Graduate Thesis Or Dissertation
 

Establishment and dynamics of methane-oxidizing microbial communities in marine sediments

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  • Marine sediments are vast sources and reservoirs of methane, a potent greenhouse gas. Most of this methane is anaerobically oxidized by archaea before it can reach the overlying ocean, though the efficiency of this process often depends on methane fluxes and mechanisms of fluid transport. Anaerobic methanotrophic archaea, or ANME, often aggregate with sulfate-reducing bacteria in consortia using sulfate as an electron acceptor, and thus are active at sulfate-methane interfaces of marine sediments. ANME are known to double on the order of months, but have not been isolated in pure culture. Their temporal responses to variation in sulfate supply or methane flux as sediment conditions change are not well understood, but would help constrain estimates of methane emission to the hydrosphere. The focus of this dissertation was to gain an integrated understanding of how ANME, sulfate reducing bacteria, microbial community composition, and rates of methane oxidation change in response to fluctuations in biogeochemical conditions. Using two field studies and one laboratory incubation, we hypothesized that methane influx or addition would increase ANME and sulfate-reducing bacteria populations and abundances as well as methane oxidation rates, but only after lag periods of several months. Samples were collected from Storfjordrenna, offshore Svalbard in the high Arctic, where increases in methane flux inferred from reactive-transport modeling brought methane into shallower sediment horizons. Sediments from this area were also incubated at in situ temperature and pressure for different lengths of time and under different methane concentrations. At the active Venere mud volcano in the Mediterranean, microbial communities from a freshly extruded summit mud breccia flow were analyzed in combination with geochemical data to infer the development of methanotrophy and examine whether deep-sourced fluids hosted unique microbial communities. At Venere mud volcano, methane and sulfate structured microbial communities to a greater extent than deep-sourced fluids, and methanotrophs present in mud flows consisted of aerobic Gammaproteobacteria. These low-biomass fresh mud flows, probably a few years old, are likely too young for active ANME populations to develop. In contrast, at Storfjordrenna, ANME were most abundant near sulfate-methane transitions where methane was moving up the sediment column, suggesting these populations can respond after a year or less of methane intrusion. This methane intrusion fueled a boom in ANME and sulfate-reducing bacteria and decreased community diversity, which reverted after methane influx stopped. Some parallels were also observed in incubations: rates of anaerobic methane oxidation and ANME percent abundances increased with added methane concentrations, but only in a sediment from an active area of seepage and after a few months. These results provide a path forward for understanding the dynamics and capacity of this globally significant microbial subseafloor methane filter, and further the understanding of how microbial community structures and activities are linked.
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  • Ongoing Research
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  • 2019-03-19 to 2021-04-20

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file details Klasek_Dissertation.final.pdf 2019-03-18 公开 下载