Carbon, nitrogen, and sulfur cycling interactions in organic carbon rich extreme environments
Bowles, Marshall Wayne
MetadataShow full item record
The interactions of dissimilatory processes were studied in organic carbon rich extreme environments, on the basis of geochemical signatures, molecular microbiology, and microbial activity assessments. In cold seep and hydrothermally-altered sediments dissimilatory processes such as nitrate reduction (DNF), sulfate reduction (SR), anaerobic oxidation of methane (AOM), and bicarbonate reduction based-methanogenesis (MOG) were measured. Sediments with bacterial mats and exposed gas hydrates, which are surficial features associated with the active seepage of reduced compounds (e.g. sulfide and methane), were targeted during this work and typify the general sample description. In a Gulf of Mexico cold seep we measured the highest reported rates of SR, and noted a large disparity in comparison to the lower AOM rates. These findings led to a global compilation of all ex situ rates of simultaneously reported SR and AOM rates. The current process based understanding of AOM suggests that the ratio of SR to AOM should be 1:1 based on the reaction stoichiometry of one mol sulfate reduced and methane oxidized. In the global compilation (n = 53 cores) of ex situ rates the median ratio of SR to AOM was 10.7:1. The median global integrated rate of AOM, representing a general cold seep AOM rate, was only 5% of the previously published estimate for AOM rates at cold seeps. These ex situ AOM rate estimates are performed at atmospheric pressure, where saturated methane concentrations might be significantly lower than methane concentrations in situ. Based on reaction kinetics at higher methane concentrations rates of AOM should be higher. To address these issues we developed a novel method wherein in situ methane concentrations could be achieved by applying pressure to a sample amended with gaseous headspace. The method was described and a pressure and methane concentration effect was seen for processes such as SR, AOM, and MOG in samples from cold seeps in the Gulf of Mexico and Monterey Bay. These results explicitly show that degassed sediments used for ex situ rate estimates do not describe microbial activities in deep sea sediments, particularly methane rich environments.