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Evaluating Aerobic Microbial Methane Cycling under Archaean-Proterozoic Environmental Conditions

Final Report Summary - EMCAPE (Evaluating Aerobic Microbial Methane Cycling under Archaean-Proterozoic Environmental Conditions)

The overall goal of this project was to generate high-resolution data for understanding patterns in microaerophilic biological methane cycling across the Archaean/Proterozoic boundary, linked to the rise of atmospheric O2 and climate ~2.9-1.8 billion years ago (Ga). Specific aims were to reconstruct microaerophilic microbial methane oxidation kinetics and ultimately to derive empirical models for the activity of these microorganisms with the evolution of ocean chemistry. Copper geochemistry and traditional microbiological culture work was to be used to answer five key questions:

(1) Does methanotrophic bacterial growth on various Cu-doped ferrihydrite minerals result in significant Cu stable isotope fractionation between biomass and mineral phases?
(2) Did dissolved oceanic Cu concentrations change, following the development of oxygenated surface waters during the early stages of the Great Oxidation Event?
(3) Are copper isotopes fractionated between ferrihydrite minerals and organic matter deposited in a range of Precambrian banded iron formations?
(4) Do copper isotopic signatures in various iron formations correlate with methanotrophic bacterial activity?
(5) Can Cu isotopic fractionation signatures be used as biomarkers for tracing the link between Precambrian atmospheric CH4 concentrations and the rise of atmospheric O2?

(i) Microbial culture analysis. Various Cu-doped ferrihydrite minerals were synthesized and analyzed by powdered X-ray diffraction (XRD) for mineralogical composition. Cu/Fe molar ratios were determined using inductively coupled plasma optical emission spectrometry (ICP-OES). Two types of methanotrophs were successfully grown on these various minerals, which acted as a sole source of copper. Two key aspects pertinent to the project became clear.

(1) The Cu/Fe molar ratios conditionally affect growth and methane oxidation rates, controlling the type of enzyme used for methane oxidation. Sufficient mineral-Cu favoured the expression of the di-Cu enzyme. In low Cu the di-Fe enzyme was employed for methane oxidation.
(2) Cells fractionate copper during methane oxidation. The lighter Cu isotopes are associated with biomass and heavy Cu enriched in the residual ferrihydrite mineral. These fractionation patterns were proportional to growth and methane oxidation rates.

(ii) Rock analysis: Copper distribution in shale samples deposited between ∼2.6 and ∼1.6 Ga, indicated that oceanic copper concentrations might have increased dramatically after the great oxidation event (GOE), at ~2.45 Ga when oxygen concentrations rose from nothing to ∼5% of present day levels (Holland, 2006). This was followed by a dramatic decline at ~2.1 Ga, towards pre-GOE levels. The Cu trends were suggested to reflect patterns in organic matter deposition through the same time interval. Together with records in banded iron formations, deposited ∼3.7-0.6 billion years ago, a three-stage change in the evolution of Cu in the Archaean/Proterozoic oceans could be inferred: A low Archaean Cu stage, a Cu-rich Palaeoproterozoic stage which terminates with a return to low levels in the Neoproterozoic.

Potential impact of results: Potential autotrophic pathways that would likely dominate in a methane-rich atmosphere, as was the case in the early atmosphere (Kasting, 2005; Konhauser et al. 2009; Zahnle et al. 2006), especially before the rise of atmospheric oxygen, was studied. We found that pathways favouring Cu-dependent methane oxidation likely were important for organic carbon deposition, especially in the Palaeoproterozoic. Combining this information with the results gained on the growth of the various archetypical methanotrophs on different Cu/Fe molar ratios and the Cu/Fe molar ratios measured for the geological materials, including those reported in the public literature for banded ironstones deposited ∼3.0-1.6 billion years ago, three key scenarios arose:

(1) Methanotrophic bacteria activity likely increased substantially at the onset of the GOE, coupled to increased copper bioavailability.
(2) This activity likely peaked at ~2.1 Ga coinciding with a global episode of anomalous high deposition of marine organic carbon known as the Lomagundi event (Kump et al., 2011; Maheshwari et al., 2010).
(3) They were active from the start to the end of the Palaeoproterozoic glaciations (Kopp et al., 2005), suggesting a role in the regulation of both atmospheric methane concentrations, facilitation of the rise of atmospheric oxygen and dramatic fluctuations in the Palaeoclimate


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Contact: Ernest Chi Fru (