MOLECULAR CHARACTERIZATION OF MICROBIAL MATS FROM ALKALINE LAKES IN WARNER VALLEY, OREGON

Alkaline brackish lakes and wetlands are the dominant environments in the northern part of Warner Valley which is closed-drainage half-graben within the tectonic province of the Oregon-Nevada Great Basin. Water, mat, and sediment samples were collected from a chain of alkaline lakes located adjacent to cliffs comprising layers of andesitic basalt that rise 500-1000 meters above the valley floor. Water chemistries in the lakes are dominated by Na-Cl-SO₄. From early summer through fall, most of the lakes are covered by fibrous algal-cellulose mats overlying bacterial mats and anaerobic muddy sediments. Both algae and bacteria in these lakes are tolerant of arsenic. Concentrations of arsenic are highest (up to 700 μg/l) in seeps on the eastern margin of Anderson Lake where gelatinous microbial mats dominated by purple-sulfur bacterial blanket the marginal wetland. Algal and bacterial mats were characterized using solvent extraction of both untreated and pyrolyzed samples separation of the saturated hydrocarbons and gas chromatograph/mass spectrometry. Saturated hydrocarbons from untreated mat samples are dominated by n-C₂₃, n-C₂₅, n-C₂₇, and n-C₂₉, and have a Carbon Preference Index (CPI)=4.6. Saturated hydrocarbons from pyrolyzed mat samples are dominated by branched alkanes eluting in the range of n-C₁₈ to n-C₂₀, with a CPI=1.5. Hydrous pyrolysis yields 250 to 900 mg of extractable organic matter per gm of mat suggesting a high potential for preserving microbial biomarkers and for sourcing petroleum from alkaline lake deposits in extensional tectonic settings.