I. Formate Dehydrogenase Gene Diversity in Lignocellulose-Feeding Insect Gut Microbial Communities. II. Metabolic Impacts on the Hydrogen Isotope Content of Bacterial Lipids

Citation

Zhang, Xinning (2010) I. Formate Dehydrogenase Gene Diversity in Lignocellulose-Feeding Insect Gut Microbial Communities. II. Metabolic Impacts on the Hydrogen Isotope Content of Bacterial Lipids. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ETXQ-D671. https://resolver.caltech.edu/CaltechTHESIS:05072010-142709280

Abstract

I.) Symbiotic CO₂-reducing acetogens are important bacterial members of lignocellulose-feeding termite and roach gut communities. Acetogens are the major consumers of H₂ derived from lignocellulose fermentation and can contribute up to 1/3 of the acetate that serves as fuel for the insect host. Many acetogens in wood-feeding termites belong to a diverse group of relatively unstudied, uncultured spirochetes within the genus Treponema. Here, I use the gene sequence for hydrogenase-linked formate dehydrogenase, an enzyme utilized in sugar fermentation and the acetogenic metabolism of the spirochete isolate Treponema primitia, to investigate the diversity, evolution, and activity of uncultured acetogenic spirochetes in lignocellulose-feeding insect guts. The results suggest that (a) the trace element selenium has shaped the gene content of acetogenic spirochetes in gut communities over evolutionary time scales, (b) acetogenic spirochete populations have undergone extinctions and radiations associated with an evolutionary bottleneck, convergent evolutions, and possibly even invasion during termite evolution, and (c) termite gut acetogenesis is largely mediated by only a few spirochete species, which represent a small portion of total acetogenic spirochete diversity.

II.) The hydrogen-stable isotope compositions (D/H) of lipids in the environment vary greatly. All variations have been assumed to result from changes in the D/H of water, a source of lipid hydrogen. However, several studies suggest that water D/H may not be the only influential factor. In this study, I report that lipid D/H values can vary by 500‰ in bacterial cultures despite constant water D/H. This indicates variations in lipid/water fractionation need to be considered when interpreting environmental data. More significantly, I demonstrate that lipid D/H values are systematically related to the utilization of different central metabolic pathways in bacteria. The results suggest that different cellular mechanisms for NADPH synthesis result in lipids with characteristic D/H. Implications for the use of lipid D/H as an isotopic marker of energy metabolism are discussed.

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