"Earth’s surficial chemistry evolved in close association with life, exemplified by the incorporation of trace metals into the active sites of many enzymes involved in major biogeochemical processes and as structural components of proteins. Consequently, trace metal bioavailability has likely exerted a major influence upon rates of specific microbial processes throughout Earth history. Changes in the bioavailability of trace metals would have had a significant impact on microbial processes and biogeochemical elemental cycling across Earth’s first Great Oxidation Event (GOE) ca. 2.3 billion years ago (Ga), when atmospheric O2 rose from essentially nothing to ca. 1-5% of present atmospheric levels (PAL). At this time, ocean chemistry evolved from anoxic and Fe-rich to the widespread occurrence of deeper water sulphidic conditions. During this early oxygenation period, dominant microbial pathways would have shifted considerably. For example, microaerophilic CH4 oxidation predicted to have evolved by 2.7 Ga, may have become more important as surface waters became oxygenated, and could have reduced CH4 emissions into the Precambrian atmosphere by up to 90%. This could have potentially contributed to the development of widespread glaciations by 2.3 Ga, and to the depletion of a significant sink for O2, thereby facilitating the rise of atmospheric O2. However, rates of microbial activity and the interaction of various key microorganisms under the chemical conditions existing at that time remain poorly constrained. In this proposal, microbial experiments will be combined with Cu (regulates the synthesis, expression and activity of CH4 monooxygenases) isotopic analyses of key late Archaean to mid-Proterozoic geologic successions, to test hypotheses pertinent to aerobic biological CH4 cycling under Archaean-Proterozoic environmental conditions. New insights linking aerobic biological CH4 oxidation to the rise of atmospheric O2 and climate in the deep past will be established."