Beneath the surface of the earth are microbial communities that live independent from the sun-driven surface and are sealed off from the atmosphere's oxygen. Because the subsurface is shut off from oxygen in the atmosphere, these microbes need to find alternative compounds to "breathe" (reduction), in order to carry out the energetic cycles necessary to drive life. Sulfur and oxygen share many characteristics, including their ability to serve as "breathable" compounds. To better understand sulfur reduction, this project will incorporate different studies that will look at sulfur chemistry; protein biochemistry; and the genetics of sulfur-based "breathing" (respiration) in a hot, petroleum-rich subsurface environment. Together, these studies will help with understanding how these living systems are able to develop deep in the earth, how life might have evolved on the early oxygen-free earth, and even how life might evolve in other places in the universe. Undergraduate researchers will conduct the studies, with experienced undergraduates mentoring young researchers through successful programs designed for the retention of underrepresented and lower income students in science; therefore creating a stronger multicultural scientific teaching and research community. These studies will also be integrated into undergraduate courses in Microbial Ecology and Bioinformatics, in which data obtained in the Microbial Ecology laboratory experiments will be analyzed by the students in the Bioinformatics course.
Sulfur-based respiration is suggested to have been one of the earliest energy conserving pathways for life on earth. It remains important to the sulfur and elemental cycles in the atmosphere, oceans, sediments, and deep subsurface. It is also of specific interest in petrochemical and other fields due to the extremely corrosive and toxic effect of microbially-produced sulfides. It is not at all clear which forms of sulfur contribute to the metabolism mechanisms of microbial sulfur respiration in situ, or how these enzymes mechanistically carry out this transformation. The relative levels of sulfur-reducing enzymes and microbes in many environments remain unknown. These overarching questions will be approached by focusing on a specific environment - a deep, hot, hydrocarbon-rich reservoir - and by integrating studies: (1) at the level of the microbial community by characterizing the microbes in the deep, hot subsurface environment and identifying sulfur-reducing microbes and enzymes through metagenomics and metatranscriptomics; (2) at the enzymatic level by determining the mechanisms of sulfur-reducing enzymes by kinetic and structural techniques; and (3) at the geochemical level by using cyclic voltammetry to determine the chemical speciation of sulfur in situ and during reduction by microbes and enzymes. A transformative aspect of this work is a way in which cyclic voltammetry will be used to obtain a "snapshot" of the entire range of sulfur species present during the reduction of sulfur by enzymes, isolated microbial species, and microbial populations. This work will also provide a view of the sulfur chemical species present in deep subsurface fluids in situ. Having a clear picture of sulfur metabolism in the deep subsurface will broaden our understanding of biogeochemical sulfur cycling, life in extreme environments, and evolutionary processes. It has been estimated that the subsurface environment contains 40-60% of the bacterial cells on earth, accounting for at least one third of the earth's carbon biomass. However, because of its hidden nature, this huge reservoir of biodiversity has only begun to be explored.
