Japan Geoscience Union Meeting 2016

Presentation information


Symbol M (Multidisciplinary and Interdisciplinary) » M-IS Intersection

[M-IS06] Biogeochemistry

Sun. May 22, 2016 9:00 AM - 10:30 AM A03 (APA HOTEL&RESORT TOKYO BAY MAKUHARI)

Convener:*Muneoki Yoh(Tokyo University of Agriculture and Technology), Hideaki Shibata(Field Science Center fot Northern Biosphere, Hokkaido University), Naohiko Ohkouchi(Japan Agency for Marine-Earth Science and Technology), Youhei Yamashita(Faculty of Environmental Earth Science, Hokkaido University), Chair:Naohiko Ohkouchi(Japan Agency for Marine-Earth Science and Technology), Tomoya Iwata(Faculty of Life and Environmental Sciences, University of Yamanashi), Urumu Tsunogai(Graduate School of Environmental Studies, Nagoya University)

9:30 AM - 9:45 AM

[MIS06-03] Influences of carbon and sulfur cycle inside the hypersaline microbial mat on the chemical composition of overlying brine water

*Yuta Isaji1, Hodaka Kawahata1, Junichiro Kuroda2, Toshihiro Yoshimura1, Nanako O. Ogawa2, Atsushi Suzuki3, Takazo Shibuya2, Francis J. Jimenez-Espejo2, Stefano Lugli4, Vinicio Manzi5, Marco Roveri5, Naohiko Ohkouchi2 (1.Atmosphere and Oceanic Research Institute University of Tokyo, 2.Japan Agency for Marine-Earth Science and Technology, 3.Geological Survey of Japan National Institute of Advanced Industrial Science and Technology, 4.Dipartimento di Scienze Chimiche e Geologiche, Univ. degli Studi di Modena e Reggio Emilia, 5.Physics and Earth Science Dept., Univ. of Parma)

Keywords:hypersaline environment, microbial mat, biogeochemical cycle

Salinity is one of the most important environmental factors limiting the habitability of the marine organisms. As the seawater evaporates and the salinity increases, unique ecosystem is formed by specific organisms tolerable to substantial fluctuations in osmotic pressure, pH, and redox condition. In particular, microbial mat often formed in shallow hypersaline aquatic environment is known to be densely inhabited by microorganisms such as cyanobacteria, phototrophic and chemotrophic sulfur bacteria, sulfate reducing bacteria, methanogens, and others (e.g. Ollivier et al., 1994). Intensive studies have been made to reveal the biogeochemical processes of these microorganisms and their mutual interactions (e.g. Des Marais, 2003), but only few studies have focused on its effect on the chemical composition of the overlying brine water. In this study, we analyzed brine water collected from the modern saline pan in Trapani, Sicily, to understand the influence of the microbial activity on the overlying brine water with different salinity.
Major ion concentrations, DIC (dissolved inorganic carbon) concentration, and its carbon isotopic composition (δ13CDIC) were measured on 9 brine water samples with different salinity (38-276). Although intensive carbon fixation by photosynthesis occurs in the ponds where microbial mat is formed (salinity 97-159), corresponding DIC concentration showed little decrease (1250-1000 µmol L-1). This may be because mineralization of the fixed carbon by sulfate reduction occurs in the mat, releasing DIC in the brine water (e.g. Des Marais, 2003). This interpretation is supported by the low δ13CDIC value (~-10‰), because sulfate reduction supplies relatively 13C-depleted DIC. Alternative explanation is the chemically enhanced atmospheric invasion of CO2, which can cause considerable depletion in δ13CDIC (Baertschi, 1952; Lazar et al., 1992). By contrast, increase in the δ13CDIC value (~7.2‰) is observed in the ponds with higher salinity (>270) where no microbial mat is developing. This may be attributed to non-equilibrium escape of CO2 due to higher salinity and lower pH (Stiller et al., 1985; Barkan et al., 2001). These results suggest that δ13CDIC of hypersaline environment varies substantially due to both biological and physical processes.
Despite the intense sulfate reduction taking place in the mat, variation in SO42- concentration follows the ideal seawater evaporation path (Timofeeff et al., 2001). This may be because sulfide originating from the sulfate reduction is re-oxidized biotically by sulfur oxidizing bacteria or abiotically by the oxygen produced by cyanobacteria inhabiting the upper layer (e.g. Oren et al., 2009). Taken together, although DIC and SO42- are utilized intensively by the microorganisms in the mat, their content in the brine water are relatively unchanged due to efficient recycling in the mat.