Metabolic activities in deep subseafloor sediments
Microbes in subseafloor sediments account for a significant fraction of the Earth's total biomass. It has been argued that these communities are sustained at extremely low respiration rates, based on estimates of sulfate reduction rates and cell abundances. It is conceivable, however, that respiration rates may be much higher due to iron reduction. Iron reduction is difficult to directly quantify due to the removal of dissolved Fe(II) from pore waters. Additionally, hypothetically competing respiration pathways have been suggested to co-occur in deep subseafloor sediments based on qualitative analysis of chemical profiles. Yet, the vertical distributions of these processes and the extent of their co-occurrence have not been objectively quantified. The controls on these co-occurrences have been relatively unexplored. ^ First, an objective and robust approach for quantifying metabolic reaction rates was developed that is applicable to deep sediments. The approach was successfully tested using synthetic concentration profiles. It was also applied to dissolved sulfate, sulfide, methane, and manganese from a 420-meter-thick sediment column, ODP Site 1226 in the eastern equatorial Pacific. Our results provide quantitative evidence of co-occurrence of multiple hypothetically competing respiration reactions in deep subseafloor sediments. ^ Second, a method was developed to quantify rates of subseafloor carbon oxidation and iron reduction based on mass-balance of CO2, alkalinity, and dissolved sulfate. At Site 1226, the subseafloor microbes live at extremely low respiration rates. Iron reduction accounts for 6 to 16% of the total C oxidized. ^ Third, thermodynamic properties of multiple respiration reactions were investigated at depths where dissolved sulfide is detectable (7 to 275 m below seafloor). Co-occurring sulfate reduction and acetotrophic methanogenesis are energetically favorable and mediated throughout these depths to their biologically utilizable energy minima. Sulfate-reducing methanotrophy is energetically favorable and may co-occur cryptically. The average energy of iron reduction is approximately equal to that of sulfate reduction. We infer that microbial iron reduction occurs in the depth intervals where sulfate reduction occurs. This subseafloor sedimentary ecosystem appears to be a thermodynamic homeostat, which maintain multiple respiration pathways for millions of years close to their biological energy minima through feedbacks between these reactions.* ^ *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Microsoft Office; MatLab.^
"Metabolic activities in deep subseafloor sediments"
Dissertations and Master's Theses (Campus Access).