Upside down sulphate dynamics in a saline inland lake.

The sulphur cycle has a key role on the fate of nutrients through its several interconnected reactions. Although sulphur cycling in aquatic ecosystems has been thoroughly studied since the early 70's, its characterisation in saline endorheic lakes still deserves further exploration. Gallocanta Lake (NE Spain) is an ephemeral saline inland lake whose main sulphate source is found on the lake bed minerals and leads to dissolved sulphate concentrations higher than those of seawater. An integrative study including geochemical and isotopic characterization of surface water, porewater and sediment has been performed to address how sulphur cycling is constrained by the geological background. In freshwater and marine environments, sulphate concentration decreases with depth are commonly associated with bacterial sulphate reduction (BSR). However, in Gallocanta Lake sulphate concentrations in porewater increase from 60 mM at the water-sediment interface to 230 mM at 25 cm depth. This extreme increase could be caused by dissolution of the sulphate rich mineral epsomite (MgSO4·7H2O). Sulphur isotopic data was used to validate this hypothesis and demonstrate the occurrence of BSR near the water-sediment interface. This dynamic prevents methane production and release from the anoxic sediment, which is advantageous in the current context of global warming. These results underline that geological context should be considered in future biogeochemical studies of inland lakes with higher potential availability of electron acceptors in the lake bed compared to the water column.

Drivers of variability in mercury and methylmercury bioaccumulation and biomagnification in temperate freshwater lakes.

The four largest freshwater lakes in southwestern France are of both ecological and economic importance. However, some of them are subjected to mercury (Hg) contamination, resulting in the ban of human consumption of piscivorous fish. Moreover, beyond predatory fish, little information exist regarding Hg levels in other species of these ecosystems. In this context, we used a food web analytical approach to investigate Hg bioaccumulation and biomagnification in relation to the trophic structure of these four lakes. More specifically, various organisms (macrophytes, epiphyton, invertebrates and fish) were collected at the four lakes and analysed for carbon and nitrogen stable isotopes as well as for total Hg (THg) and methylmercury (MeHg). A spatial variability of bioaccumulation in organisms was observed, particularly in carnivorous fish, with higher Hg levels being found in the two more northern lakes (median±SE: 3491 ± 474 and 1113 ± 209 ng THg.g-1 dw in lakes HC and L, respectively) than in the southern pair (600 ± 117 and 911 ± 117 ng THg.g-1 dw in lakes CS and PB, respectively). Methylmercury biomagnification was observed through the food webs of all four lakes, with different trophic magnification slopes (HC = 0.16; L = 0.33; CS = 0.27; PB = 0.27), even though the length of the food chains was similar between the lakes. Our results suggest that rather than the food web structure, anthropogenic inputs (sulfate in northern lakes and phosphorus inputs in southern ones) may have a strong impact, more or less directly, on Hg methylation in freshwater environments, and lead to concentrations exceeding environmental recommendations despite low Hg backgrounds in sediment and water.

Importance of the vegetation-groundwater-stream continuum to understand transformation of biogenic carbon in aquatic systems - A case study based on a pine-maize comparison in a lowland sandy watershed (Landes de Gascogne, SW France).

During land-aquatic transfer, carbon (C) and inorganic nutrients (IN) are transformed in soils, groundwater, and at the groundwater-surface water interface as well as in stream channels and stream sediments. However, processes and factors controlling these transfers and transformations are not well constrained, particularly with respect to land use effect. We compared C and IN concentrations in shallow groundwater and first-order streams of a sandy lowland catchment dominated by two types of land use: pine forest and maize cropland. Contrary to forest groundwater, crop groundwater exhibited oxic conditions all-year round as a result of higher evapotranspiration and better lateral drainage that decreased the water table below the organic-rich soil horizon, prevented the leaching of soil-generated dissolved organic carbon (DOC) in groundwater, and thus limited consumption of dissolved oxygen (O2). In crop groundwater, oxic conditions inhibited denitrification and methanogenesis resulting in high nitrate (NO3-; on average 1140 ±â€¯485 µmol L-1) and low methane (CH4; 40 ±â€¯25 nmol L-1) concentrations. Conversely, anoxic conditions in forest groundwater led to lower NO3- (25 ±â€¯40 µmol L-1) and higher CH4 (1770 ±â€¯1830 nmol L-1) concentrations. The partial pressure of carbon dioxide (pCO2; 30,650 ±â€¯11,590 ppmv) in crop groundwater was significantly lower than in forest groundwater (50,630 ±â€¯26,070 ppmv), and was apparently caused by the deeper water table delaying downward diffusion of soil CO2 to the water table. In contrast, pCO2 was not significantly different in crop (4480 ±â€¯2680 ppmv) and forest (4900 ±â€¯4500 ppmv) streams, suggesting faster degassing in forest streams resulting from greater water turbulence. Although NO3-concentrations indicated that denitrification occurred in riparian-forest groundwater, crop streams nevertheless exhibited important signs of spring and summer eutrophication such as the development of macrophytes. Stream eutrophication favored development of anaerobic conditions in crop stream sediments, as evidenced by increased ammonia (NH4+) and CH4 in stream waters and concomitant decreased in NO3- concentrations as a result of sediment denitrification. In crop streams, dredging and erosion of streambed sediments during winter sustained high concentration of particulate organic C, NH4+ and CH4. In forest streams, dissolved iron (Fe2+), NH4+ and CH4 were negatively correlated with O2 reflecting the gradual oxygenation of stream water and associated oxidations of Fe2+, NH4+ and CH4. The results overall showed that forest groundwater behaved as source of CO2 and CH4 to streams, the intensity depending on the hydrological connectivity among soils, groundwater, and streams. CH4 production was prevented in cropland in soils and groundwater, however crop groundwater acted as a source of CO2 to streams (but less so than forest groundwater). Conversely, in streams, pCO2 was not significantly affected by land use while CH4 production was enhanced by cropland. At the catchment scale, this study found substantial biogeochemical heterogeneity in C and IN concentrations between forest and crop waters, demonstrating the importance of including the full vegetation-groundwater-stream continuum when estimating land-water fluxes of C (and nitrogen) and attempting to understand their spatial and temporal dynamics.

Spectrophotometric determination of manganese in acidified matrices from (pore)waters and from sequential leaching of sediments.

Manganese (Mn) is a major redox reactive element in marine sediments and it plays an important role in the biogeochemical cycles of carbon, nitrogen, phosphorus, or trace metals. Mn cycle in marine sediments is characterized by an alternation of oxidation and reduction processes depending on physicochemical and biological conditions: assessing the quantification and the speciation of Mn is thus an essential issue to understand redox reaction-transport processes in sedimentary deposits. Solid Mn phases can be determined through chemical extractions techniques that permits selective leaching of operationally defined Mn fractions. Mn oxides and oxyhydroxides are extracted with an ascorbate leaching solution, while the whole Mn (oxyhydr-)oxides and Mn associated to carbonates is extracted with HCl. An existing spectrophotometric method allows to quantify Mn dissolved in (sea) water. We present here a modified version, which permits to measure Mn in acidified matrices, including ascorbate and HCl solutions. A metallic substitution occurs between a Cd-TCPP complex and Mn at pH 7.5-8.0, with imidazole as catalyst. We propose here to use a NaHCO3 solution to dilute the samples in order to be in the necessary pH range to perform the metal substitution. Using this method, Mn(II,III) concentrations were determined in standard solutions with a precision of 3% within a concentration range of 0.5-80 µM. The procedure was successfully applied to determine Mn in acidified pore waters and in ascorbate and HCl sequential extractions from muddy sediments of the Bay of Biscay. Spectrophotometric results agreed closely with results from atomic absorption spectrometry, validating the proposed method.

High-Content Screening of Plankton Alkaline Phosphatase Activity in Microfluidics.

One way for phytoplankton to survive orthophosphate depletion is to utilize dissolved organic phosphorus by expressing alkaline phosphatase. The actual methods to assay alkaline phosphate activity-either in bulk or as a presence/absence of enzyme activity-fail to provide information on individual living cells. In this context, we develop a new microfluidic method to compartmentalize cells in 0.5 nL water-in-oil droplets and measure alkaline phosphatase activity at the single-cell level. We use enzyme-labeled fluorescence (ELF), which is based on the hydrolysis of ELF-P substrate, to monitor in real time and at the single-cell level both qualitative and quantitative information on cell physiology (i.e., localization and number of active enzyme sites and alkaline phosphatase kinetics). We assay the alkaline phosphatase activity of Tetraselmis sp. as a function of the dissolved inorganic phosphorus concentration and show that the time scale of the kinetics spans 1 order of magnitude. The advantages of subnanoliter-scale compartmentalization in droplet-based microfluidics provide a precise characterization of a population with single-cell resolution. Our results highlight the key role of cell physiology to efficiently access dissolved organic phosphorus.