Geochemical Requirements of the anaerobic oxidation of methane in the Eel River Basin

Open Access
Author:
Beal, Emily J
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 27, 2009
Committee Members:
  • Christopher Howard House, Dissertation Advisor
  • Christopher Howard House, Committee Chair
  • Kevin Patrick Furlong, Committee Chair
  • Charles Raymond Fisher Jr., Committee Member
  • Katherine Haines Freeman, Committee Member
Keywords:
  • thermal history
  • Anaerobic Methane Oxidation
  • early Earth
  • ferrihydrite
  • birnessite
  • sulfate
Abstract:
Although the anaerobic oxidation of methane (AOM) is a widely studied process, many of the geochemical requirements for it remain a mystery, in part because the responsible organisms are not in pure culture. It has been shown that freshwater AOM proceeds with nitrite and nitrate. However, before this study the only known electron acceptor in marine AOM was sulfate. The work of this study helps to illuminate some of the requirements of marine AOM in the Eel River Basin (ERB), CA, focusing on the methane source and electron acceptors which allow for this globally significant process to proceed. In Chapter 2, I use a finite difference thermal history model to indicate areas within the ERB that are capable of thermogenic methane production. Using the model results, I propose a correlation between areas with high rates of hydrocarbon production, methane seep location, and thus the areas within the ERB where high rates of AOM occur. The results of this study not only provide a potential link between geophysics/tectonics and microbiology, but also provide target areas within the ERB that could be used for microbiologic studies. Chapters 3 and 4 are incubation studies, targeted at understanding the role of electron acceptors, using sediment from methane seeps in the ERB. Methane oxidation is monitored by measuring the incorporation of 13C, from 13CH4, into the carbon dioxide in the headspace. In Chapter 3, I examine how the rate of AOM changes at varying sulfate concentrations, with a focus on concentrations lower than 1 mM. Although it is often stated that methane oxidation occurs in a 1:1 ratio with sulfate reduction, I find that at these low sulfate concentrations, methane oxidation and sulfate reduction are uncoupled, with methane oxidation rates sometimes an order of magnitude higher than sulfate reduction rates. Our experimentally determined rates of AOM are then put into an early Earth atmospheric photochemical model where it is shown that AOM causes a faster rise of oxygen and faster re-rise of methane than models that do not contain AOM. In Chapter 4, I test whether electron acceptors other than sulfate can be used in marine AOM. My results show the first direct evidence that both manganese (in the form of birnessite) and iron (in the form of ferrihydrite) can be used in marine AOM. Although the rates of manganese- and iron-dependent AOM are slower than sulfate-dependent AOM, these processes have the potential to gain more energy from methane oxidation. In addition, manganese- and iron-dependent AOM have the potential to be significant processes on early Earth when sulfate levels were extremely low. Chapter 5 continues the study of manganese- and iron-dependent AOM using phylogenetics and fluorescence in situ hybridization (FISH). In addition we incubated the experiments demonstrating manganese-dependent AOM with 15NH4Cl, during which active cells incorporate the 15N, and measured target aggregates from the incubation using FISH coupled to secondary ion mass spectrometry (FISH-SIMS) to determine the active cells in our incubation. Based on phylogenetic analysis, we find that both manganese- and iron-dependent AOM appear to be performed by distinct microbial assemblages and/or mechanism as compared to sulfate dependent AOM. SIMS analysis of aggregates in the manganese incubation indicate that mixed and mixed-cluster aggregates (of archaea and bacteria) and archaea of sarcina morphology are active and thus are likely responsible for manganese-dependent AOM.