Microbial responses to ocean deoxygenation

Abstract

The oceans are losing oxygen (O2) as the combined result of rising global temperatures and increasing anthropogenic nutrient inputs. This deoxygenation impacts directly on marine ecology, including increased mortality rate, impaired growth, and altered behaviour of marine organisms, and indirect effects such as climate feedbacks and altered biogeochemical cycles. With progressive deoxygenation, heterotrophic respiration is increasingly channelled through microbial metabolism, and capacity to predict this respiration response is key to forecasting ocean deoxygenation and ensuing impacts on marine ecosystems and biogeochemical cycles. Despite its importance to ocean deoxygenation, respiratory responses by microorganisms, in particular microeukaryotes, remain underexplored. Microbial respiration and growth are considered as the base of the food web influencing the ecology and biogeochemistry of marine environments. Reliable prediction of ocean deoxygenation and associated impacts hence relies on models with robust and comprehensive descriptions of microbial responses. Current descriptions of these responses are, however, oversimplified due to limited empirical information and this represents one of the major uncertainties in model predictions. This thesis hence aims to reduce these uncertainties by providing new empirical knowledge on microbial responses to ocean deoxygenation. To create this new knowledge on microbial responses to declining O2, and develop predictive rate equations, I explored respiration kinetics in model bacteria, a microeukaryote, and in coastal microbial communities using ultra-sensitive optical O2 sensors. Additionally, the influence of dissolved O2 concentrations on the growth kinetics of yeast, a model microeukaryote, were studied. The new knowledge created will help to predict and mange ocean O2 loss through its incorporation into future model developments.