Dissertation/Thesis Abstract

The anaerobic oxidation of short-chain alkanes in hydrothermal vents
by Adams, Melissa Marie, Ph.D., Harvard University, 2014, 124; 3626342
Abstract (Summary)

Microorganisms are central to the cycling of methane on Earth. The anaerobic oxidation of methane (AOM) is a globally important process in anoxic marine sediments, which is often coupled to the reduction of sulfate by anaerobic methanotroph (ANME) archaea and sulfate reducing bacteria (SRB). However, the environmental and geochemical conditions that constrain these microbial communities remain largely uncharacterized. In this dissertation, I present evidence that methane and C2-C4 alkanes are substantial sources of metabolic energy in sedimented hydrothermal vent systems. Furthermore, these studies demonstrate that AOM and C2-C4 alkane oxidation linked to sulfate reduction (SR) are governed by temperature and substrate availability.

Using continuous-flow sediment bioreactors and batch incubations, rate measurements revealed a striking uncoupling of AOM from SR across the thermal gradient characteristic of hydrothermal vent sediments, with AOM occurring at the highest temperature (90°C) in the absence of SR. I discovered that ANMEs were present in sediments at all temperatures investigated, including a unique thermophilic ANME clade, while SRB were only detected in cooler regimes. Next, I present data from batch incubations demonstrating for the first time that substantial C2-C4 alkane oxidation occurs over a broad temperature range (25 - 75°C) in hydrothermal vent sediments and is coupled to SR up to 75°C with maximum rates at 55°C. Furthermore, there was preferential degradation of C2-C4 alkanes at 55°C, indicating that the active alkane oxidizers are thermophilic. At the rate maxima, 16S rRNA pyrotag sequence data revealed that a novel SRB lineage was the likely phylotype mediating the anaerobic oxidation of C 2-C4 alkanes.

Finally, I present a comparative study of methane- and sulfur-cycling ecotypes in geographically separated hydrothermal vent and hydrocarbon seep sediments. By employing high throughput sequencing of 16S rRNA genes coupled to geochemical data, I was able to establish the environmental constraints that determine ANME and SRB ecotype diversity and distribution in these biogeochemically distinct deep sea habitats. In summary, this dissertation sheds light on the pervasive effects of temperature, substrate availability, and habitat type on these metabolic processes that are critical for the cycling of carbon and sulfur in deep sea hydrothermal vent sediments.

Indexing (document details)
Advisor: Girguis, Peter R.
Commitee: Cavanaugh, Colleen M., Johnston, David T., Knoll, Andrew H.
School: Harvard University
Department: Biology, Organismic and Evolutionary
School Location: United States -- Massachusetts
Source: DAI-B 75/10(E), Dissertation Abstracts International
Subjects: Microbiology, Geochemistry
Keywords: Hydrocarbon, Hydrothermal, Metabolism, Microbial, Sulfur, Vent
Publication Number: 3626342
ISBN: 9781321012538
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