Life has a profound impact on the bulk composition of the oceans and atmosphere, with every major species (except Argon) directly or indirectly under biological control. N₂, O₂, and CO2 are byproducts of and substrates for various microbial guilds, which drive biogeochemical cycling in marine and terrestrial environments, the balance of which depends on the availability of primary substrates and limiting nutrients. In low-oxygen regions that are analogs for the early Earth (such as the deep Black Sea and oxygen minimum zones), other more reducing metabolic products (H₂, H₂S, CH₄, etc.) are also consumed and/or produced. These examples attest to Bass Becking's classic microbiological dogma "Everything is everywhere, but the environment selects", a predictive concept that microbial communities (or more broadly - metabolisms) will exist wherever the necessary nutrients, substrates, and ambient conditions for growth are available.

This project will numerically explore this idea by creating a 1-D biogeochemical model to quantify air-sea gas fluxes produced by oxygen-limited biospheres. The model will follow C, S, and O cycling through biological transformations in the water column and sediments and report fluxes of CH₄, CO₂, O2, H₂S, SO₂, DMS, and CH₃SH across the air-sea interface. These fluxes will ultimately be coupled to atmospheric chemistry models in order to make predictions about ancient atmospheric chemistry. External variables will be temperature and trace metal (e.g., Mo, Ni, Cu) concentrations, and metabolisms (initially oxygenic and anoxgenic phototrophs, methanogens, methanotrophs, and sulfate reducers) will be formulated based on kinetic rates from biological literature and available free energy (e.g., Hoehler 2007).

The model differs from existing biogeochemical models that generally prescribe specific trophic levels. Instead, a "Flask World"-type approach (Williams, 2007) will allow model organisms to migrate to maximize substrate/nutrient availability. The modelling will build on previous Archean ecosystems models (Canfield 2006; Hoehler 2007) and expand upon those providing a weak coupling with Archean photochemical models (Kharecha 2005). Ground truth will be provided by geomicrobiological data from modern anoxic ecosystems, such as the Black Sea and stratified lakes. Further training in modelling/Earth Systems theory will arise from coupling the fluxes predicted by the ecosystem model into a photochemical model, which will be used to interpret geochemical records from the Archean.

This PhD studentship is tied to a NERC funded project coupling geochemical records from the late Archean with atmospheric models to investigate hypotheses related to biological regulation of Earth's early atmosphere (e.g., Zerkle et al., 2012). The student will join an interdisciplinary team of experts from UEA, Newcastle, and the University of Maryland, enabling other training opportunities in field work, isotope geochemistry, and geobiology. A 1st class (or Masters) degree in a Physical science and numerical experience are preferred, although candidates with exceptionally strong 2nd class degrees will be considered.

• Canfield D. et al. (2006) Early anaerobic metabolisms. Phil.Trans. Roy. Soc. B 361(1474), 1819-1834.

• Hoehler T. (2007) An energy balance concept for habitability. Astrobiology. 7(6), 824-838.

• Kharecha P., et al. (2005) A coupled atmosphere-ecosystem model of the early Archean Earth. Geobiology. 3, 53-76.

• Williams, H. and Lenton, T. (2007) The Flask model: emergence of nutrient-recycling microbial ecosystems and their disruption by environment-altering 'rebel' organisms. Oikos. 116, 7 1087-1105

• Zerkle A., et al. (2012) A bistable organic-rich atmosphere on the Neoarchaean Earth. Nature Geoscience. 5,5 359-363.