| The atmospheric interface at the surface of rocks has long been recognized as a difficult environment for microbial survival. These subaerial surfaces are exposed to temperature extremes, elevated solar radiation (insolation), and low levels of water and nutrients. However, a diverse array of microbes still manages to establish subaerial lithic habitats (SLHs) in every corner of this planet. These organisms survive by establishing intimate relationships with the minerals they dwell upon and the elements brought to them by the wind and rain. They form interdependent communities that offer survival strategies more versatile than anything they could achieve on their own. Chapter I explores the existing literature to see how organisms interact with the global geochemistry to establish a network of interrelated biogeochemical cycles. These cycles determine the bioavailability of nutrients and energy for all living organisms. Chapter II documents an assessment of the test program for the Mars Science Laboratory's (MSL) Sample Acquisition / Sample Processing and Handling (SA/SPaH) system. We determined that the planned tests should adequately assess the surface sampling capabilities of the MSL. In Chapter III we applied biogeochemical concepts to propose methods for detecting life on Mars. Chapter IV details the design, development, construction, test, and operation of an arid conditions environmental simulator (ACES) for the study of a rock coating (varnish) that may be formed by lithic organisms on Earth and Mars. This work led to a new model that may answer long standing questions about rock varnish. We detail in Chapter V how lithic organisms can establish subaerial biofilms when adequate surface water is available. Organisms, aeolian dusts, and ferromanganese metabolic byproducts are deposited into layers of clay-enriched extracellular polymeric secretions (EPS) to eventually form a protovarnish. Finally, in Chapter VI we begin the characterization of a microbial community in the surface grains of sandstones collected in the Dry Valleys of Antarctica. We proposed a model where the biogeochemical cycling of iron may form the metabolic backbone for this community. The study of rock varnish-forming biofilms and Antarctic iron-cycling endolithic communities present interesting analogs to help us better understand the astrobiological possibilities of life. |
