Diversity and activity of aerobic thermophilic carbon monoxide-oxidizing bacteria on Kilauea Volcano, Hawaii

Aerobic carbon monoxide (CO) oxidation is used by phylogenetically and physiologically diverse microorganisms inhabiting a variety of terrestrial and aquatic ecosystems. Activity assays, culture-based studies, and molecular-based approaches targeting the coxL gene, encoding the large subunit of CO dehydrogenase, were used to investigate the role of temperature in structuring CO-oxidizing communities at Kilauea Volcano, Hawai’i. CO uptake activity was assessed for unvegetated and vegetated temperate volcanic deposits that experience different temperature regimes due to plant development during ecosystem succession. Both CO-oxidizing communities had similar short-term responses to temperature; however, results from extended incubations (30 d) at elevated temperature (55 °C) indicate that succession expanded the capacity of the vegetated community to adapt to high temperature. Aerobic CO uptake was also examined for geothermal sites including two soils and two microbial biofilms at the Puhimau geothermal area and Kilauea Iki crater. CO oxidation occurred at elevated temperatures for all sites assessed; however, cardinal temperatures for CO activity were not strongly correlated to in situ temperatures. These results also extended the known upper temperature limit (80 °C) for aerobic CO oxidation. Culture-based methods targeting thermophiles at these sites yielded 31 newly isolated thermophilic CO-oxidizing strains in 8 genera. Two strains were formally described as novel species. The isolation of multiple Thermogemmatispora strains (Class: Ktedonobacteria) led to exploration of CO oxidation in this group; CO oxidation was found to be a common trait among Ktedonobacteria. Additionally, geothermally-heated biofilms at Puhimau were dominated by Ktedonobacteria as determined from coxL clone libraries, 16S rRNA gene pyrosequencing, and analysis of coxL fragments from a biofilm metagenome. Thermophiles are known to exist in temperate environments, but their maintenance and activity remain unclear. We examined the activity of thermophilic CO-oxidizing bacteria under different temperature regimes including 25 °C, 55 °C, and an oscillating temperature regime (20 – 55 °C). Three hours per day above 45 °C was sufficient for growth and CO oxidation activity. CO oxidation at moderate temperatures could contribute to maintenance metabolism and survival of thermophiles under suboptimal conditions. Collectively these studies show that thermophilic CO oxidizers are active and abundant in thermal systems on Kilauea Volcano