Biomarker analysis of microbial diversity in sediments of a saline groundwater seep of Salt Basin, Nebraska
Introduction
Interest in the chemistry and biology of inland alkaline salt basins is growing for several reasons. First, they are discrete ecosystems with well-defined physical limits, contributing to the ease of studying the chemical and microbiological interactions in them. Second, they are extreme environments, offering the opportunity to study relatively unique aspects of biodiversity, microbial physiology and metabolism. Finally, the last surface aquatic habitats for life on Mars were likely to have been highly saline, making modern terrestrial salt basins good analogs for conditions under which life might have evolved on both Mars and Earth (Moore and Bullock, 1999, Joeckel and Ang Clement, 2001, Joeckel and Ang Clement, 2005, Schouten et al., 2001, Zettler et al., 2002). If so, bio-molecules found in organisms adapted to high salt and high-pH environments on Earth may be reliable biomarkers for detecting life on Mars and for studying microbial evolution in general.
Lipid biomarkers, the preserved structural skeletons of biological molecules, can be used to identify potential source organisms (e.g., Summons et al., 1999), to establish possible links between modern microbial communities and their ancient counterparts (Brocks et al., 1999, Brocks et al., 2003, Jahnke et al., 2004), and to illuminate the evolutionary histories of organisms and their environments (e.g., Brocks et al., 2003). Analysis of phospholipid fatty acids (PLFA), which are present in the form of lipid bilayers in microbial membranes, can provide a quantitative measure of the viable biomass and biological diversity of microbial communities in environmental samples (White et al., 1997). Glycolipids are unique membrane lipid components in photosynthetic organisms (Collins and Ferrier, 1995). Therefore, the analysis of glycolipids provides information on the activity of photosynthetic microorganisms in the environment (Sinninghe Damsté et al., 2001). Other lipid biomarkers, including sterols and hopanoids, are better preserved in the extended geological record and therefore offer insights into the presence of microorganisms in paleoenvironments (see Jahnke et al., 2004 and references therein).
The Salt Basin in Lancaster County, Nebraska is an inland saline basin developed around Salt Creek and its tributaries (Fig. 1). Historically at least, it incorporated multiple saline wetlands and salt flats. The Salt Basin is different from many other known salt basins in that it lies in a subhumid climate and farther northward and eastward than other well-known surficial salt accumulations in the United States (Joeckel and Ang Clement, 1999). Salt seeps and springs result from the upward discharge of groundwater through Late Pleistocene–Holocene alluvium (Joeckel and Ang Clement, 1999). Saline waters move upward into this alluvium from the Cretaceous Dakota Formation, but much of the dissolved salt content may have originated in Paleozoic strata, and probably migrated some distance (Gosselin et al., 2001). The oxidation and leaching of pyrite-bearing glacial tills underlying adjacent uplands could have contributed to the accumulation of dissolved sulfate, whereas sodium and chloride are likely to have been transported longer distances through the regional aquifer. Black sulfidic sediments are generated as a result of the saturation of surficial sediments and soils by through-flowing, sulfate-bearing groundwater around seeps and in small wetlands, where surface-water salinity varies seasonally.
Soil-surface salt efflorescences in the basin are dominated by halite (NaCl) with minor amounts of thenardite (Na2SO4). Sediment porewater in Salt Basin is characterized by high pH (7.8–12.1) and low redox potential (as low as −200 mV). Microbial sulfate reduction and iron sulfide precipitation are the prominent biogeochemical processes in Salt Basin. H2S (detected by odor) and accompanying strong effervescence were produced in a reaction of sulfidic sediment with 1 N HCl (Joeckel and Ang Clement, 1999). However, iron sulfide minerals are labile and can be oxidized readily when the water table drops or surface water evaporates. Algal and cyanobacterial mats have been observed in Salt Basin (Joeckel and Ang Clement, 1999).
In the present study, we analyzed lipid biomarkers in sediments of the Salt Basin in order to provide the first-ever comprehensive assessment of the microbial community in sulfidic soils around seeps and in the overlying water column.
Section snippets
Sampling and geochemical analysis
The sediment core analyzed in this study was taken from a groundwater-intercepting drainage ditch immediately west of Capitol Beach Lake, a manmade impoundment that now covers what was a large salt flat at the time of initial Euramerican settlement in the 1860s (Joeckel and Ang Clement, 1999). At the study site (Fig. 1), patches of black sulfidic sediment under saline water, associated with seeps and small springs, were distributed along the ditch. A fifteen-centimeter push core was collected
Sedimentary geochemistry
The groundwater of Salt Basin contains high levels of Na+ (10.4–18.0 g/l) and Cl− (15.3–22.6 g/l). A surface water sample taken near a saline seep contained 82 and 114 g/l of Na+ and Cl−, respectively. Other major ions included Ca2+ (0.22–0.32 g/l), (0.66–0.84 g/l), and (0.4–0.77 g/l) (Joeckel and Ang Clement, 1999). Porewater extracted from Section 1 of the sediment core contained elevated concentrations of Cl− (20.9 g/l), (1.7 g/l), S2− (0.024 g/l), and alkalinity (0.53 g/l). The
Discussion
Because lipid profiles of many extant organisms are lacking and the abundance and distributions of biomarkers represent a mixture of contributions from all organisms present in the environment (Brocks et al., 2003, Pancost and Sinninghe Damsté, 2003), caution must be exercised in using lipid biomarkers for microbial community analysis. Nevertheless, certain molecular biomarkers are unique and diagnostic for specific microorganisms and can provide insight on overall community structure and on
Conclusions
Biomarkers of cyanobacteria, phototrophic sulfur bacteria, SRB, sulfide-oxidizing bacteria, and bacterivorous ciliates were identified in sediments near saline seeps in the Salt Basin of Lancaster County, Nebraska. The identification of these compounds verifies and greatly amplifies the results achieved by more conventional means (Joeckel and Ang Clement, 1999). Our results also demonstrate that distinct and comparatively unique microbial assemblages exist in shallow geological environments and
Acknowledgements
We thank Kelly Crowley, Carrie Carlson, and Seth Chamberlain for their assistance with laboratory analyses. Comments from Associate Editor Richard Pancost and Helen Talbot greatly improved the manuscript. D.A.B. is supported by US National Science Foundation grant EAR-0311950.
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