Effects of Impact Cratering on the Microbial Biosphere of the Deep Terrestrial Subsurface

Gronstal, Aaron Lee (2009). Effects of Impact Cratering on the Microbial Biosphere of the Deep Terrestrial Subsurface. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f257

Abstract

The 2005 ICDP-USGS deep drilling of the Chesapeake Bay Impact Structure (CBIS) returned the first complete core through an impact structure. A strict set of contamination assessment measures were implemented during sample collection to ensure that materials could be confidently used in geobiology, molecular biology and microbiology studies. Through direct cell counting, culturing and molecular analysis, samples offered a unique opportunity to characterize the subsurface microbial community present at depth in an impact structure. This work outlines how subsurface habitats can recover after impacts, and how impacts act to generate new microenvironments where microorganisms can colonize.

Geobiology studies revealed a pattern of microbial abundance that corresponds to lithological transitions within the crater structure. Three 'zones' of abundance were defined, with the first showing a steeper logarithmic decline in cell numbers than seen in other deep subsurface environments. This is followed by a zone of cell numbers below the detection limit of the methods used. Finally, the deepest section of the core shows an increase in cell numbers, indicating that recolonisation has occurred following the impact event.

Culturing studies were consistent with the results of enumerations, with successful cultures retrieved from microbiological zones 1 and 3. The majority of cultures were acquired using heterotrophic media, although cultures were also returned with media for iron reducers, iron oxidizers, sulfate reducers and humic acid utilisers. Culturing studies and molecular studies showed that a diverse consortium of microorganisms is present in the deep subsurface of the CBIS. Finally, the ability of microorganisms to access nutrients and minerals from meteoritic material was analyzed. The results of these studies add to our knowledge of how impacts events can affect subsurface microbial habitats; both directly by kinetic disruption of the environment, and through the delivery of exogenous materials.

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