Enhanced mercury reduction in the South Atlantic Ocean during carbon remineralization

Highlights • Dissolved gaseous mercury can be calculated from modeled dissolved inorganic carbon. • Modeled dissolved gaseous mercury agrees well with worldwide observations. • Dissolved gaseous mercury is related to depth and macronutrients concentrations. Mercury (Hg) in seawater is subject to interconversions via (photo)chemical and (micro)biological processes that determine the extent of dissolved gaseous mercury (DGM) (re)emission and the production of monomethylmercury. We investigated Hg speciation in the South Atlantic Ocean on a GEOTRACES cruise along a 40°S section between December 2011 and January 2012 (354 samples collected at 24 stations from surface to 5250 m maximum depth). Using statistical analysis, concentrations of methylated mercury (MeHg, geometric mean 35.4 fmol L−1) were related to seawater temperature, salinity, and fluorescence. DGM concentrations (geometric mean 0.17 pmol L−1) were related to water column depth, concentrations of macronutrients and dissolved inorganic carbon (DIC). The first-ever observed linear correlation between DGM and DIC obtained from high-resolution data indicates possible DGM production by organic matter remineralization via biological or dark abiotic reactions. DGM concentrations projected from literature DIC data using the newly discovered DGM–DIC relationship agreed with published DGM observations

Advancing methodology for sub-zero temperature application of DGT technique on sea ice samples for two-dimensional imaging of biogenic metals

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Advancing methodology for sub-zero temperature application of DGT technique on sea ice samples for two-dimensional imaging of biogenic metals

Ice-associated communities colonize the brine-filled spaces and are exposed to major biogeochemical and physical changes: temperature fluctuations, salinity, dissolved oxygen, light, pH, the surrounding organic matrix, and nutrient stress. A key adaptive response is the formation of biofilms, which play a major role in macro- and micro-nutrient storage, transformation and mobilization. Considerable enrichment of Fe and other trace metals has been recorded in sea ice, supposedly being adsorbed onto organic matter. Current methods for collecting pristine ice samples mostly involve melting an ice core, erasing any spatial information and discrimination between solid, liquid and gaseous phases. As a result, sea ice analytical methods have an insufficient spatial resolution to detect or describe microbial processes at submillimetre scale (in biofilms within the brine network), without any existing alternative. We have developed a new Diffusive Gradients in Thin-films (DGT) procedure for sea ice application, based on DGT capacity for imaging 2-dimensional distribution of total labile metal concentrations in soil/sediment. During the optimization process, we considered atypical conditions for DGT application at sub-zero temperatures; hydrogel freezing, slow diffusion, high brine salinity. We defined diffusive coefficients at water freezing temperatures and assured contact with hydrogel and thus diffusion. Using Peltier element to precisely control ambient temperature, slow equilibration to in situ temperature of -1.8°C successfully maintained the brine liquid, ice remained solid, and the hydrogel did not freeze. This allowed diffusion to occur, and importantly, allowed sea ice to de-gas. Without gradual equilibration, gases from sea ice were trapped between hydrogel and ice, separating the two and preventing diffusion. Our result are the first two-dimensional images of biogenic metal micronutrients in the sea ice, revealing a clear spatially diverse signal. Fe, Zn and Mn were associated with organic matter-rich micro-locations where the biofilm communities were clearly visible. The new procedure has muchpotential to advance our understanding of the sea ice biogeochemistry. It could provide missing empirical evidence to connect hypothesized reductive conditions in biofilm with trace element and organic matter growth/remineralization on a fine spatial scale, thus increasing understanding of processes occurring in polar oceans and its feedback on the ongoing global change.info:eu-repo/semantics/nonPublishe

Advancing methodology for sub-zero temperature application of DGT technique on sea ice samples for two-dimensional imaging of biogenic metals

Ice-associated communities colonize the brine-filled spaces and are exposed to major biogeochemical and physical changes: temperature fluctuations, salinity, dissolved oxygen, light, pH, the surrounding organic matrix, and nutrient stress. A key adaptive response is the formation of biofilms, which play a major role in macro- and micro-nutrient storage, transformation and mobilization. Considerable enrichment of Fe and other trace metals has been recorded in sea ice, supposedly being adsorbed onto organic matter. Current methods for collecting pristine ice samples mostly involve melting an ice core, erasing any spatial information and discrimination between solid, liquid and gaseous phases. As a result, sea ice analytical methods have an insufficient spatial resolution to detect or describe microbial processes at submillimetre scale (in biofilms within the brine network), without any existing alternative. We have developed a new Diffusive Gradients in Thin-films (DGT) procedure for sea ice application, based on DGT capacity for imaging 2-dimensional distribution of total labile metal concentrations in soil/sediment. During the optimization process, we considered atypical conditions for DGT application at sub-zero temperatures; hydrogel freezing, slow diffusion, high brine salinity. We defined diffusive coefficients at water freezing temperatures and assured contact with hydrogel and thus diffusion. Using Peltier element to precisely control ambient temperature, slow equilibration to in situ temperature of -1.8°C successfully maintained the brine liquid, ice remained solid, and the hydrogel did not freeze. This allowed diffusion to occur, and importantly, allowed sea ice to de-gas. Without gradual equilibration, gases from sea ice were trapped between hydrogel and ice, separating the two and preventing diffusion. Our result are the first two-dimensional images of biogenic metal micronutrients in the sea ice, revealing a clear spatially diverse signal. Fe, Zn and Mn were associated with organic matter-rich micro-locations where the biofilm communities were clearly visible. The new procedure has muchpotential to advance our understanding of the sea ice biogeochemistry. It could provide missing empirical evidence to connect hypothesized reductive conditions in biofilm with trace element and organic matter growth/remineralization on a fine spatial scale, thus increasing understanding of processes occurring in polar oceans and its feedback on the ongoing global change.info:eu-repo/semantics/nonPublishe

Trace metal speciation in North Sea coastal waters

International audienc

Sea-ice biogeochemistry: from micro-environment to the scale of Antarctic Sea Ice. Symposium on sea ice in the Earth System: A multidisciplinary perspective

Observations over recent decades suggest that sea ice plays a significant role in global biogeochemical cycles, providing an active biogeochemical interface at the ocean-atmosphere boundary. Sea ice is a semisolid matrix permeated by a network of channels and pores. The brine-filled spaces are colonized by sympagic (ice-associated) communities that are both taxonomically diverse and metabolically active, with multiple trophic levels, efficiently consuming, reprocessing, and redistributing chemicals within the ice and exchanging with both the overlying atmosphere and the underlying ocean. Analyzing biogeochemical properties in sea ice is fundamentally complicated by its inherent heterogeneity and multiphase nature. This is especially illustrated by the lack of robust estimates for the most basic biogeochemical fluxes, such as primary production and carbon export. Measurements of sea-ice primary production are scarce and challenging. Accumulation of organic matter being trapped within sea ice during the growth season is likely to provide a conservative estimate of the net community production. More than 20 years ago, Legendre et al. [1] used the few available observations to infer Antarctic sea-ice primary productivity. We will revisit this estimation by using a much larger database (n = 421 ice-cores). Based on this compilation, a preliminary estimate for Antarctic sea-ice primary productivity is 35 Tg C yr-1, representing roughly 20% of the primary productivity in the seasonal ice zone. Sympagic communities are exposed to major biogeochemical and physical changes during their lifetime into the ice. A key adaptive response is the formation of biofilms, which play multiple roles in the entrapment, retention and survival of microorganisms. We will review the growing body of evidence that suggests that the biofilm is also playing a major role in sea-ice biogeochemical dynamics (e.g. macro- and micro-nutrient storage; microenvironments with distinct biogeochemical properties; …). Current methods for collecting sea-ice samples mostly involve melting an ice core, disrupting both geochemical continuum and equilibrium of the sample and erasing any spatial information. Although analyzing bulk liquids may provide information on centimeter-scale, this is insufficient to detect or describe microbial processes, operating on submillimetre scale in biofilms, yet there is currently no alternative option for these measurements. To overcome this issue, we will also introduce a passive sampling technique of Diffusive Gradients in Thin-films for 2-dimensional images of labile metals in soil/sediment that we are developing for sea-ice application. This new approach may provide new conceptual and quantitative understanding of biogeochemical processes regulating the distribution of key elements and solutes in the structurally multiphase complex sea-ice matrix.info:eu-repo/semantics/nonPublishe

The biogeochemical role of a microbial biofilm in sea ice: Antarctic fast ice as a case study

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The biogeochemical role of a microbial biofilm in sea ice: Antarctic landfast sea ice as a case study

A paradox is commonly observed in productive sea ice in which an accumulation in the macro-nutrients nitrate and phosphate coincides with an accumulation of autotrophic biomass. This paradox requires a new conceptual understanding of the biogeochemical processes operating in sea ice. In this study, we investigate this paradox using three time series in Antarctic landfast sea ice, in which massive algal blooms are reported (with particulate organic carbon concentrations up to 2600 µmol L-1) and bulk nutrient concentrations exceed seawater values up to 3 times for nitrate and up to 19 times for phosphate. High-resolution sampling of the bottom 10 cm of the cores shows that high biomass concentrations co-exist with high concentrations of nutrients at the sub-centimetre scale. Applying a nutrient-phytoplankton-zooplankton-detritus (i.e., NPZD) model approach to this sea-ice system, we propose the presence of a microbial biofilm as a working hypothesis to resolve this paradox. By creating microenvironments with distinct biogeochemical dynamics, as well as favouring nutrient adsorption onto embedded decaying organic matter, a biofilm allows the accumulation of remineralization products (nutrients) in proximity to the sympagic (ice-associated) community. In addition to modifying the intrinsic physico-chemical properties of the sea ice and providing a substrate for sympagic community attachment, the biofilm is suggested to play a key role in the flux of matter and energy in this environment.YROSIA