The habitability of Earth is dependent upon the global recycling of elements\udessential for life, such as nitrogen, sulfur and carbon. Nutrient-cycling by micro-organisms\udis vital to these biogeochemical cycles because many key steps are\udmediated primarily, or exclusively, by microbial life. The dynamics of these cycles\udare highly complex, and environmental perturbations (such as changes in the\udoceanic oxygen concentration) can have unexpected or catastrophic effects; often\udcausing abrupt switches between chemical states. Despite the importance of these\udenvironmental perturbations however, few theoretical models have addressed how\udthey affect the dynamical behaviour of nutrient-cycling microbial ecosystems.\udIn this work, we investigate the effect of environmental perturbations on\udmicrobially-mediated nutrient cycles and assess the likelihood of "sudden transitions"\udbetween chemical states of the ecosystem occurring in a variety of ecological\udcontexts. To do this, we first use computational modelling of microbial nutrient-cycling,\udusing a "box model" approach. We then move on to an experimental\udstudy using the microbial sulfur cycle as a model ecosystem, with freshwater\udpond sediment/water microcosms. These microcosms have the advantage\udof retaining many of the features of the real ecosystem (such as microbial\uddiversity, spatial structure, and abiotic interactions) while allowing the controlled\udmanipulation of environmental perturbations. We study these microcosms using\uda combination of chemical measurements and high-throughput sequencing of\udthe microbial community. Finally, we return to the computational side, and\udattempt to reproduce chemical data from our experiments in a mathematical\udmodel containing realistic abiotic chemical interactions.
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