Articles | Volume 12, issue 12
Biogeosciences, 12, 3713–3724, 2015
Biogeosciences, 12, 3713–3724, 2015

Research article 17 Jun 2015

Research article | 17 Jun 2015

Redox regime shifts in microbially mediated biogeochemical cycles

T. Bush1, I. B. Butler2, A. Free3, and R. J. Allen1 T. Bush et al.
  • 1SUPA, School of Physics and Astronomy, University of Edinburgh, King's Buildings, Edinburgh EH9 3FD, UK
  • 2School of Geosciences, University of Edinburgh, King's Buildings, Edinburgh EH9 3FE, UK
  • 3Institute of Cell Biology, School of Biological Sciences, University of Edinburgh, King's Buildings, Edinburgh EH9 3BF, UK

Abstract. Understanding how the Earth's biogeochemical cycles respond to environmental change is a prerequisite for the prediction and mitigation of the effects of anthropogenic perturbations. Microbial populations mediate key steps in these cycles, yet they are often crudely represented in biogeochemical models. Here, we show that microbial population dynamics can qualitatively affect the response of biogeochemical cycles to environmental change. Using simple and generic mathematical models, we find that nutrient limitations on microbial population growth can lead to regime shifts, in which the redox state of a biogeochemical cycle changes dramatically as the availability of a redox-controlling species, such as oxygen or acetate, crosses a threshold (a "tipping point"). These redox regime shifts occur in parameter ranges that are relevant to the present-day sulfur cycle in the natural environment and the present-day nitrogen cycle in eutrophic terrestrial environments. These shifts may also have relevance to iron cycling in the iron-containing Proterozoic and Archean oceans. We show that redox regime shifts also occur in models with physically realistic modifications, such as additional terms, chemical states, or microbial populations. Our work reveals a possible new mechanism by which regime shifts can occur in nutrient-cycling ecosystems and biogeochemical cycles, and highlights the importance of considering microbial population dynamics in models of biogeochemical cycles.

Short summary
Despite their global importance, redox reactions mediated by microorganisms are often crudely represented in biogeochemical models. We show that including the dynamics of microbial growth in such a model can cause sudden shifts between redox states in response to an environmental change. We identify the conditions required for these redox regime shifts, and predict that they are likely in the modern day sulfur and nitrogen cycles, and potentially the iron cycle in the ancient ocean.
Final-revised paper