater and sediment) and host-associated (estuarine fish hindgut microbiome, Pelates sexlineatus) communities across six temperate Australian estuaries spanning ∼500km. We find that spatial and environmental factors have different influences on these communities, with seawater demonstrating strong distance-decay relationships (R = -0.69) and significant associations with a range of environmental variables. Distance-decay relationships were weak for sediment communities but became stronger over smaller spatial scales (within estuaries, R = -0.5), potentially reflecting environmental filtering across biogeochemical gradients or stochastic processes within estuary sediments. Finally, P. sexlineatus hindgut microbiome communities displayed weak distance-decay relationships (R = -0.36), and limited variation explained by environmental variables, indicating the significance of host-related factors in driving community variation. Our findings provide important ecological insights into the spatial distributions and driving forces of both free-living and host-associated bacterial patterns across temperate estuarine systems.

Pharmaceutical Attenuation Differs within Woodchip-Based Lignocellulose Bioreactors across Nitrate- and Sulfate-Reducing Conditions

Lignocellulosic bioreactors use solid-phase substrates such as woodchips to sustain microbial respiration and have been applied to treat agricultural runoff, stormwater, and other impaired waters. Here, we query how respiration of different soluble electron acceptors impacts the degradation of environmentally relevant pharmaceuticals associated with treated municipal wastewater discharge. Laboratory-scale columns containing a mixture of woodchips and alfalfa were manipulated across nitrate- and sulfate-reducing conditions using residence time and influent composition. Under steady-state conditions, bioreactors dominated by nitrate reduction harbored a distinct phylogenetic profile containing the genera Denitratisoma and increases in the denitrification gene nirS. In contrast, bioreactors where sulfate reduction dominated exhibited increased relative abundance of fermenters (e.g., Obscuribacteriales) and putative sulfate reducers (e.g., Desulfobulbus). Atenolol attenuation and biotransformation to carboxy-metoprolol accelerated under nitrate-reducing conditions; in contrast, trimethoprim attenuation and biotransformation to desmethyl trimethoprim was nearly an order of magnitude faster under sulfate-reducing conditions. Modest sulfamethoxazole attenuation occurred under all tested conditions. Denitrification-associated rate constants for atenolol were comparable to those reported in constructed wetlands and aquifer recharge, suggesting commonality in biotransformation mechanisms. Collectively, results suggest that manipulation of biogeochemical gradients during nature-based treatment can be applied to attenuate nitrate and trace quantities of pharmaceuticals.

Uniform selective pressures within redox zones drive gradual changes in microbial community composition in hadal sediments.

Microbial communities in marine sediments are highly diverse, yet the processes that give rise to this complexity are unclear. It has been proposed that benthic microbial communities must be continuously re-seeded from the water column because dispersal within the sediment is severely limited. Previous studies consistently report that the composition of the microbial community gradually changes with sediment depth. However, the relative contributions of the processes that underlie these compositional gradients have not been determined, and it is unknown whether microbial dispersal is indeed too slow to outpace burial. Here, we applied ecological statistical frameworks to 16S rRNA gene amplicon-based community composition data from Atacama Trench sediments to investigate the links between biogeochemistry, burial, and microbial community assembly processes. We confirm that dispersal limitation affects microbial communities and find that gradual changes in community composition are driven by selective pressures that change abruptly across the discrete boundaries between redox zones rather than along continuous biogeochemical gradients, while selective pressures are uniform within each zone. The gradual changes in community composition over centimetres of depth within a zone hence reflects a decades-long response to the abruptly changing selective pressures.

Terrigenous dissolved organic matter input and nutrient-light-limited conditions on the winter microbial food web of the Beagle Channel

The viral shunt and microbial loop are trophic pathways in the microbial food web, recognized as major drivers of carbon and nutrient cycles in the sea. The Beagle Channel, located at the southernmost extreme of South America, is a subantarctic marine environment influenced by terrestrial runoff and glacial meltwater with high temporal and spatial variability. There is a knowledge gap on the trophic pathways and the microbial food web in the eastern part of the Channel. We herein present for the first time a comprehensive and spatially wide study of the effects of physicochemical and bio-optical parameters on the microbial food web along the Eastern Beagle Channel. The study took place under winter hydrographic conditions, featuring weak vertical stratification of the water column and low chlorophyll a concentrations (0.13 ± 0.07 mg m−3). Biogeochemical gradients were notable between the inner and outer (easternmost) sectors of the Channel, separated by a shallow sill at Mackinlay Strait. As a chemical water mass tracer, the presence of the highest concentrations in bottom waters of NO3 + NO2 in the inner channel and the difference in the T-S diagram indicate the Subantarctic Pacific origin of this water mass. Structural equation models showed that the specific UV absorbance of water (SUVA254, a proxy for the aromatic carbon content) and NO3 + NO2 were the main parameters affecting the microbial food web in surface/subsurface waters suggesting that prokaryotes biomass was partially supported by allochthonous dissolved organic matter (DOM) derived from river discharges. Our results show a major role of the viral shunt in surface/subsurface waters retaining nutrients in the basal levels of the food web. We suggest that the viral shunt in surface/subsurface waters and the water residence time west of Mackinlay Strait allows infected cells and viral lysis products to reach bottom waters propelling the production of recalcitrant and labile DOM there.

Saturated, Suffocated, and Salty: Human Legacies Produce Hot Spots of Nitrogen in Riparian Zones

AbstractThe compounding effects of anthropogenic legacies for environmental pollution are significant, but not well understood. Here, we show that centennial‐scale legacies of milldams and decadal‐scale legacies of road salt salinization interact in unexpected ways to produce hot spots of nitrogen (N) in riparian zones. Riparian groundwater and stream water concentrations upstream of two mid‐Atlantic (Pennsylvania and Delaware) milldams, 2.4 and 4 m tall, were sampled over a 2 year period. Clay and silt‐rich legacy sediments with low hydraulic conductivity, stagnant and poorly mixed hydrologic conditions, and persistent hypoxia in riparian sediments upstream of milldams produced a unique biogeochemical gradient with nitrate removal via denitrification at the upland riparian edge and ammonium‐N accumulation in near‐stream sediments and groundwaters. Riparian groundwater ammonium‐N concentrations upstream of the milldams ranged from 0.006 to 30.6 mgN L−1 while soil‐bound values were 0.11–456 mg kg−1. We attribute the elevated ammonium concentrations to ammonification with suppression of nitrification and/or dissimilatory nitrate reduction to ammonium (DNRA). Sodium inputs to riparian groundwater (25–1,504 mg L−1) from road salts may further enhance DNRA and ammonium production and displace sorbed soil ammonium‐N into groundwaters. This study suggests that legacies of milldams and road salts may undercut the N buffering capacity of riparian zones and need to be considered in riparian buffer assessments, watershed management plans, and dam removal decisions. Given the widespread existence of dams and other barriers and the ubiquitous use of road salt, the potential for this synergistic N pollution is significant.

Shelled pteropod abundance and distribution across the Mediterranean Sea during spring

Thecosome pteropods are a dominant group of calcifying pelagic molluscs and an important component of the food web. In this study, we characterise spring pteropod distribution throughout the Mediterranean Sea, an understudied region for this common group of marine calcifying organisms. This semi-enclosed sea is rapidly changing under climatic and anthropogenic forcings. The presence of surface water biogeochemical gradients from the Atlantic Ocean/Gibraltar Strait to the Eastern Mediterranean Sea allowed us to investigate pteropod distribution and their ecological preferences. In the ultra-oligotrophic Eastern Mediterranean Sea, we found the mean upper 200 m pteropod standing stock of 2.13 ind. m−3 was approximately 5x greater than the Western basin (mean 0.42 ind. m−3). Where standing stocks were high, pteropods appeared largely in the same family grouping belonging to Limacinidae. Temperature, O2 concentration, salinity, and aragonite saturation (Ωar) explain 96 % of the observed variations in the community structure at the time of sampling, suggesting that pteropods might show a preference for environmental conditions with a lower energetic physiological demand. We also document that pteropods and planktonic foraminifera have an opposite geographical distribution in the Mediterranean Sea. Our findings indicate that in specific pelagic ultra-oligotrophic conditions, such as the Eastern Mediterranean Sea, different feeding strategies could play an important role in regulating calcifying zooplankton distribution.

Changes of organic matter composition in surface sediments from the Pearl River estuary to the coastal South China Sea revealed by rapid molecular screening using FTICR-MS

The Pearl River drains the second largest watershed in China, funnelling large amounts of freshwater and organic matter into the northern part of the South China Sea through an estuary characterized by pronounced biogeochemical gradients. In this study we used a newly developed single injection Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) method to analyze organic extracts of surface sediments transitioning from freshwater to marine settings in the Pearl River estuary and coastal South China Sea. Results show a complex mixture of molecular markers, with distinct fingerprints characterizing the freshwater and saline sites. Notable examples include steroidal and thiophenic species at the freshwater site, indicative of possible anthropogenic organic matter inputs from the continent; on the other hand, samples from the coastal site are uniquely characterized by putative tetrapyrrole species, indicative of phaeopigments sourced from marine phytoplankton. This study demonstrates the potential application of single injection FTICR-MS in rapidly probing the dynamic changes of organic matter sources and fates occurring in estuaries influenced by anthropogenic activities.

Effects of Microbial-Mineral Interactions on Organic Carbon Stabilization in a Ponderosa Pine Root Zone: A Micro-Scale Approach

Soil microbial communities affect the formation of micro-scale mineral-associated organic matter (MAOM) where complex processes, including adhesion, aggregate formation, microbial mineral weathering and soil organic matter stabilization occur in a narrow zone of large biogeochemical gradients. Here we designed a field study to examine carbon stabilization mechanisms by using in-growth mesh bags containing biotite that were placed in a ponderosa pine root zone for 6 months and compared to the surrounding bulk soil. We sought to determine the composition of the microbial community in the mesh bags compared to the surrounding soils, analyze the direct interactions between microbes and biotite, and finally identify the nature of the newly formed MAOM within the mesh-bags. Our results revealed that minerals in the mesh bags were colonized by a microbial community that produced organic matter in situ. The 16S rRNA gene sequencing and ITS2 region characterization showed phylogenetic similarity between the mesh bag and bulk soil archaea/bacteria and fungi microbiomes, with significant differences in alpha- and beta-diversity and species abundances. Organic matter pools in the mesh bags, analyzed by Fourier transform ion cyclotron resonance mass spectrometry, contained protein- (peptides) and lipid-like compounds while the bulk soil OM was comprised of lignin-like and carboxyl-rich alicyclic molecules. These results support that the newly formed biotite associated organic compounds have a microbial signature in the mesh bags. High-resolution electron microscopy documented strongly adhered organic compounds to biotite surfaces, formation of microaggregates, elemental uptake at the microbe (organic matter)-mineral interface, and distortion of biotite layers. Overall, this study shows the direct and indirect involvement of soil microbial communities from the root zone of ponderosa pine in the formation of MAOM, soil organic carbon stabilization, microaggregation, and mineral weathering at micro- and nano-scales.

Estuarine dissolved speciation and partitioning of trace metals: a novel approach to study biogeochemical processes

Estuaries are complex systems involving numerous biogeochemical gradients and processes that influence the behavior of trace metals. Lead (Pb), cadmium (Cd), and copper (Cu) speciation and partitioning were studied in the Gironde Estuary (SW France), using a multi-method approach in which data from innovative sensors and laboratory-based techniques were combined. For the first time in this system, the so-called dynamic fractions of the target metals (dissolved forms that are potentially bioavailable) were recorded on-board through voltammetry using unique antifouling gel-integrated microelectrode arrays (GIME) incorporated in a submersible sensing probe (TracMetal). Trace metals in the operationally defined dissolved <0.2 μm and <0.02 μm fractions, as well as complexed with suspended particles (collected after centrifugation) were quantified through sampling/laboratory-based techniques. High spatial resolution trace metal concentrations were monitored along the salinity gradient (S = 0.10 to S = 34.0) together with master bio-physicochemical parameters providing robust cruise-specific information on how well-known abiotic and biotic processes control the Gironde estuarine trace element partitioning, (i.e. conservative behavior, addition/removal). Combining conventional methods with GIME measurements showed: (i) the dominance of Cd dynamic species in the intra-estuarine total dissolved fraction (up to 90%), (ii) the importance of small colloids as trace metal carrier phases, desorbing and complexing dynamic fractions of Pb and Cu, and (iii) the potential influence of photo-redox processes remobilizing Pb under their dynamic forms (up to 80%). Data also suggest trace metal release/sorption by phytoplankton with an increase of dissolved Cu concentrations in the riverine branch, as well as Cu and Cd particulate concentrations showing higher levels towards productive coastal waters. This complete approach allowed to monitor key estuarine biogeochemical processes and highlighted the valuable use of the TracMetal to record subtle variations of potentially bioavailable dissolved metal fractions.

Archaeal tetraether lipids and their biphytane carbon isotope composition in sediments along an estuarine biogeochemical gradient

The stable carbon isotopic composition of Thaumarchaeota-produced crenarchaeol (δ13Ccren) is thought to reflect the δ13C of dissolved inorganic carbon (DIC) and is valuable in carbon cycle research. Recently, an isotope flux-balance model was proposed that δ13Ccren is dependent on passive CO2 uptake and a slow rate of intracellular conversion to HCO3−, which has been validated in open ocean environments. However, further studies are required in environments with larger ranges of aqueous CO2 and diverse archaeal communities to examine whether the δ13Ccren value could still be predicted by the model. Here, we report δ13C values of biphytanes chemically released from the archaeal tetraether lipids (i.e., GDGTs) in sediments along a transect exhibiting shifts in the archaeal community and showing gradients of salinity, pH, and CO2 in the Pearl River Estuary, South China. Methanogenic and methanotrophic Euryarchaeota are present in the upper estuary, although not significant, as revealed by the elevated relative abundances of GDGT-0 and higher values of methane index in suspended particulate matter (SPM) and surface sediments, as well as the more 13C-depleted C40:0, C40:1, and C40:2 biphytanes than the C40:3 biphytane released from sedimentary GDGTs. Toward the lower estuary and coastal sea, GDGT distributions in SPM and sediments tend to indicate archaeal communities under normal marine conditions. However, there are still differences in δ13C values between C40:0 and C40:3 biphytanes by up to 2.4‰ in sediments, although C40:1, C40:2, and C40:3 were similar in δ13C, likely due to the existence of marine group II Euryarchaeota. Along such a physicochemical and biogeochemical gradient, the values of sedimentary δ13C40:3 increase monotonically with the increase of mean annual pH and the decrease of mean annual CO2. This phenomenon resembles the reported δ13CGDGT profiles, along with the variations in pH and CO2, in water column in the open ocean, and hence consistent in direction with the isotope flux-balance model for Thaumarchaeota lipids. However, our estimated apparent biosynthetic isotope effect (εAr) of crenarchaeol, ranging from 15.2‰ to 19.7‰, is mostly lower than those estimated in the open ocean. With the constraints of thaumarchaeal growth rates in such a dynamic estuarine environment as well as those in the open ocean, we suggest slight reductions in catalytic efficiency and intracellular kinetic isotope effect of intracellular CO2 hydration relative to previously employed values. Such modifications would slightly change the sensitivities of εAr to archaeal growth rate and ambient CO2 concentration but might be more suitable for variable hydrology and community environments.

Neutron computed laminography yields 3D root system architecture and complements investigations of spatiotemporal rhizosphere patterns

PurposeRoot growth, respiration, water uptake as well as root exudation induce biogeochemical patterns in the rhizosphere that can change dynamically over time. Our aim is to develop a method that provides complementary information on 3D root system architecture and biogeochemical gradients around the roots needed for the quantitative description of rhizosphere processes.MethodsWe captured for the first time the root system architecture of maize plants grown in rectangular rhizotrons in 3D using neutron computed laminography (NCL). Simultaneously, we measured pH and oxygen concentration using fluorescent optodes and the 2D soil water distribution by means of neutron radiography. We co-registered the 3D laminography data with the 2D oxygen and pH maps to analyze the sensor signal as a function of the distance between the roots and the optode.ResultsThe 3D root system architecture was successfully segmented from the laminographic data. We found that exudation of roots in up to 2 mm distance to the pH optode induced patterns of local acidification or alkalization. Over time, oxygen gradients in the rhizosphere emerged for roots up to a distance of 7.5 mm.ConclusionNeutron computed laminography allows for a three-dimensional investigation of root systems grown in laterally extended rhizotrons as the ones designed for 2D optode imaging studies. The 3D information on root position within the rhizotrons derived by NCL explained measured 2D oxygen and pH distribution. The presented new combination of 3D and 2D imaging methods facilitates systematical investigations of a wide range of dynamic processes in the rhizosphere.

Modeling mixoplankton along the biogeochemical gradient of the Southern North Sea

The ecological importance of mixoplankton within marine protist communities is slowly being recognized. However, most aquatic ecosystem models do not include formulations to model a complete protist community consisting of phytoplankton, protozooplankton and mixoplankton. We introduce PROTIST, a new module for the aquatic ecosystem modelling software Delft3D-WAQ that can model a protist community consisting of two types of phytoplankton (diatoms and green algae), two types of mixoplankton (constitutive mixoplankton and non-constitutive mixoplankton) and protozooplankton. We employed PROTIST to further explore the hypothesis that the biogeochemical gradient of inorganic nutrient and suspended sediment concentrations drives the observed occurrence of constitutive mixoplankton in the Dutch Southern North Sea. To explore this hypothesis, we used 11 1D-vertical aquatic ecosystem models that mimic the abiotic conditions of 11 routine monitoring locations in the Dutch Southern North Sea. Our models result in plausible trophic compositions across the biogeochemical gradient as compared to in-situ data. A sensitivity analysis showed that the dissolved inorganic phosphate and silica concentrations drive the occurrence of constitutive mixoplankton in the Dutch Southern North Sea.

Underway Hyperspectral Bio-Optical Assessments of Phytoplankton Size Classes in the River-Influenced Northern Gulf of Mexico

High inflows of freshwater from the Mississippi and Atchafalaya rivers into the northern Gulf of Mexico during spring contribute to strong physical and biogeochemical gradients which, in turn, influence phytoplankton community composition across the river plume–ocean mixing zone. Spectral features representative of bio-optical signatures of phytoplankton size classes (PSCs) were retrieved from underway, shipboard hyperspectral measurements of above-water remote sensing reflectance using the quasi-analytical algorithm (QAA_v6) and validated against in situ pigment data and spectrophotometric analyses of phytoplankton absorption. The results shed new light on sub-km scale variability in PSCs associated with dynamic and spatially heterogeneous environmental processes in river-influenced oceanic waters. Our findings highlight the existence of localized regions of dominant picophytoplankton communities associated with river plume fronts in both the Mississippi and Atchafalaya rivers in an area of the coastal margin that is otherwise characteristically dominated by larger microphytoplankton. This study demonstrates the applicability of underway hyperspectral observations for providing insights about small-scale physical-biological dynamics in optically complex coastal waters. Fine-scale observations of phytoplankton communities in surface waters as shown here and future satellite retrievals of hyperspectral data will provide a novel means of exploring relationships between physical processes of river plume–ocean mixing and frontal dynamics on phytoplankton community composition.

Biogeochemical Gradients in a Serpentinization‐Influenced Aquifer: Implications for Gas Exchange Between the Subsurface and Atmosphere

AbstractSerpentinization involves the hydration and alteration of ultramafic rocks, which produces hydrogen (H2) and methane (CH4) and results in distinctive groundwater chemistries. As reacted fluids mix with recharging surface water, gradients in chemistry and microbiology develop in the subsurface. We present a comprehensive analysis of biogeochemical gradients in the water column of a serpentinite‐hosted well, CSW1.1, at the Coast Range Ophiolite Microbial Observatory (CROMO) in California, USA. Samples for geochemistry, 16S rRNA gene sequencing, and metagenomics were collected at four discrete depths from the top of the well corresponding to 100%, 50%, 15%, and 0% of atmospheric oxygen (O2) levels, and from the well base at 19.5 m depth. Gibbs energy calculations assessed the energy available for a suite of reactions coupled to O2, sulfate (SO42−), and nitrate (NO3−). Metagenomic data from the profile was used to construct metagenome assembled genomes (MAGs) to evaluate the completeness of biochemical pathways and compare the relative abundance of key diagnostic genes. Bioenergetic data point to the favorability of CH4 oxidation reactions despite little genetic evidence for this. Amplicon sequencing results highlight the abundance of key taxa affiliated with the genera Truepera, Serpentinomonas, and Dethiobacter. Although concentrations of NO3− and H2 are low, genes for NO3− reduction and oxidation of H2 and carbon monoxide (CO) were found in high abundance. Conceptual modeling results demonstrate the net depletion of H2 and CO in the groundwater, the consumption of CO2 and O2, and the potential for CH4 emission into the atmosphere at this terrestrial site of serpentinization.

Distribution of Sprattus sprattus phalericus (Risso, 1827) and zooplankton near the Black Sea redoxcline.

Understanding what environmental drivers influence marine predator-prey relationships can be key to managing and protecting ecosystems, especially in the face of future climate change risks. This is especially important in environments such as the Black Sea, where strong biogeochemical gradients can drive marine habitat partitioning and ecological interactions. We used underwater video recordings in the north-eastern Black Sea in November 2013 to observe the distribution and behaviour of the Black Sea sprat (Sprattus sprattus phalericus, Risso 1827) and its zooplankton prey. Video recordings have shown that the Black Sea sprat S. sprattus phalericus tolerates severely hypoxic waters near the redoxcline. The school was distributed in the 33-96 m layer [oxygen concentration (O2 ) 277-84 μmol L-1 ]. Some individuals were observed to leave the school and descended 20 m deeper for foraging on copepods in the 119-123 m layer (O2 12-10μmol L-1 ). Zooplankton appeared concentrated on the upper boundary of the suboxic zone (O2 < 10μmol L-1 ). No zooplankton were observed below O2 6-7μmol L-1 (128 m). Understanding the ability of this species to tolerate low oxygen waters is crucial to predicting future responses to natural and anthropogenic changes in hypoxia.

Coastal riverine wetland biogeochemistry follows soil organic matter distribution along a marsh-to-mangrove gradient (Florida, USA)

Many subtropical coastal wetland vegetation communities are transitioning from herbaceous marsh to woody mangrove, often facilitated by sea-level rise. This study investigated the relationships between vegetation community (upstream marsh, ecotone/transition, and downstream mangrove), salinity (S), and soil biogeochemistry in wetlands along three rivers on the Florida Gulf coast (the Little Manatee, Peace, and Fakahatchee Rivers). Vegetation was surveyed, and soil and water samples were collected during both the dry and the wet season and analyzed for biogeochemical properties (soil: bulk density, pH, organic matter, extractable inorganic and total nutrients, dissolved organic carbon (DOC), and microbial biomass carbon; water: inorganic nutrients and DOC) and processes (greenhouse gas production) while salinity and water level were continuously monitored in the field. Results indicated landscape-scale patterns in soil biogeochemistry differed significantly by river and were most strongly correlated with soil organic matter content, regardless of vegetation community or salinity regime. Contrary to expectations, soil organic matter content gradients were not always inversely related to salinity gradients, and methane production was observed in moderate- (S = 12) and high- (S = 34) salinity mangrove communities. The vegetation ecotone experienced seasonally variable salinity and did not serve as a true biogeochemical intermediate between the marsh and mangrove communities. This study demonstrates the need for site-specific studies of biogeochemical gradients in coastal wetlands and indicates the marsh-to-mangrove ecotone is not a proxy for salinity or biogeochemical tipping points. Instead, soil organic matter content is suggested as the most relevant indicator of biogeochemical properties and processes in wetlands along coastal rivers, superseding vegetation community or salinity.