CLOSED-BASIN LACUSTRINE SYSTEMS AS TERRESTRIAL ANALOGS FOR POSSIBLE AQUATIC HABITATS ON MARS AND OTHER WATER-LIMITED PLANETARY BODIES

Terrestrial analogs for aquatic habitats on water-limited planets generally focus on hydrothermal, acidic, and saline environments. Relatively little is known about alkalophilic microbial communities and associated concentration gradients for sulfate, nitrate, and acetate. Basalt-hosted evaporative lakes are particularly interesting as analogs for poorly differentiated planetary bodies. Alkalophilic bacteria are often tolerant of wide fluctuations in ionic composition and strength resulting from seasonal to decadal desiccation cycles in semi-arid settings. Warner Valley, Oregon is an alluvial system containing numerous geothermal springs and evaporative lakes underlain and hosted by andesitic basaltic flows, minor silicic tuffs, and deposits from Pleistocene Lake Warner. Modern lakes in Warner Valley undergo significant changes in water balance resulting from seasonal to decadal scale droughts. Lake waters are evaporatively concentrated from late spring through the summer, with a wide variation in chemistries and thermal structure resulting from interplay of supply/evaporation and geothermal inputs. Algal/bacterial mats are segregated by temperature (psychrophiles, mesophiles, thermophiles, and hyperthermophiles) and chemical (alkaliphiles and halophiles) variations of the lakes, playas, and springs. Seasonally dynamic microbial populations respond to wide swings in Na⁺, pH, Cl^-, temperature, and metal concentration. Water chemistries range from moderately to highly alkaline to sulfate-chloride dominated with a range in pH from 8.3 to 10.5 and total dissolved solids from 25 ppm (dilute) to 300,000 ppm (hypersaline). This environment favors vertically structured algal/bacterial mat communities dominated by spore-forming organisms that can tolerate a rapid change in surface aridity and water chemistry. These environments are in marked contrast to a chemically buffered and relatively uniform oceanic system.