Methanosarcinales biogeochemistry, implications for methane cycling

Open Access
Author:
Moran, James Joseph
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 28, 2007
Committee Members:
  • Katherine Haines Freeman, Committee Chair
  • Christopher Howard House, Committee Chair
  • James Kasting, Committee Member
  • Kenneth James Davis, Committee Member
Keywords:
  • anaerobic methane oxidation
  • dimethyl sulfide
  • methanogens
  • RNA selection
  • stable isotope analysis
Abstract:
The anaerobic oxidation of methane (AOM) is a poorly understood process. It consumes large amounts of methane and is a significant component of the global carbon and methane cycles. I employed an interdisciplinary approach to help enlighten AOM by looking at both the activity of related processes and incubations of AOM-containing sediments. Trace methane oxidation (TMO) is a process, potentially related to AOM, by which archaeal methane producers (methanogens) oxidize small amounts of methane. I used a stable isotope tracer to examine this process in multiple archaeal sulfate reducer and methanogen cultures (with emphasis on Methanosarcina acetivorans cultures). I provide new evidence that co-enzyme M reductase, a key enzyme of methanogenesis, is likely involved in AOM. I also observed that a suite of oxidizing agents included in the incubations did not enhance TMO rates. Finally, I demonstrated that, under specific culture conditions, the rate of TMO to methyl sulfide products in M. acetivorans is nearly seven times greater than to any product ever previously reported for TMO. I used incubations of active AOM sediment to demonstrate that hydrogen does not play a metabolic role in AOM consortia. These experiments, combined with thermodynamic and geochemical considerations, suggest that methyl sulfides may be the long-sought intermediate metabolite and crucial to the AOM metabolism. The potential role of methyl sulfides in AOM increases the need for understanding their formation in related pathways. I demonstrate the first instance of energy conservation through methyl sulfide production by a methanogen, M. acetivorans, and the first documented instance of methyl sulfide production from a carbon monoxide substrate. I also developed a new approach for isolating taxon-specific RNA sequences in preparation for stable isotope analysis. I used a DNAzyme containing a catalytic motif with variable recognition domains. I used M. acetivorans RNA to develop and test the approach in preparation for potential application in AOM investigations. Finally, in testing the potential influence of oxidizing agents on TMO I discovered that M. acetivorans was capable of growth under a microaerophilic atmosphere. I tested and compared the ability of other strict anaerobes to withstand similar oxidative stress.