Geochemical energy sources for microbial primary production in the ...

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C. Schmidt et al. / Marine Chemistry 108 (2008) 18–31 23 where ΔG T and ΔG T ° (J mol − 1 ) denote the change in Gibbs free energy and the change in standard Gibbs free energy (all species activities are set to 1 at standard state), respectively. Πa i product /Πa i reactive represents the activity quotient of the products and reagents of the chemical reaction. The temperature dependence of ΔG° can be determined applying the Van't Hoff law: ln K T K 298K ¼ DH- R d 1 T 1 T 298K ð2Þ with K, the equilibrium constant, defined as ΔG T °=−RT ln K. R represents the ideal gas constant (8.314 kJ mol − 1 ), ΔH° the change in standard enthalpy of the reaction and T is temperature in Kelvin. The total amounts of chemical energy available from a redox reaction, therefore potentially disposable for microbially mediated processes, were determined by the following relationship: Fig. 3. Schematic representation of the shrimp environment. The swarm position in the mixing zone between seawater and a local source of the hydrothermal fluid on the flank of the chimney. were compared to their modeled trends at equilibrium with sulfide, ferrous iron and hydrogen. Variable contribution of these electron donors in the source fluid were considered. The 200 ml mixing zone samples are composed of multiple mixing fractions with highly diverse compositions and temperature. pH is a more reliable tracer to asses the contribution of the hydrothermal end-member fluid to such samples than temperature at the sampling inlet which only reflects punctual conditions. For this reason and considering the lack of temperature data for most samples, pH was first chosen as the correlation factor in order to constrain the model. Equilibrium in samples after recovery on-board was modeled for a temperature of 25 °C. To model the in situ oxygen variation required to account for the real temperature of the mix and for the electron donors that achieve equilibrium in situ with oxygen. For TAG, only the correlation of iron II concentration with temperature could be constrained by means of in situ data. As hydrogen is unlikely to be present in the shrimp environment at this site, only sulfide was accounted to impact on the oxygen-pH relation in samples. The sulfide content in the mixing zone was estimated on this basis. 2.5. Energy budget calculation Energy calculation were performed for the mild part of the mixing zone (temperature below 30 °C). The energy release per mole of reactants for a given reaction at temperature T is determined by: DG T ¼ DG T - þ RT ln Pa product i =Pa reactive i ð1Þ DGðtotÞ T ¼ DG T d½e Š ð3Þ ΔG(tot) T in kJ kg −1 represents the maximum energy amount that can be liberated per mass unit of solution. It depends on both, the Gibbs free energy ΔG T per mole [kJ mol − 1 ]and[e − ], the concentration of electron donor that can be oxidized, considering that this quantity is limited by the availability of oxygen and the number of electron needed to oxidize one compound (mol kg − 1 ). The reactions considered in this study are the oxidation of methane, sulfide, hydrogen and ferrous iron. Both, the complete oxidation of sulfide (as HS − )to sulfate or partial oxidation to S° are investigated. To quantify the energy budget available from ferrous iron oxidation, the definition of thermodynamic properties of the oxidized form of iron is of main importance. Two-line ferrihydrite is the mineral that was shown to be formed dominantly in association with the shrimp epibionts that are harbored in the branchial cavity (Gloter et al., 2004). The thermodynamic properties of this compound have only recently been assessed in experimental conditions. Majzlan et al. (2004) defined ΔG 298K ° ranging from 26 to 29 kJ mol − 1 and ΔH° of−48 to−47 kJ mol − 1 for the formation of 2-line ferrihydrite, depending on the duration of precipitation process (Table 1). 3. Results 3.1. Temperature ranges for the shrimp habitat Temperature measurements performed in a shrimp swarm at TAG and Rainbow in 2005 are summarized in Table 2 and the corresponding sampling locations are shown in Fig. 2a, b. The temperature ranged between 2.8–17.4 °C at TAG and 5.3–18.2 °C at Rainbow. At both sites, Rainbow and TAG, the lowest temperature was recorded at the boundary of the shrimp swarm and the unpopulated rock, while more elevated
24 C. Schmidt et al. / Marine Chemistry 108 (2008) 18–31 Iron distribution appears conservative in first approximation in the studied environment, with a roughly linear correlation with temperature. The observed trend still lies significantly below the reference end-member dilution model. This depletion reflects, either a reduction of the end-member iron content, or more likely, iron removal by precipitation from the secondary source fluid before its emission into seawater. From the in situ iron II measurements performed in 2005, a ratio of 51 μMFe II °C − 1 was established. This estimate depicts an iron depletion in the local source fluid of about 23%, with regard to the end-member dilution measured in 1997 (i.e. 18.4 mM instead of 24 mM when extrapolated to the end- Fig. 4. (a) Iron as a function of temperature at Rainbow showing the reference end-member dilution model (dotted line) and the model defined from 2005 in situ data (solid line). Open circles: total Fe in samples (2001), triangles: in situ Fe II measurements (2005), crosses: in situ Fe II measurements (2001). (b) Sulfide as a function of iron at Rainbow. Bold dotted line: reference end-member dilution model, solid line: empirical model, thin dotted line: model allowing equilibrium with FeS precipitate. Open circles: sample contents, Close circles: Fe II in situ data (ATOS 2001). temperatures were measured in the center of the swarms. A temperature gradient was observed along a vertical axis within the shrimp assemblages. Similar ranges and patterns were obtained during the ATOS cruise (2001) for two different swarms at the Rainbow site (Table 3). These features and the turbulent currents of shimmering fluid observed around the bottom of the swarm suggest a hot fluid emission below the swarm as schematized in Fig. 3. The direct outflow source was not observable, as it was masked by a group of mineral spires. 3.2. Comparison of empirical data with the end-member dilution model 3.2.1. Rainbow Iron concentrations in the swarm environment have been determined, both, in situ (Fe II alone) and from fluid sampling (total labile Fe after reduction of Fe III with ascorbic acid). The iron–temperature correlation for the data sets obtained in 2001 and 2005 is rather consistent (Fig. 4a). Furthermore, the total Fe-temperature correlation in fluid samples does not significantly depart from in situ data (Fig. 4a). This suggests that the contribution of ferric iron to the overall in situ iron budget in the shrimp surrounding is minor. Fig. 5. (a) pH as a function of temperature, (b) CO 2 as a function of pH for Rainbow samples. Solid line: without Fe II oxidation, line with crosses: allowing Fe II oxidation in samples. Open squares: in the shrimp habitat (2001), triangles: local hot fluid source (2001), close squares: in the shrimp habitat (2005). (c) CH 4 as a function of pH for Rainbow samples. Thin line: reference end-member dilution model, solid line: empirically fitted model.
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