Primary Energy Metabolism in Geothermal Environments:The Role of Carbon Monoxide

Abstract

Chemolithotrophic metabolism fuels primary production in many hydrothermal ecosystems, representing energy conservation strategies that may be independent of sunlight. Many autotrophic metabolic pathways in geothermal communities depend on H2; however, in this paper we focus on anaerobic carboxydotrophs that are capable of using CO as a carbon and energy source, producing H2 and CO2. We have recently completed the genome sequence of an autotrophic carboxydotroph, Carboxydothermus hydrogenoformans, which appears to be a specialist in CO-oxidation, encoding five unlinked genetic loci capable of expressing CO dehydrogenase. Because of their global occurrence in geothermal environments, we propose that CO-utilizing organisms play important roles in anaerobic microbial consortia. To understand the role of CO-utilizing bacteria in Yellowstone hot springs we isolated and described a novel H2-producing bacterium (strain Nor1) from an Fe-rich site in Norris Geyser Basin, YNP. Strain Nor1 is a low-G+C Gram-positive bacterium belonging to the division Firmicutes, and grows chemolithotrophically at 75°C on CO (generation time 1.5 h), producing equimolar quantities of H2 and CO2. Strain Nor1 is also capable of chemoautotrophic growth on FeIII and Se, as well as growing heterotrophically on glucose and several sugars, producing acetate, H2, and CO2. We have proposed that strain Nor1 be assigned to a new genus, Thermosinus gen. nov. The type species is Thermosinus carboxydivorans sp. nov. (type strain, Nor1T =DSM 14886T; Sokolova et al. 2004). It is evident that CO-dependent hydrogenogenic metabolism occurs in a diverse phylogenetic context and can be accompanied by a varied repertoire of alternative trophic strategies