The roles of microbial selenate reduction and selenium sorption on selenium immobilization in littoral sediment from the hypersaline Salton Sea, California
The Salton Sea in California was formed between 1905-1907 by an accident that diverted Colorado River water to the Salton Sea Basin of the Colorado desert. Since 1924 the Salton Sea serves as an agricultural drainage reservoir maintained by agricultural and municipal wastewater inputs from the Coachella and Imperial Valleys in California and the Mexicali Valley in Mexico. Today, the Salton Sea is California's largest lake by area (975 km2) and constitutes a vital habitat for more than a million birds belonging to more than 400 species, including migratory and endangered populations. The Salton Sea, however, has been called an “ecological time bomb” given the threat that the convergence of selenium accumulation, chemical pollution, water diversion and climate change poses to this unique environment.
Selenium (Se) is considered an “essential toxin” due to its role as both an essential nutrient and an environmental contaminant. Selenium contamination is a concern in important areas of the United States, Canada, Mexico, Egypt and Israel where mining, agriculture and the extraction and refinement of fossil fuels and metals lead to selenium release from seleniferous parent rock material and selenium bioaccumulation in wildlife.
In aquatic ecosystems, selenium bioaccumulation is preceded by the accumulation of selenium in sediment which is driven by sorption and microbial activity, more specifically, the assimilatory and dissimilatory reduction of water-soluble selenium oxyanions. For the Salton Sea it remains unclear whether the sorptive and reductive capacity of sediments will suffice to prevent dissolved selenium concentrations in water from increasing in the future.
In order to characterize the transfer of selenium from water to underlying sediments in the Salton Sea, in this dissertation I describe the spatial variability in sediment parameters controlling selenate reduction rates across seven geographical locations and four depth intervals. I identify organic carbon content in sediment as an important driver of selenium accumulation in Salton Sea littoral sediment due to its dual nature as a sorbant for selenium oxyanions and as a source of reducing power for sediment microbial communities capable of selenate reduction. In the the first chapter, I quantify organic carbon content, selenate reducer abundance, selenium content and potential selenate reduction rates using sediment slurries in sediment collected from seven Salton Sea littoral locations. I show a broad diversity of sediment characteristics and the effect of sediment organic carbon content on selenate reduction potential.
In the second chapter, I show a range of Km and Rmax values for selenate and selenate reduction, respectively, comparable to that obtained in sediment collected from eleven geographical locations in the states of California and Nevada. Km values for selenate in evaluated sediment are well above ambient selenate concentrations and therefore microbial communities capable of selenate reduction operate well below their selenate reduction potential. I also show unexpectedly high selenate sorption which, under present conditions, contributes more than microbial activity to selenium retention in sediment.
In the third chapter, I use sediment slurries and a microbial consortium recovered from Salton Sea littoral sediment to suggest that nitrate reducers play an important role in selenium accumulation in the Salton Sea. I discuss this data in the context of our current knowledge regarding the microbial pathways of selenate reduction and the thermodynamic constraints associated with selenate respiration and microbial biomass synthesis.