Kartchner Caverns: Habitat Scale Community Diversity and Function in a Carbonate Cave

Abstract

This dissertation examines the microbial and functional diversity in Kartchner Caverns, a limestone cave in Arizona, USA. Kartchner is highly oligotrophic due to the lack of photosynthesis and the limited inputs of organic material from the surface. This characteristic poses a challenge for microbial life in the cave. The first objective of this work was to evaluate the bacterial richness, diversity and taxonomic composition of speleothems surfaces within Kartchner Caverns in order to gain insight into the distribution patterns associated with these communities. Secondly, the metabolic strategies used by cave communities to survive harsh cave conditions were investigated based on phylogenetic associations and metagenomics. Both objectives were directed toward answering the questions "who are there?" and "what are they doing?". The 454-pyrotag analysis of the V6 region of the 16S rRNA gene revealed an unexpectedly high bacterial diversity with each speleothem supporting a unique bacterial community profile. A focused study on one room of the cave revealed three community types: Type 1 was dominated by the phylum Proteobacteria; Type 2 by Actinobacteria; and Type 3 by Acidobacteria. Phylogenetic associations of the sequences generated by the 454 sequencing and by a Sanger clone library suggested cave microbial communities are supported by chemoautotrophic activities such as nitrite and iron oxidation. Results from the phylogenetic associations guided the metagenomic analysis which supports the presence of chemoautotrophic activities in the cave. Genes for two complete CO2 fixation mechanisms, the Calvin-Benson-Bashan and the rTCA cycles were identified in the cave metagenome, as well as genes for ammonia and nitrite oxidation. These genes are associated with both Bacteria and Archaea suggesting members of both domains are acting as primary producers in the cave ecosystem. Comparative analysis of cave samples to other environments suggests an overabundance of DNA repair mechanisms which could be potentially used by cave communities to overcome the toxicity due to high concentrations of calcium on the speleothem surfaces. This work provides the first comprehensive analysis of the microbial diversity and potential strategies used by microbial communities to survive under the extreme conditions found in a semi-arid limestone cave environment.