Originally published as introduction to chapter 12, Microbiology and Biogeochemistry of Hypersaline Environments.

Marvin-DiPasquale M., Oren A., Cohen Y., Oremland R.S., 1999, Radiotracer studies of bacterial methanogenesis in sediments from the Dead Sea and Solar Lake (Sinai). Chapter 12, in Oren A, ed., Microbiology and Biogeochemistry of Hypersaline Environments: Boca Raton, CRC Press, p 149-160.

INTRODUCTION:
Methanogenesis has been studied extensively in freshwater and marine ecosystems, but much less is known about its occurrence in hypersaline environments (Oremland, 1988; Oremland and King, 1988). Of these, most research has been done in moderately hypersaline systems (e.g., salinity range = ~ 50 - 150 g/L), such as evaporative bacterial mats and the sediments of alkaline lakes (Oremland et al., 1982; King, 1988; Ollivier et al., 1994). Moderately hypersaline environments harbor methanogenic populations which produce methane by fermentation of precursors like methanol, methylated amines, and dimethyl sulfide, but show little affinity for acetate fermentation or the reduction of carbon dioxide with hydrogen (Oremland et al., 1982; Giani et al., 1984; 1989; Kiene et al., 1986; King, 1988). The reason for this constraint rests with the effectiveness of sulfate-reducing bacteria in out-competing methanogens for hydrogen and acetate, and the fact that hypersaline brines contain an abundance of sulfate several-fold greater than seawater. However, there have been very few radiotracer experiments with 14C-methanogenic precursors conducted in hypersaline environments to confirm whether these substances are robustly or weakly metabolised by the resident methanogens (e.g., Oremland et al., 1993). In addition, sulfate-reduction in the profoundal sediments of highly productive soda lakes occurs to such an extent that a complete depletion of porewater sulfate can be observed with depth in cores (Oremland et al., 1987). Under these conditions, methanogenesis via reduction of carbon dioxide is theoretically possible, but it proceeds at slow rates and it is problematic to detect because of the large dissolved carbonate + bicarbonate pool diluting any added 14HCO3= radiotracer (Oremland et al., 1993; 1994).
Biogeochemical investigations of extremely hypersaline systems (i.e., salinity > 300 g/L) have focused upon three large water bodies: the Orca Basin in the Gulf of Mexico, the Dead Sea, and the Great Salt Lake. Detailed studies on the ecology of methanogenesis are lacking for all three, but aspects of related research in these locations suggest the possibility of active, but constrained methane cycles. Thus, the anoxic Orca Basin (salinity = ~ 300 g/L) has supersaturated levels of dissolved methane which are strongly depleted in 13C, thereby indicating a bacterial origin (Wiesenburg et al., 1985). Although simple incubations of Orca Basin sediment slurries failed to demonstrate discernable methanogenic actvity (Oremland and King, 1988), nonetheless its waters have appreciable levels of archaebacterial lipids which suggests the presence of active methanogens (Dickins and van Vleet, 1992). Dissolved methane profiles in the Great Salt Lake (salinity = ~ 333 g/L) are indicative of a source in the sediments (Baedecker, 1985; Schinck et al., 1983), and bacterial formation of methane in the sediments from precursors like methionine have been briefly noted in the context of a review on a broader topic (Zeikus, 1983). The Dead Sea (salinity = ~ 322 g/L) has been the object of numerous investigations, albeit that none deal with methane (e.g., Nissenbaum, 1975; Nissenbaum et al., 1972; Nissenbaum et al., 1990; Cohen et al., 1983; Oren, 1983; 1988 a & b; 1990; 1993; 1994; 1995; Oren and Gurevich, 1994). Bacterial growth in the Dead Sea is severely constrained by the abundance of divalent cations (Cohen et al., 1983). Initial attempts to enrich for methanogens from the Dead Sea were unsuccessful (Oren, 1988 b). Nonetheless, extremely halophilic methanogens have been isolated from lagoonal sediments of the Crimea, suggesting that such organisms may inhabit the sediments of other extremely hypersaline systems (Zhilina, 1986; Zhilina and Zavarzin, 1987).
In this investigation we compare the methanogenic capability of sediments from the Dead Sea, whose waters are at salt-saturation concentrations (> 300 g/L), to those from the less saline Solar Lake (salinity < 180 g/L; Cohen et al., 1977). The organic-rich Solar Lake sediments were quite active and readily able to metabolise a diversity of 14C-methylotrophic methanogenic precursors to methane and carbon dioxide. In contrast, Dead Sea sediments only demonstrated a weak ability to form methane only from 14C- methanol, and not all samples were able to achieve this activity. Nonetheless, the detection of even this small amount of methanogenic activity from Dead Sea materials constitutes the first report of methanogenesis in this extreme environment.


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