Adaptation of methanogens to carbon monoxide: Implications for life on Mars

The search for life on Mars has been a source of great interest throughout history. Any organism capable of surviving in Mars' apparently hostile environment must be capable of utilizing the few things Mars has to offer. A group of organisms that meets this requirement is the methanogens. Much is known about the H2-dependent CO2 reduction pathway used by methanogens. Less is known about methanogenesis from carbon monoxide, a possible alternative methanogenic pathway. Methanosarcina barkeri and Methanobacterium formicicum have been acclimated and adapted to growth on carbon monoxide as an energy source. Several approaches to adaptation were taken in attempt to successfully create carbon monoxide-dependent cultures and to gather information regarding the effects of carbon monoxide on the methanogens. For each of these approaches, a basic experimental procedure was followed, then modified according to each experimental protocol. Test tubes containing 10 mL of organic or inorganic medium were inoculated with 1 mL of M. barkeri or 1 mL of M. formicicum. Carbon monoxide was added to the tubes in varying amounts, according to experimental protocol. Initially, the tubes were pressurized with hydrogen gas; later, this step was omitted. For some experiments, tubes were pressurized with carbon dioxide. The test tubes were incubated at 37°C. Headspace gas was analyzed weekly. Methane production was observed in most tubes and ranged from 0.02% to nearly 50%. This was accompanied by an increase in turbidity and a decrease in carbon monoxide in some tubes. Ultimately, these methanogens were weaned off of molecular hydrogen as an energy source and began using carbon monoxide in inorganic medium. Following the acclimation and adaptation to carbon monoxide, 1 mL aliquots of carbon monoxide-dependent cultures of M. barkeri or M. formicicum were transferred to test tubes containing 5g of Mars soil simulant. These tubes were pressurized with carbon monoxide or carbon monoxide plus carbon dioxide and incubated at 37°C. Headspace gas was analyzed weekly. Methane production has not been observed. Since carbon monoxide as well as methane is found in the Martian atmosphere, these results take on increasing significance to life on Mars research.