Dissimilatory perchlorate reduction linked to aerobic methane oxidation via chlorite dismutase
Abstract
The presence of methane (CH4) in the atmosphere of Mars is controversial yet the evidence has aroused scientific interest, as CH4 could be a harbinger of extant or extinct microbial life. There are various oxidized compounds present on the surface of Mars that could serve as electron acceptors for the anaerobic oxidation of CH4, including perchlorate (ClO4-). We examined the role of perchlorate, chlorate (ClO3-) and chlorite (ClO2-) as oxidants linked to CH4 oxidation. Dissimilatory perchlorate reduction begins with reduction of ClO4- to ClO2- and ends with dismutation of chlorite to yield chloride (Cl-) and molecular oxygen (O2). We explored the potential for aerobic CH4 oxidizing bacteria to couple with oxygen derived from chlorite dismutation during dissimilatory perchlorate reduction. Methane (0.2 kPa) was completely removed within several days from the N2-flushed headspace above cell suspensions of methanotrophs (Methylobacter albus strain BG8) and perchlorate reducing bacteria (Dechloromonas agitata strain CKB) in the presence of 5 mM ClO2-. Similar rates of CH4 consumption were observed for these mixed cultures whether they were co-mingled or segregated under a common headspace, indicating that direct contact of cells was not required for methane consumption to occur. We also observed complete removal of 0.2 kPa CH4 in bottles containing dried soil (enriched in methanotrophs by CH4 additions over several weeks) and D. agitata CKB and in the presence of 10 mM ClO2-. This soil (seasonally exposed sediment) collected from the shoreline of a freshwater lake (Searsville Lake, CA) demonstrated endogenous CH4 uptake as well as perchlorate, chlorate and chlorite reduction/dismutation. However, these experiments required physical separation of soil from the aqueous bacterial culture to allow for the partitioning of O2 liberated from chlorite dismutation into the shared headspace. Although dissimilatory reduction of ClO4- and ClO3- could be inferred from the accumulation of chloride ions either in spent media or in slurries prepared from Searsville Lake soil, neither of these oxyanions evoked methane oxidation when added to either anaerobic mixed cultures or soils enriched in methanotrophs. This result leads us to surmise that the release of O2 during enzymatic perchlorate reduction was low, and that the oxygen produced was unavailable to the aerobic methanotrophs. This was borne out by patterns of O2 and CO2 production during experiments with lake soil, growth media, and pure cultures of dissimilatory perchlorate reducing bacteria. We observed that O2 release during incubation of D. agitata CKB with 10 mM ClO4- or ClO3- was decoupled from metabolism. More O2 was released during incubations without added acetate than with 10 mM acetate and an even greater amount of O2 was released during incubation with heat-killed cells. This suggests a chemical mechanism of O2 production during reaction with ClO4- and ClO3-. Hence, perchlorate reducing bacteria need not be present to facilitate O2 release from the surface of Mars, in support of recent interpretations of Viking LR and GEx experiments.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.P51F1790O
- Keywords:
-
- 0406 BIOGEOSCIENCES Astrobiology and extraterrestrial materials;
- 0456 BIOGEOSCIENCES Life in extreme environments;
- 0471 BIOGEOSCIENCES Oxidation/reduction reactions;
- 5200 PLANETARY SCIENCES: ASTROBIOLOGY