Role of micro-organisms in karstification

Summary

INTRODUCTION

Whilst chemolithoautotrophic micro-organisms are found in nearly every environment on Earth, they are more abundant in dark habitats where competition by photosynthetic organisms is eliminated. Caves, particularly, represent dark but accessible subsurface habitats where the importance of microbial chemolithoautotrophy to biogeochemical and geological processes can be examined directly. At Lower Kane Cave, WY, USA, hydrogen sulfide-rich springs provide a rich energy source for chemolithoautotrophic micro-organisms, supporting a surprisingly complex consortium of micro-organisms, dominated by sulfur-oxidizing bacteria. Several evolutionary lineages within the class ‘Epsilonproteobacteria’ dominate the biovolume of subaqueous microbial mats, and these microbes support the cave ecosystem through chemolithoautotrophic carbon fixation. The anaerobic interior of the cave microbial mats is a habitat for anaerobic metabolic guilds, dominated by sulfate-reducing and -fermenting bacteria. Biological controls of speleogenesis had not been considered previously and it was found that cycling of carbon and sulfur through the different microbial groups directly affects sulfuric acid speleogenesis and accelerates limestone dissolution. This new recognition of the contribution of microbial processes to geological processes provides a better understanding of the causal factors for porosity development in sulfidic groundwater systems.

Karst landscapes form where soluble carbonate rocks dissolve by chemical solution (karstification), resulting in numerous geomorphic features, including caves and subterranean-conduit drainage systems (e.g. White, 1988; Ford & Williams, 1989). This has traditionally been viewed as an abiotic, chemical process that occurs near the water table, with biologically produced CO₂ as the principal reactive component.