People: Astrobiology Faculty

David Stahl
Civil and Environmental Engineering

Our research combines studies in microbial ecology and evolution to better understand the origins and diversification of microorganisms that now comprise the foundation of our biosphere, and that have contributed to the emergence of complex higher life forms. Since the evolution of the biosphere and geosphere are intimately connected, we also collaborate with other research teams in the Astrobiology Institute to better resolve the emergence of key functional groups of microorganisms in the biogeochemical cycling of elements. Much our astrobiology-related research has focused on the sulfur cycle, the origin and radiation of sulfate-respiring microorganisms, and the characterization of microbial mat communities in which the sulfur cycle is a major process. A more recent project is examining the role of bacterial symbiosis with invertebrate animals (annelids) in determining co-evolutionary processes.

Extreme environments such as hot springs and hypersaline ponds provide favorable habitats for the growth of complex microbial mat communities. These mat communities are morphologically dominated by photosynthetic bacteria, but include microbes engaged in most major biogeochemical pathways including the sulfur, nitrogen and carbon cycles. Additionally, microbial mats are thought to be modern Earth analogs of prokaryotic assemblages that formed stromatolite fossils dating back 2-3.5 billion years. As members of both the UW's Center for Astrobiology and Early Evolution and the NASA Astrobiology Institute, members of our lab are participating in collaborative field research projects. One project is studying microbial mat communities in saline ponds at the Exportadora de Sal Corporation, Guerrero Negro, Baja California S., Mexico. This project is a multi-faceted investigation into the microbiology and biogeochemistry of these systems. Related studies are investigating microbial mat communities that develop in the hot springs of Yellowstone National Park, WY, USA. Our main goals at both field sites are to develop a better understanding of the contribution of sulfate-reducing bacteria (SRB) to microbial mat community structure and activity, and how the evolutionary radiation of sulfate-reducing bacteria relates to their ecology. These analyses combine methods in molecular phylogeny, microscopy, physiology, and analytical chemistry.

Symbiotic associations between bacteria and eukaryotes have contributed to major events in the evolution of the biosphere. The most spectacular examples are the bacterial origins of the mitochondrium (necessary for the later evolution of complex multicellular forms having high energy requirements) and the chloroplast (derived from a captive cyanobacterium). Thus, the development of simple models of symbiotic interaction between bacteria and eukaryotes are essential to understanding the more general role symbiosis has played in the diversification of complex life forms on earth.

Publications

Zverlov, V., M. Klein, S. Lucker, M. Friedrich, J. Kellerman, D.A. Stahl, A. Loy, and M. Wagner. Lateral gene transfer of dissimilatory (bi)sulfite reductase revisited. J. Bacteriol. 187: 2203-2208 (2005).

Fishbain, S., J. Dillon, H. Gough and D.A. Stahl. High rates of sulfate reduction in Yellowstone hot springs linked to unique sequence types in the dissimilatory sulfate respiration pathway. Appl. Environ. Microbiol. 69: 3663-3667 (2003).

Stahl, D.A., S. Fishbain, M. Klein, B. Baker, and M. Wagner. Origins and diversification of sulfate-respiring microorganisms. Anton Leeuw. Int. J. Gen. Molec. Microbiol. 81: 189-195 (2002).