HR: 0800h
AN: B21A-0857    [Abstracts]
TI: The Effect of Temperature and Hydrogen Limited Growth on the Fractionation of Sulfur Isotopes by {\it Thermodesulfatator indicus}, a Deep-sea Hydrothermal Vent Sulfate-Reducing Bacterium
AU: * Hoek, J
EM: hoekj@pdx.edu
AF: Portland State Unniversity, Dept. of Biology 1719 SW 10th Ave, Portland, OR 97201 United States
AU: Reysenbach, A
EM: reysenbacha@pdx.edu
AF: Portland State Unniversity, Dept. of Biology 1719 SW 10th Ave, Portland, OR 97201 United States
AU: Habicht, K
EM: khabicht@biology.sdu.dk
AF: University of Southern Denmark, Institute of Biology Campusvej 55, Odense, 5230 Denmark
AU: Canfield, D E
EM: dec@biology.sdu.dk
AF: University of Southern Denmark, Institute of Biology Campusvej 55, Odense, 5230 Denmark
AB: Sulfate-reducing bacteria fractionate sulfur isotopes during dissimilatory sulfate reduction, producing sulfide depleted in $^{34}$S. Although isotope fractionation during sulfate reduction of pure cultures has been extensively studied, most of the research to date has focused on mesophilic sulfate reducers, particularly for the species {\it Desulfovibrio desulfuricans}. Results from these studies show that: 1) fractionations range from 3-46$\permil$ with an average around 18$\permil$, 2) when organic electron donors are utilized, the extent of fractionation is dependent on the rate of sulfate reduction, with decreasing fractionations observed with higher specific rates, 3) fractionations are suppressed with low sulfate concentrations, and when hydrogen is used as the electron donor. High specific sulfate-reduction rates are encountered when sulfate-reducing bacteria metabolize at their optimal temperature and under non-limiting substrate conditions. Changes in both temperature and substrate availability could shift fractionations from those expressed under optimal growth conditions. Sulfate reducers may frequently experience substrate limitation and sub-optimal growth temperatures in the environment. Therefore it is important to understand how sulfate-reducing bacteria fractionate sulfur isotopes under conditions that more closely resemble the restrictions imposed by the environment. In this study the fractionation of sulfur isotopes by {\it Thermodesulfatator indicus} was explored during sulfate reduction under a wide range of temperatures and with both hydrogen-saturating and hydrogen-limited conditions. {\it T. indicus} is a thermophilic (temperature optimum = $70\deg$ C) chemolithotrophic sulfate-reducing bacterium, which was recently isolated from a deep-sea hydrothermal vent on the Central Indian Ridge. This bacterium represents the type species of a new genus and to date is the most deeply branching sulfate-reducing bacterium known. {\it T. indicus} was grown in carbonate-buffered salt-water medium with H$_{2}$ as the sole electron donor, and CO$_{2}$ as primary carbon source. The fractionation of sulfur isotopes was measured in batch cultures and in a thermal gradient block over the full temperature range of growth (40-$80\deg$ C). For experiments in the gradient block, cell-specific rates of sulfate reduction increased with increasing temperatures to $70\deg$ C after which sulfate-reduction rates rapidly decreased. The range of fractionations (1.5-10$\permil$) was typical for growth with hydrogen as the electron donor. Fractionations decreased with increasing temperature from 40--$60\deg$ C, and increased with increasing temperatures from 60-$80\deg$ C. Growth under H$_{2}$-limited conditions in a fed-batch culture revealed high fractionations of 24-37$\permil$. This is the first report of sulfur isotope fractionation under H$_{2}$ limited growth and indicates that large fractionations are produced when H$_{2}$ is supplied as a limiting substrate. Our results suggest that fractionation is controlled by the competition of forward and reverse enzymatic reaction rates during sulfate reduction and by sulfate transport into the cell.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1615 Biogeochemical processes (4805)
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting