Porewater Sulfide Concentrations Research Articles

Lack of detectable chemosynthesis at a sponge dominated subarctic methane seep

We used high-resolution imagery within a Geographic Information System (GIS), free gas and porewater analyses and animal bulk stable isotope measurements to characterize the biotic and abiotic aspects of the newly discovered Vestbrona Carbonate Field (VCF) seep site on the Norwegian shelf (63°28′N, 6° 31′E, ∿270 m water depth). Free gas was mainly composed of microbial methane. Sediment porewater sulfide concentrations were in the millimolar range and thus high enough to sustain seep chemosymbiotrophic animals. Nonetheless, the VCF lacked chemosymbiotrophic animals despite an abundance of methane-derived carbonate crusts which are formed by the same anaerobic processes that sustain chemosymbiotrophic animals at seeps. Furthermore, none of the sampled taxa, across various trophic guilds exhibited a detectable contribution of chemosynthetically fixed carbon to their diets based on bulk stable isotope values, suggesting a predominantly photosynthetic source of carbon to the VCF seep food web. We link the absence of chemosymbiotrophic animals to highly localized methane flow pathways, which may act as a “shunt-bypass” of the anaerobic oxidation of methane (AOM) and by extension sulfide generation, thus leading to sediment sulfide concentrations that are highly heterogeneous over very short lateral distances, inhibiting the successful colonization of chemosymbiotrophic animals at the VCF seep. Instead, the seep hosted diverse biological communities, consisting of heterotrophic benthic fauna, including long lived taxa, such as soft corals (e.g., Paragorgia arborea) and stony corals (i.e., Desmophyllum pertusum, formerly known as Lophelia pertusa). Compared to the surrounding non-seep seafloor, we measured heightened megafaunal density at the seep, which we attribute to increased habitat heterogeneity and the presence of a variety of hard substrates (i.e., methane-derived authigenic carbonates, dropstones and coral rubble), particularly since the most abundant taxa all belonged to the phylum Porifera. Compared to the surrounding non-seep seafloor, marine litter was denser within the VCF seep, which we link to the more variable local topography due to authigenic carbonates, which can rip off parts of bottom trawling nets thereby making the seep act as catchment area for marine litter.

Controls on sulfide accumulation in coastal soils during simulated sea level rise

A likely consequence of sea level rise during the next century will be progressive inundation of seawater through the current unsaturated zone in coastal-margin soils. It is unclear how this will change the biogeochemical cycling of iron, sulfur and carbon over different time scales and soil types. A long-term (540 day) laboratory experiment, slowly inundating intact coastal soil cores with seawater from the ‘bottom up’, was conducted to observe how coupled iron and sulfur dynamics change in different soil types. Pore water was extracted from the 60 cm cores at 10 cm depth intervals every 90 days and analysed for pH, electrical conductivity (EC), Fe2+, HS− and dissolved metals (Fe, Mn, As, Cd, Cu, Ni, Pb, Co, Zn) as well as stable sulfur (34S/32S) and radiogenic strontium (87Sr/86Sr) isotopes. Destructive solid phase analyses (for reactive Fe, acid volatile and chromium reducible sulfur (AVS and CRS) and total organic carbon (TOC)) were made at the start and end of the experiment. Iron and sulfate reduction was induced in soils with readily available TOC as anoxic conditions established, evidenced by statistically significant increases (P < 0.001) in dissolved Fe2+ and sulfide concentrations in porewaters, as well as simultaneous and progressive increase in the sulfur isotope (34S/32S) ratios. An increase in AVS indicated formation of metastable iron sulfide (FeS) minerals resulting from Fe2+ and sulfide in porewaters post inundation. This was supported by PHREEQC modelling of local mineral saturation states/indices in the system. Overall, the Fe2+ concentration increased to a peak at between 270 and 360 days, and then began decreasing at some depths, indicating slowing iron reduction, presumably because reactive iron (oxyhydro)oxides became progressively consumed by microbially driven reductive processes. However, microbially mediated sulfate reduction continued as abundant sulfate was still available from seawater. In the absence of free Fe2+, sulfide accumulated in the porewater, but only in those soils inundated the longest (>360 days), and with greater than 5% TOC. The combined use of sulfur (34S/32S) and strontium (87Sr/86Sr) isotopes supported the observed results, and represents a new and robust technique to quantify progressive sulfate reduction and seawater mixing phenomena in coastal soils. This study gives new insights into the biogeochemical cycling of sulfur and iron in soils experiencing seawater inundation from sea level rise over longer timescales. It is likely that sulfidisation (due to in-situ sulfate reduction) will begin to affect coastal wetland soils, especially in areas where TOC is high and where reactive iron depletes over time. This has potential consequences for sulfide toxicity in coastal soils and environments globally.

The Mo- and U-isotope signatures in alternating shales and carbonate beds of rhythmites: A comparison and implications for redox conditions across the Cambrian-Ordovician boundary

The Green Point Formation (GPF) of the Cow Head Group in western Newfoundland (Canada) includes the Global Boundary Stratotype Section and Point (GSSP) for the Cambrian-Ordovician (Є-O) boundary on Laurentia. This formation is composed primarily of thick, deep subtidal to slope deposits (rhythmites) of alternating lime mudstones and shale interbeds. Here, we employ isotopic and elemental data of Mo and U of these shales and carbonates to better understand their depositional environments and the open ocean redox conditions at the Є-O transition. The variability of δ98Moauth (−0.40 to +3.16‰) indicates variable aqueous sulfide concentrations ([H2S]aq) in bottom waters or porewaters during deposition of the shales. A significant positive correlation of shale [Mo]auth with [U]auth coupled with low Mo concentrations compared to Phanerozoic euxinic shales further points to weakly reducing (~dysoxic/suboxic to anoxic) benthic conditions with H2Saq primarily confined to porewaters. For the lime mudstones, the δ98Mo variability (−0.30 to +2.34‰) suggests variable [H2S]aq in porewaters during early diagenesis. Low Mo concentrations ([Mo]carb = 0.25 ± 0.80 μg/g, 2σ) and a poor correlation of [Mo]carb with δ98Mocarb indicate weak sulfidic ([H2S]aq < 11 μM) porewater conditions and possibly dysoxic to suboxic bottom environments. Thus, the δ98Mocarb may not track the δ98Mo of basin waters because of low [H2S]aq in porewaters and insignificant accumulation of authigenic Mo during early diagenesis. This explanation is consistent with a poor correlation between δ98Mocarb and the δ238U of basin waters (estimated from δ238Ucarb). A cautious examination of MoEF/UEF, δ98Moauth, and δ238Uauth of the shales suggests that the GPF was likely deposited in a basin with topographic barriers. During deposition of the lower and middle part of the studied interval, the barriers in combination with sea-level fluctuations may have caused partial isolation of the local basin from the open ocean. Rapid sea-level rises in the earliest Ordovician greatly increased the connectivity between the local basin and the open ocean, resulting in remarkable increases in MoEF/UEF, [Mo]auth, and [U]auth of the shales. The δ238U of the earliest Ordovician open oceans, inferred from δ238Ucarb, exhibited perturbations and an average value of −0.78 ± 0.33‰ (2σ), reflecting oceanic redox oscillations and expanded ocean euxinia compared to the modern oceans.

Nitrogen Fixation in Subtropical Seagrass Sediments: Seasonal Patterns in Activity in Santa Rosa Sound, Florida, USA

Seagrass beds are important coastal habitats that are diminishing globally. Nitrogen, a key nutrient, often limits seagrass growth. Nitrogen fixation provides new, bioavailable nitrogen to the plants. This study explores its importance and factors controlling rates in sediments colonized by two dominant taxa in Northwest Florida, Thalassia testudinum and Halodule wrightii, compared to unvegetated sediments. We hypothesized that nitrogen fixation rates would be greater in seagrass colonized sediments, particularly during high growth periods. We expected to observe a positive relationship between rates and porewater sulfide concentrations because sulfate reducers were the dominant diazotrophs in similar studies. Rates were higher in vegetated areas. In H. wrightii beds, nitrogen fixation was driven by the decreased availability of porewater ammonium relative to phosphorus. In T. testudinum beds, rates were highest during winter. Organic matter may be a controlling factor in all substrate types albeit the exact mechanism driving nitrogen fixation differs slightly. During the summer and fall, nitrogen fixation provided between 1–15% of T. testudinum nitrogen demand. Annually, nitrogen fixation provided 4% and 1% of T. testudinum and H. wrightii nitrogen demand, respectively. Nitrogen fixation was an important source of nitrogen during periods of senescence and dormancy when organic matter content was high.

Diverse Communities of hgcAB+ Microorganisms Methylate Mercury in Freshwater Sediments Subjected to Experimental Sulfate Loading.

Methylmercury (MeHg) is a bioaccumulative neurotoxin produced by certain sulfate-reducing bacteria and other anaerobic microorganisms. Because microorganisms differ in their capacity to methylate mercury, the abundance and distribution of methylating populations may determine MeHg production in the environment. We compared rates of MeHg production and the distribution of hgcAB genes in epilimnetic sediments from a freshwater lake that were experimentally amended with sulfate levels from 7 to 300 mg L-1. The most abundant hgcAB sequences were associated with clades of Methanomicrobia, sulfate-reducing Deltaproteobacteria, Spirochaetes, and unknown environmental sequences. The hgcAB+ communities from higher sulfate amendments were less diverse and had relatively more Deltaproteobacteria, whereas the communities from lower amendments were more diverse with a larger proportion of hgcAB sequences affiliated with other clades. Potential methylation rate constants varied 52-fold across the experiment. Both potential methylation rate constants and % MeHg were the highest in sediments from the lowest sulfate amendments, which had the most diverse hgcAB+ communities and relatively fewer hgcAB genes from clades associated with sulfate reduction. Although pore water sulfide concentration covaried with hgcAB diversity across our experimental sulfate gradient, major changes in the community of hgcAB+ organisms occurred prior to a significant buildup of sulfide in pore waters. Our results indicate that methylating communities dominated by diverse anaerobic microorganisms that do not reduce sulfate can produce MeHg as effectively as communities dominated by sulfate-reducing populations.

The Effect of Fertilization on Biomass and Metabolism in North Carolina Salt Marshes: Modulated by Location‐Specific Factors

AbstractThe resilience of salt marshes to sea level rise depends on vertical accretion through belowground biomass production and sediment deposition to maintain elevation above sea level. Increased nitrogen (N) availability from anthropogenic sources may stimulate aboveground biomass production and sediment deposition and, thus, accretion; however, increased N may also negatively impact marsh accretion by decreasing belowground biomass and increasing net CO2 emissions. A study was conducted in Spartina alterniflora‐dominated salt marshes in North Carolina, USA, to determine how responses to fertilization vary across locations with different physical and chemical characteristics. Pore water residence time, inundation time, and proximity to tidal creeks drove spatial differences in pore water sulfide, ammonium, and dissolved carbon concentrations. Although annual respiration and gross primary production were greater at the creek edge than interior marsh sites, net ecosystem CO2 exchange (NEE) was nearly balanced at all the sites. Fertilization decreased belowground biomass in the interior sites but not on the creek edge. Aboveground biomass, respiration, gross primary production, and net CO2 emissions increased in response to fertilization, but responses were diminished in interior marsh locations with high pore water sulfide. Hourly NEE measured by chambers were similar to hourly NEE observed by a nearby eddy covariance tower, but correcting for inundation depth relative to plant height was critical for accurate extrapolation to annual fluxes. The impact of fertilization on biomass and NEE, and thus marsh resilience, varied across marsh locations depending upon location‐specific pore water sulfide concentrations.

First Field-Based Evidence That the Seagrass-Lucinid Mutualism Can Mitigate Sulfide Stress in Seagrasses

Seagrass meadows form vital ecological components of coastal zones worldwide, but are rapidly declining. Large-scale seagrass diebacks have been related to accumulation of toxic sulfide in the sediment, a phenomenon predicted to occur more frequently in the near future due to ongoing global warming and increasing organic loading of coastal systems worldwide. Recently, a facultative mutualism between seagrasses and lucinid bivalves with endosymbiotic sulfide-consuming gill bacteria was discovered that may prevent toxic sulfide accumulation in seagrass sediments. Yet, direct field-based evidence for the importance of this mutualism in alleviating sulfide stress in seagrasses is currently lacking, as well as how its role may change when sediment sulfide levels increase due to environmental change. Here, we investigated the sulfide detoxification function of this seagrass-lucinid mutualism and its resilience to organic-loading induced sulfide stress in a temperate lagoon system (Thau lagoon, France), using a correlative field survey and a full factorial field experiment. The field survey revealed a strong positive correlation between seagrass above-ground biomass and lucinid densities, and pore water sulfide concentrations close to zero at all sites. Furthermore, the field experiment revealed that addition of organic matter (starch mixed with sucrose) increased sedimentary sulfide intrusion into seagrass (Zostera noltei) leaves (a proxy for sulfide stress), while experimentally enhanced lucinid densities reduced sulfide intrusion, regardless of addition of organic matter. Moreover, addition of organic matter reduced seagrass rhizome biomass and increased pore water sulfide levels, lucinid tissue sulfur content, lucinid condition (expressed as flesh/shell dry weight ratio), and total lucinid biomass, while enhancement of lucinid densities reduced lucinid condition. These results provide the first field-based evidence that lucinid bivalves and their sulfide-oxidizing gill symbionts mitigate sulfide stress in seagrasses, and suggests that the dependence of seagrass on this seagrass-lucinid mutualism will increase under conditions of enhanced sediment sulfide production, as predicted for the near future. Therefore, we suggest that awareness of the ecological importance of the seagrass-lucinid mutualism may be instrumental for designing new measures for improving long-term restoration success and seagrass resilience to global change.

Assessment of the ameliorating effect of sedimentary iron inputs on sulfide stress in eelgrass beds

Iron is recognized as an efficient method to alleviate sulfide stress. This study tested the response of Zostera marina plants to different levels of sedimentary sulfides (100.0–818.7 μmol L−1) and iron inputs (590.0–825.3 μg L−1) in a field experiment performed over an eighty-day period. We measured plant responses in terms of shoot density and plant morphology and productivity. The relationship between the propagation effort (PE, in %) and sulfide content (S, in μmol L−1) was expressed as: PE = −14.01 × ln (S) + 86.86 (R2 = 0.99, p < .01), which indicates that the toxic limit of the pore-water sulfide concentration for the survival of eelgrass is 493 μmol L−1. The addition of iron can reduce the toxicity of sulfides to eelgrass beds, resulting in an increase in plant density and productivity, and can even reverse the decline of eelgrass beds exposed to high sulfide concentrations.

A new colorimetric DET technique for determining mm-resolution sulfide porewater distributions and allowing improved interpretation of iron(II) co-distributions.

Measurement of sulfide in pore waters is critical for understanding biogeochemical processes, especially within coastal sediments. Here we report the development of a new colorimetric DET (diffusive equilibration in thin films) technique for determining mm-resolution, two-dimensional sulfide distributions in sediment pore waters. This colorimetric sulfide DET method was based on the standard spectrophotometric methylene blue assay, but modified to allow quantitation of sulfide by computer imaging densitometry. The method detection and effective upper measurement limits of the optimised technique were 3.7 and 1000μmolL-1, respectively. The optimised sulfide DET method was combined with the colorimetric iron(II) DET method to obtain co-distributions in coastal seagrass (Zostera muelleri) colonised sediment under light and dark conditions. In the dark, seagrass sediments were more reduced than in the light, with large areas being dominated by high porewater sulfide concentrations. These co-distributions were compared with those obtained using the previously described DET-DGT (diffusive gradients in thin films) method for measuring iron(II) and sulfide co-distributions. There was less overlap of iron(II) and sulfide distributions using the sulfide DET as the two DET methods are influenced most by the later hours of deployment, whereas the sulfide-DGT measurement integrates concentrations over the whole deployment period. Overlap was most apparent in very dynamic sediment zones, such as burrow wall sediments.

Cumulative Sulfate Loads Shift Porewater to Sulfidic Conditions in Freshwater Wetland Sediment.

It is well established that sulfide can be toxic to rooted aquatic plants. However, a detailed description of the effects of cumulative sulfate loads on sulfide and iron (Fe) porewater geochemistry, plant exposure, and ecological response is lacking. Over 4 yr, we experimentally manipulated sulfate loads to self‐perpetuating wild rice (Zizania palustris) populations and monitored increases in the ratio of sulfur (S) to Fe in sediment across a range of sulfide loading rates driven by overlying water sulfate. Because natural settings are complicated by ongoing Fe and S loads from surface and groundwater, this experimental setting provides a tractable system to describe the impacts of increased S loading on Fe–S porewater geochemistry. In the experimental mesocosms, the rate of sulfide accumulation in bulk sediment increased linearly with overlying water sulfate concentration up to 300 µg‐SO4 cm–3. Seedling survival at the beginning of the annual life cycle and seed mass and maturation at the end of the annual life cycle all decreased at porewater sulfide concentrations between 0.4 and 0.7 µg cm–3. Changes to porewater sulfide, plant emergence, and plant nutrient uptake during seed production were closely related to the ratio of S to Fe in sediment. A mass balance analysis showed that porewater sulfide remained a small and relatively transient phase compared to sulfate in the overlying water and Fe in the sediment solid phase. The results illuminate the evolution of the geochemical setting and timescales over which 4 yr of cumulative sulfate loading resulted in a wholesale shift from Fe‐dominated to sulfide‐dominated porewater chemistry. This shift was accompanied by detrimental effects to, and eventual extirpation of, self‐perpetuating wild rice populations. Environ Toxicol Chem 2019;38:1231–1244. © 2019 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Creek Dynamics Determine Pond Subsurface Geochemical Heterogeneity in East Anglian (UK) Salt Marshes

Salt marshes are complex systems comprising of ephemerally flooded, vegetated platforms hydraulically fed by tidal creeks. Where drainage is poor, formation of saline-water ponds can occur. Within East Anglian (UK) salt marshes, two types of sediment chemistries can be found beneath these ponds; iron-rich sediment, which is characterized by high ferrous iron concentration in subsurface porewaters (up to 2 mM in the upper 30 cm); and sulfide-rich sediment, which is characterized by high porewater sulfide concentrations (up to 8 mM). We present five years of push-core sampling to explore the geochemistry of the porewater in these two types of sediment. We suggest that when carbon is present in quantities sufficient to exhaust the oxygen and iron content within pond sediments, conditions favour the presence of sulfide-rich sediments. In contrast, in pond sediments where oxygen is present—primarily through bioirrigation—reduced iron can be reoxidised and thus recycled for further reduction, favouring the perpetuation of iron-rich sedimentary conditions. We find these pond sediments can alter significantly over an annual timescale. We carried out a drone survey, with ground-truthed measurements, to explore the spatial distribution of geochemistry in these ponds. Our results suggest a pond’s proximity to the creek partially determines the pond subsurface geochemistry with iron-rich ponds tending to be closer to large creeks relative to sulfide-rich ponds. We suggest differences in surface delivery of organic carbon, due to differences in the energy of the ebb flow, or the surface/ subsurface delivery of iron may control the distribution. This could be amplified if tidal inundations flush ponds closer to creeks more frequently, removing carbon and flushing with oxygen. These results suggest that anthropogenic creation of drainage ditches could shift the distribution of iron- and sulfide-rich ponds and thus have an impact on nutrient, trace metal and carbon cycling in salt marsh ecosystems.

The Microbial Communities of Leaves and Roots Associated with Turtle Grass (Thalassia testudinum) and Manatee Grass (Syringodium filliforme) are Distinct from Seawater and Sediment Communities, but Are Similar between Species and Sampling Sites.

Seagrasses are vital members of coastal systems, which provide several important ecosystem services such as improvement of water quality, shoreline protection, and serving as shelter, food, and nursery to many species, including economically important fish. They also act as a major carbon sink and supply copious amounts of oxygen to the ocean. A decline in seagrasses has been observed worldwide, partly due to climate change, direct and indirect human activities, diseases, and increased sulfide concentrations in the coastal porewaters. Several studies have shown a symbiotic relationship between seagrasses and their microbiome. For instance, the sulfur, nitrogen, and carbon cycles are important biochemical pathways that seem to be linked between the plant and its microbiome. The microbiome presumably also plays a key role in the health of the plant, for example in oxidizing phyto-toxic sulfide into non-toxic sulfate, or by providing protection for seagrasses from pathogens. Two of the most abundant seagrasses in Florida include Thalassia testudinum (turtle grass) and Syringodium filliforme (manatee grass), yet there is little data on the composition of the microbiome of these two genera. In this study, the microbial composition of the phyllosphere and rhizosphere of Thalassia testudinum and Syringodium filiforme were compared to water and sediment controls using amplicon sequencing of the V4 region of the 16S rRNA gene. The microbial composition of the leaves, roots, seawater, and sediment differ from one another, but are similar between the two species of seagrasses.

Effects of Zostera marina rhizosphere and leaf detritus on the concentration and distribution of pore-water sulfide in marine sediments

The relationship between seagrasses and pore-water hydrogen sulfide in the sediment is complex due to the opposing effects of the seagrass root system, which can reduce sulfide concentration by promoting its oxidation to sulfate, and the burial of organic matter from the plant itself, which can increase sulfide concentration by stimulating anaerobic sulfate reduction. Here, field samples of pore-water sulfide were collected in areas with and without Zostera marina shoots using Diffusive Gradients in Thin-Films (DGTs) to obtain two-dimensional visualizations of sulfide concentrations within the sediment in relation to location of eelgrass detritus and the rhizosphere. To decouple the effects of leaf detritus and the eelgrass rhizosphere on sulfide, manipulative laboratory studies were conducted over 4 weeks using 10 aquaria with or without eelgrass and different quantities of eelgrass detritus planted at 4 cm and 11 cm depths. Spatial distribution of pore-water sulfide was again measured via DGT. We found high spatial variation in pore-water sulfide concentration depending on the location analyzed relative to the rhizosphere or detritus. In the field study, pore-water sulfide concentration was greater in samples with eelgrass present, and the presence of leaf detritus accounted for higher than average sulfide concentrations in the sediment. In the laboratory experiment, sulfide concentrations localized around the leaf detritus increased with higher mass of added detritus, and sediment within the rhizosphere exhibited reduced sulfide concentrations. These results suggest that on the large scale the presence of eelgrass increases sulfide concentration, but on the smaller scale the pattern varies, with higher concentrations near buried detritus and lower concentrations near root tips.

An evaluation of sedimentary molybdenum and iron as proxies for pore fluid paleoredox conditions

Iron speciation and trace metal proxies are commonly applied together in efforts to identify anoxic settings marked by the presence of free sulfide (euxinia) or dissolved iron (ferruginous) in the water column. Here, we use a literature compilation from modern localities to provide a new empirical evaluation of coupled Fe speciation and Mo concentrations as a proxy for pore water sulfide accumulation at non-euxinic localities. We also present new Fe speciation, Mo concentration, and S isotope data from the Friends of Anoxic Mud (FOAM) site in Long Island Sound, which is marked by pore water sulfide accumulation of up to 3 mM beneath oxygen-containing bottom waters. For the operationally defined Fe speciation scheme, ‘highly reactive’ Fe (FeHR) is the sum of pyritized Fe (Fepy) and Fe dominantly present in oxide phases that is available to react with pore water sulfide to form pyrite. Observations from FOAM and elsewhere confirm that Fepy/FeHR from non-euxinic sites is a generally reliable indicator of pore fluid redox, particularly the presence of pore water sulfide. Molybdenum (Mo) concentration data for anoxic continental margin sediments underlying oxic waters but with sulfidic pore fluids typically show authigenic Mo enrichments (2–25 ppm) that are elevated relative to the upper crust (1–2 ppm). However, compilations of Mo concentrations comparing sediments with and without sulfidic pore fluids underlying oxic and low oxygen (non-euxinic) water columns expose non-unique ranges for each, exposing false positives and false negatives. False positives are most frequently found in sediments from low oxygen water columns (for example, Peru Margin), where Mo concentration ranges can also overlap with values commonly found in modern euxinic settings. FOAM represents an example of a false negative, where, despite elevated pore water sulfide concentrations and evidence for active Fe and Mn redox cycling in FOAM sediments, sedimentary Mo concentrations show a homogenous vertical profile across 50 cm depth at 1 to 2 ppm. A diagenetic model for Mo provides evidence that muted authigenic enrichments are derived from elevated sedimentation rates. Consideration of a range of additional parameters, most prominently pore water Mo concentration, can replicate the ranges of most sedimentary Mo concentrations observed in modern non-euxinic settings. Together, the modern Mo and Fe data compilations and diagenetic model provide a framework for identifying paleo-pore water sulfide accumulation in ancient settings and linked processes regulating seawater Mo and sulfate concentrations and delivery to sediments. Among other utilities, identifying ancient accumulation of sulfide in pore waters, particularly beneath oxic bottom waters, constrains the likelihood that those settings could have hosted organisms and ecosystems with thiotrophy at their foundations.

Hypersalinity as a trigger of seagrass (Thalassia testudinum) die-off events in Florida Bay: Evidence based on shoot meristem O2 and H2S dynamics

The mechanisms initiating rapid, large-scale (>50 km2) seagrass die-off events globally remain elusive. Thalassia testudinum, a dominant habitat-forming tropical seagrass of the Caribbean and tropical Atlantic region, experiences recurrent die-off events in Florida Bay. Thus, T. testudinum provides an excellent case study to identify triggers of large-scale seagrass mortality. Presently, seagrass die-off events in Florida Bay are correlated to high total sulfide (H2S, HS−, S2−) concentrations in the sediment porewater (≥2000 μM), water column hypoxia, high temperature and hypersalinity (salinity > 45). Because the mortality appears to be initiated at the shoot base within the meristem, we examined the response of shoot meristematic O2 and H2S dynamics using microsensors over a 5-hour nighttime simulation. Shoots were held at salinity 35 (ambient) or 65 (hypersaline) using microsensors in intact cores with plants from the field. The rate of H2S intrusion in the dark and oxidation in the light were similar at 35 and 65 salinity. However, the tissue O2 consumption rate was significantly higher under hypersaline conditions (−11.07 ± 4.32 kPa pO2 h−1) compared to ambient (−3.93 ± 0.88 kPa pO2 h−1) in the dark. Consequently, the meristematic pO2 threshold (~1.5 kPa O2) where H2S intrusion occurred in the meristem was reached more rapidly, increasing the time H2S accumulated (1.5 to 2.8 h). Longer H2S accumulation time significantly increased maximum meristem H2S levels at 65 (536 ± 330 μM H2S) compared to 35 salinity (121 ± 62 μM H2S). These results in T. testudinum provide evidence for a triggering mechanism that links an enhanced respiratory rate under hypersaline conditions to sulfide toxicity. We propose that hypersalinity in Florida Bay, or any stressor that significantly increases nighttime respiration rates, can subject seagrasses to longer and higher concentrations of sulfide, a known phytotoxin, within the shoot meristem. This mechanism likely explains large-scale mass mortality of T. testudinum in Florida Bay during periods of high salinity and elevated porewater sulfide levels, although field experiments are required to further validate this supposition.

Characterization of benthic biogeochemistry and ecology at three methane seep sites on the Northern U.S. Atlantic margin

Several hundred methane seeps were recently discovered along the U.S. Atlantic margin, a passive margin without a major hydrocarbon basin. These seeps represent a compelling opportunity to compare and contrast geochemical parameters, as well as the distribution and composition of associated chemosynthetic ecosystems, across geologically distinct seeps. Specifically, this study characterized the physical, chemical, geological, and biological features at Veatch Canyon, New England, and Shallop Canyon methane seep sites using a suite of state-of-the-art sampling and communication tools: AUV Sentry, HOV Alvin, a video-equipped multicore, and a real-time telepresence connection to the Inner Space Center (ISC) at the University of Rhode Island. Water column backscatter data collected by AUV Sentry confirmed previously detected gas emission and further indicated the presence of 13 actively emitting gas seeps within the Veatch Canyon survey area, 5 within the New England survey area, and 15 within the Shallop Canyon survey area. Complementary high-resolution seafloor bathymetry and backscatter mapping data indicated highly rugose morphology at all surveyed seep sites including the presence of extensive rock outcrops and bivalve beds. Video collected by HOV Alvin provided visual confirmation of gas plumes and the presence of chemosynthetic communities, including microbial mats and mussel beds, as well as large outcrops of authigenic carbonate rock surrounding locations of discrete gas emission. Bathymodiolus was the dominant species observed at the Veatch Canyon methane seep site, while the deep-sea red crab, Chaceon quinquedens, dominated the New England and Shallop Canyon seep sites. Elevated pore water sulfide concentrations suggested stimulation of sulfate-coupled anaerobic oxidation of methane (AOM) and/or degradation of organic matter in seep sediments relative to non-seep sediments; AOM was observed directly within overlying authigenic carbonate rocks. DNA sequencing of the surface sediments revealed diverse sulfide-oxidizing bacteria similar to the taxa described previously at other methane seeps, including members of the Thiotrichales and Campylobacterales. Together, these analyses provide one of the first interdisciplinary descriptions of sites within this massive seep system, setting the stage for more targeted, hypothesis-driven investigations. In this context, telepresence facilitated interdisciplinary research between specialists at the ISC and scientists and engineers at sea, enabling researchers to transfer data and maximize research efforts with sampling tools at sea. Our findings suggest that although the geological setting is distinct, the surficial microbial and macrofaunal communities closely resemble those of previously described methane seeps.

Transient biogeochemistry in intertidal sediments: New insights from tidal pools in Zostera noltei meadows of Arcachon Bay (France)

Several studies highlighted the occurrence of circular pools in intertidal flats of different coastal systems and their transient water chemistry over both tidal and diurnal cycles. However, little is known about (1) the response of benthic biogeochemical reactions and fluxes at the sediment-water interface over such short time scales, and (2) the role of these tidal pools in the biogeochemical functioning of coastal systems. Based on in situ microprofiles and water sampling, we investigated the dynamics of dissolved oxygen (O2), nutrients, sulfides and metals, and the associated fluxes at the sediment-water interface in tidal pools from the Arcachon Bay (Atlantic coast of France). Our integrative approach included several tidal and diurnal cycles over two different seasons in the presence and absence of Zostera noltei. The results show that water temperature and light irradiance were the main factors driving the biogeochemical functioning of the tidal pools, as they controlled the physiological activity of the microphytobenthos. Changes in light radiations induced diurnal fluctuations of O2 concentrations within surficial sediment, thus resulting in fluctuations of the O2 diffusive fluxes at the sediment-water interface and of the O2 penetration depth in sediment. At high tide, the increase in turbulence above the sediment induced the advection of oxygenated water within the first millimeters of sediment, resulting in a significant increase in porewater O2 concentrations and sediment O2 penetration depth. Porewater sulfide concentrations and apparition depth were concomitant with the O2 dynamic over both diurnal and tidal cycles, indicating that intermediate redox diagenetic processes were impacted by O2 dynamic over such short time-scale. The rapid changes in redox processes in the sediment column are confirmed by a significant flux of dissolved manganese toward the water column during nighttime. The consumption of nitrate and the release of ammonium and phosphate, associated to the mineralization of the organic matter in the surface sediment did not appeared related however to such short time cycles. The efflux of dissolved silica from the sediment was most likely associated with the enhanced dissolution of Si-bearing particles in surface sediment at higher temperatures, although silica uptake by Z. noltei was also noted. This study clearly shows that tidal pools function as natural incubators of transient biogeochemical processes. A rough assessment of the nutrient budget at the scale of the bay indicates the tidal pools may contribute significantly to the biogeochemical functioning of Arcachon Bay.

The Seagrass Holobiont and Its Microbiome.

Seagrass meadows are ecologically and economically important components of many coastal areas worldwide. Ecosystem services provided by seagrasses include reducing the number of microbial pathogens in the water, providing food, shelter and nurseries for many species, and decreasing the impact of waves on the shorelines. A global assessment reported that 29% of the known areal extent of seagrasses has disappeared since seagrass areas were initially recorded in 1879. Several factors such as direct and indirect human activity contribute to the demise of seagrasses. One of the main reasons for seagrass die-offs all over the world is increased sulfide concentrations in the sediment that result from the activity of sulfate-reducing prokaryotes, which perform the last step of the anaerobic food chain in marine sediments and reduce sulfate to H2S. Recent seagrass die-offs, e.g., in the Florida and Biscayne Bays, were caused by an increase in pore-water sulfide concentrations in the sediment, which were the combined result of unfavorable environmental conditions and the activities of various groups of heterotrophic bacteria in the sulfate-rich water-column and sediment that are stimulated through increased nutrient concentrations. Under normal circumstances, seagrasses are able to withstand low levels of sulfide, probably partly due to microbial symbionts, which detoxify sulfide by oxidizing it to sulfur or sulfate. Novel studies are beginning to give greater insights into the interactions of microbes and seagrasses, not only in the sulfur cycle. Here, we review the literature on the basic ecology and biology of seagrasses and focus on studies describing their microbiome.

The Evolution of Sulfide in Shallow Aquatic Ecosystem Sediments: An Analysis of the Roles of Sulfate, Organic Carbon, and Iron and Feedback Constraints Using Structural Equation Modeling

AbstractThe generation of elevated concentrations of sulfide in sediment pore waters that are toxic to rooted macrophytes is problematic in both marine and freshwaters. In marine waters, biogeochemical conditions that lead to toxic levels of sulfide generally relate to factors that affect oxygen dynamics or the sediment iron concentration. In freshwaters, increases in surface water sulfate have been implicated in decline of Zizania palustris (wild rice), which is important in wetlands across the Great Lakes region of North America. We developed a structural equation (SE) model to elucidate key variables that govern the evolution of sulfide in pore waters in shallow aquatic habitats that are potentially capable of supporting wild rice. The conceptual basis for the model is the hypothesis that dissimilatory sulfate reduction is limited by the availability of both sulfate and total organic carbon (TOC) in the sediment. The conceptual model also assumes that pore water sulfide concentrations are constrained by the availability of pore water iron and that sediment iron supports the supply of dissolved iron to the pore water. A key result from the SE model is that variations in three external variables (sulfate, sediment TOC, and sediment iron) contribute nearly equally to the observed variations in pore water sulfide. As a result, management efforts to mitigate against the toxic effects of pore water sulfide on macrophytes such as wild rice should approach defining a protective sulfate threshold as an exercise tailored to the geochemistry of each site that quantitatively considers the effects of ambient concentrations of sediment Fe and TOC.

Increase in Nutrients, Mercury, and Methylmercury as a Consequence of Elevated Sulfate Reduction to Sulfide in Experimental Wetland Mesocosms

AbstractMicrobial sulfate reduction (MSR) in both freshwater and marine ecosystems is a pathway for the decomposition of sedimentary organic matter (OM) after oxygen has been consumed. In experimental freshwater wetland mesocosms, sulfate additions allowed MSR to mineralize OM that would not otherwise have been decomposed. The mineralization of OM by MSR increased surface water concentrations of ecologically important constituents of OM: dissolved inorganic carbon, dissolved organic carbon, phosphorus, nitrogen, total mercury, and methylmercury. Increases in surface water concentrations, except for methylmercury, were in proportion to cumulative sulfate reduction, which was estimated by sulfate loss from the surface water into the sediments. Stoichiometric analysis shows that the increases were less than would be predicted from ratios with carbon in sediment, indicating that there are processes that limit P, N, and Hg mobilization to, or retention in, surface water. The highest sulfate treatment produced high levels of sulfide that retarded the methylation of mercury but simultaneously mobilized sedimentary inorganic mercury into surface water. As a result, the proportion of mercury in the surface water as methylmercury peaked at intermediate pore water sulfide concentrations. The mesocosms have a relatively high ratio of wall and sediment surfaces to the volume of overlying water, perhaps enhancing the removal of nutrients and mercury to periphyton. The presence of wild rice decreased sediment sulfide concentrations by 30%, which was most likely a result of oxygen release from the wild rice roots. An additional consequence of the enhanced MSR was that sulfate additions produced phytotoxic levels of sulfide in sediment pore water.