PIGMENT AND LIPID DESICCATION PROXIES FROM CYANOBACTERIA IN DESERT SOILS AND GREAT SALT LAKE SEDIMENTS

Cyanobacteria are common in terrestrial ecosystems and can be the dominant photosynthetic organisms in extreme environments where they colonize, for example, exposed rock surfaces and desert soils. In such environments, the combined effects of desiccation and ultraviolet radiation exposure stimulate responses such as the production of the UV screening pigment scytonemin and methyl-branched alkanes. We previously reported that in Black Sea sediments scytonemin and 7-methylheptadecane concentrations both increased markedly during the Subboreal, apparently due to increased aridity and soil erosion in the Ukraine Steppe. Developing a better understanding of the relationship between these biomarkers in desert soils and sediments will strengthen the use of both as proxies for cyanobacterial ecology in the past.

Our present study focuses on pigment and lipid production in desert soil crusts in the Great Salt Lake Desert and the preservation of biomarkers in Great Salt Lake sediments. We conducted hydration/dehydration experiments on desert soil crusts to examine changes in pigment and lipid compositions in response to physiological stress. We used high performance liquid chromatography/mass spectrometry to quantify scytonemin and intact polar lipid (IPL) concentrations during the growth experiments. Glycolipids and phospholipids were the most abundant IPLs, with probable cyanobacterial and heterotrophic bacterial sources, respectively. Glycolipid and phospholipid abundances varied on a diel cycle in hydrated soils apparently reflecting the balance between photosynthetic and heterotrophic growth. Glycolipid fatty acid moieties, the probable precursors to methyl-branched alkane biomarkers found in sedimentary rocks and oils, varied during the experiments. We also report the down-core occurrence in Great Salt Lake sediments of branched and normal alkanes and scytonemin, which was especially abundant in the early Holocene from ca. 11.5 to 10 ka (radiocarbon, calibrated) deposited following the Gilbert highstand interval.