Comparative Analysis of Anaerobic Carbon Monoxide Oxidation in Saline and Non-saline Soils and Sediments at Ambient and Elevated Temperatures.

Carbon monoxide (CO) is a ubiquitous, biogeochemically active molecule that likely played an important role in the origin of life on Earth, and that might also fuel microbial life in extraterrestrial systems. CO enters microbial biogeochemistry through oxidative reactions based on two distinct CO dehydrogenase enzyme systems (CODH). One of these requires a molybdenum co-factor (Mo-CODH), is largely confined to oxic systems, and has an uncertain evolutionary history. The second system requires a nickel co-factor (Ni-CODH), is largely confined to anaerobic systems (with some notable exceptions), and almost certainly evolved very early. Of the two, the biogeochemistry of the Mo-CODH system has been explored in modest detail, primarily due to its significance in regulating atmospheric CO. In contrast, relatively little is known about the distribution, activity and biogeochemical significance of the Ni-CODH system, although the biochemistry and microbiology of the system have been well documented. In order to understand the contemporary reach and significance of an ancient biogeochemical system, we have initiated a comparative analysis of the distribution of anaerobic CO oxidation using two levels of CO, 10 ppm and 25%, in headspaces of samples representing a range of forest and agricultural soils; freshwater lake, swamp and hot spring sediments; saline soils and muds, and salt crusts incubated at 25^oC and elevated temperatures (60-86 ^oC). Our results indicate that the capacity for oxidation of 10 ppm CO at 25^oC is widely distributed, with uptake rates typically comparable to those for the oxygen-driven Mo-CODH. Activity also occurs in anoxic hot spring sediments collected and incubated at 60^oC, but not at 86^oC. The capacity for CO oxidation at concentrations of 25% was surprisingly more limited. The most rapid activity was observed for mesothermal soils and sediments incubated at 60^oC, although in some cases activity was also noted during incubations at 25^oC. In these cases, CO oxidation appeared to proceed via a hydrogenic reaction rather than via acetogenesis. Activity did not occur in salt-saturated muds, which suggests that salinity stresses might constrain the distribution and activity of Ni-CODH. These results provide new insights into the ecology of one of life's oldest processes.