Bacterially-Mediated Formation of Rock Coatings in Karkevagge, Swedish Lapland: A Mineralogical and Micro-Environmental Analog for Mars.

摘要

The search for past or present life on Mars is, for now, limited to surface environments. An often neglected surface environment that could have served as an abode for life and could presently preserve evidence of that life is that of rock coatings. Rock coatings are mineral accretions on rock surfaces. On Earth, they are widespread and occur with considerable chemical diversity. There is growing evidence for a biotic role in their formation on Earth, particularly with respect to rock varnish. As a result, rock varnish has become a target of astrobiological interest on Mars, where varnish-like coatings have been observed. However, a number of coating types compatible with martian mineralogy exist but have yet to be investigated thoroughly. In this dissertation, I present a study of three principle rock coating types from a glacially eroded valley, Karkevagge, in northern Sweden. The coatings consist of iron films, sulfate crusts, and aluminum glazes, all with primary mineralogies that are compatible with those minerals that have been identified on Mars.;To examine the role of microbiology in these terrestrial rock coatings and what the biotic formation of coatings might tell us about observed coatings on Mars, we asked three basic questions: 1) What microbes inhabit the coatings, 2) What are those microbes contributing to the geochemistry of the coatings, and 3) How are the microbes contributing to the overall formation of the rock coating? To answer these questions, we undertook two bacterial diversity surveys - Sanger sequencing and 454 pyrosequencing. Using the results of these surveys, we were able to assess diversity, richness, and metabolic potential of the communities. Microscopy and spectroscopy were used in order to visualize microbial communities inhabiting the coatings and to observe evidence of biomineralization. Using the answers to those questions - who, what, and how - a conceptual model of coating formation was developed to relate the terrestrial process of biological rock-coating formation to what may have occurred in the martian past.