Vertical distributions of thiosulfate and sulfite in the Cariaco Basin

Water column profiles of S2O and SO were obtained in January and May 2004 in the eastern portion of the 22 22 3 3 Cariaco Basin (108309N 648409W). Mean concentrations in the chemocline (250 to 450 m) decreased from 2.3 6 1.1 mmol L21 S2O in January to 1.4 6 0.3 mmol L21 S2O in May and from 4.8 6 2.9 mmol L 21 SO in 22 22 22 3 3 3 January to 2.7 6 0.7 mmol L21 SO in May. Integrated over this same depth interval, S2O was 459 mmol m22 22 22 3 3 in January and 287 mmol m22 in May and SO was 799 mmol m22 in January and 574 mmol m22 in May. During 22 3 this time, the integrated chemoautotrophic production in the chemocline decreased from 26 to 13 mmol C m22 d21. The concurrent decreases in S2O and SO inventories and chemoautotrophic production are consistent with a 22 22 3 3 decrease in the frequency or intensity of intrusion events. S2O and SO were present in the oxic portion of the 22 22 3 3 water column, suggesting lateral transport from shallower areas containing H2S. Calculations suggest that sufficient O2 and H2S can be supplied to the interface by advection and diffusion to form S2O in sufficient quantities to 22 3 support the observed levels of chemoautotrophic production. However, apparently ,2% of the H2S needed to form the observed amount of S2O can be produced from the degradation of organic carbon sinking through the interface 22 3 at this site. This suggests either that the H2S or carbon flux must be supplied laterally, perhaps from shallower areas (not yet sampled), or that chemoautotrophic production near the interface is overestimated. The Cariaco Basin, located on the continental shelf of Venezuela, is the largest truly marine anoxic basin on earth. The basin is made up of two 1,400-m-deep subbasins separated by a saddle reaching 900 m and is connected to the southern Caribbean Sea by two channels at the eastern and western ends of the basin that reach depths of no more than ;140 m. The waters above sill depth interact with the open sea, but vertical mixing below ;100 m is inhibited by the geometry of the basin and a strong pycnocline (Astor et al. 2003). Seasonal upwelling is usually observed in the surface 1 Present address: GZA GeoEnvironmental of New York, 440 Ninth Avenue, 18th Floor, New York, New York 10001. 2 Corresponding author ([email protected]). Acknowledgments We are indebted to the personnel of the Fundación La Salle de Ciencias Naturales, Estación de Investigaciones Marinas Isla Margarita (FLASA/EDIMAR) and the crew of the R/V Hermano Ginés for their enthusiasm and professional support. Ramon Varela provided unwavering leadership in coordinating and carrying out field expeditions. Russell Cuhel provided valuable advice regarding development of the HPLC method. We also thank Laura Lorenzoni for her assistance with the figures and the reviewers of this manuscript for their many helpful comments. This work was supported by the National Science Foundation through grants OCE 0118491 and OCE 0326175 to M.I.S. and G.T.T. and, in Venezuela, by Proyecto Cariaco by FONACIT (Fondo Nacional de Ciencia, Tecnologia e Innovacion) under grant 2000001702 to Ramon Varela of Fundacion La Salle. This is contribution 1305 from the Marine Sciences Research Center, Stony Brook University. waters between November and May, and annual primary production is estimated to vary between 500 and 600 g C m22 yr21 (Muller-Karger et al. 2001). Because of the restricted circulation and the degradation of the large amount of organic matter produced in the region, oxygen is completely consumed, and the waters below ;300 m are permanently sulfidic. The pathways of organic matter decomposition in the basin are reflected in vertical distributions of the major electron acceptors and their reduced products (Ho et al. 2004). Autotrophic H2Sand S2O -oxidizing denitrifiers, S2O -oxi22 22 3 3 dizing Mn reducers, S0 disproportionaters, and SO reduc22 4 ers have all been cultivated from selective enrichment experiments with waters from near the O2/H2S interface in the Cariaco Basin (Madrid 2000). Elevated concentrations of autotrophic and heterotrophic bacteria have also been found near the interface of other anoxic water columns, including the Black Sea and Framvaren Fjord (Karl 1978; Sorokin et al. 1995; Vetriani et al. 2003). Because the interface is located well below the illuminated layer, the interface community of the Cariaco Basin must be supported by chemoautotrophs that use a variety of oxidants and reductants to obtain energy for the fixation of CO2 (Taylor et al. 2001; Ho et al. 2004). Chemoautotrophs conserve chemical energy and reducing power in the form of ATP and NADPH, respectively. Three moles of ATP and 2 mol of NADPH are needed for each mole of CO2 fixed, at a basic cost of 191 kJ (Thauer et al. 1977). ATP synthesis is coupled to the oxidation of an electron donor, whereas 281 S2O and SO in the Cariaco Basin 22 22 3 3 Fig. 1. Map of Cariaco Basin with station for CARIACO time series (108309N, 648409W) indicated by the bull’s-eye. Isobaths are in meters. NADPH is generated from reverse electron transport reactions, metabolic mechanisms required for generating the necessary negative redox potential. A variety of reactions that yield enough energy for the fixation of CO2 are listed in Eqs. 1 to 4. In particular, the most common sulfur compounds used as electron donors for ATP synthesis in chemoautotrophs are H2S, S0, and S2O (Madigan et al. 2003). 22 3 22 1 H S 1 2O → SO 1 2H 2 2 4 21 DG8 5 2798 kJ mol (1) 1 H S 1 O → S 1 H O 2 2 0 2 2 21 DG8 5 2209 kJ mol (2) 3 0 22 1 S 1 H O 1 O → SO 1 2H 2 2 4 2 21 DG8 5 2587 kJ mol (3) 22 22 1 S O 1 H O 1 2O → 2SO 1 2H 2 3 2 2 4 21 DG8 5 2818 kJ mol (4) In these reactions, O2 is the electron acceptor. However, alternative electron acceptors, such as NO , can also be used 3 for chemoautotrophic H2S oxidation (Jannasch and Mottl 1985). For chemoautotrophic production to proceed, both electron donors and acceptors are required. If vertical transport of these species controls supply, fluxes of H2S, O2, and NO calculated from vertical concentration profiles and es3 timates of vertical eddy diffusion should balance biological demand calculated from the measured chemoautotrophic production. However, in the Cariaco Basin, Taylor et al. (2001) found that measured rates of chemoautotrophic production were much higher than could be supported by the diffusive supply of H2S, O2, and NO to the interface. Ver3 tical diffusive H2S fluxes could provide between 0.7% and 2.8% of the demand for reductant, whereas O2 and NO3 supply could provide 2.5–4.3% and 1.0–1.4%, respectively, of the demand for oxidant. Horizontal intrusions of O2-rich water from the Caribbean Sea have been reported in the Cariaco Basin (Scranton et al. 2001; Astor et al. 2003), and these could increase supply of oxidant. In addition, we considered it probable that reductants other than H2S, including intermediate oxidation sulfur species like S2O and SO , might be important in che22 22 3 3 moautotrophic production and thus that measurement of H2S alone would underestimate reductant supply. The reaction of O2 with H2S in the basin could form sulfur species of intermediate oxidation states, such as S2O , SO , and S0, that 22 22 3 3 could be used in disproportionation reactions, such as those in Eqs. 5 and 6, in which a compound is transformed into two compounds of lower and higher oxidation states, or as a reductant in reactions like Eqs. 7 or 8, in which metal oxides serve as the oxidant (Nealson and Myers 1992; Zhang and Millero 1993a; Janssen et al. 1996). 22 22 S O 1 H O → SO 1 H S 2 3 2 4 2 21 DG8 5 221.9 kJ mol (5) f 1 3 1 22 1 22 SO 1 H → SO 1 H S 3 4 2 2 4 4 21 DG8 5 258.9 kJ mol (6) f 2 1 2 2 0 22 1 S 1 FeOOH → SO 1 FeS 1 H 4 3 3 3 3 21 DG8 5 234.4 kJ mol (7) f 22 1 21 22 S O 1 4MnO 1 6H → 4Mn 1 2SO 1 3H O 2 3 2 4 2 21 DG8 5 2733 kJ mol (8) f In these examples, DG is the standard Gibb’s free energy 8f at pH 7. The above reactions are schematic and might not reflect exact reaction pathways. However, these and similar reactions would yield sufficient energy to support chemoautotrophic growth. To investigate the potential importance of S2O and SO in the redox budget of the Cariaco Basin, 22 22 3 3 measurements of S2O and SO were made during January 22 22 3 3