Interactions between microorganisms and oxic-anoxic transitions

Hypoxia and anoxia are detrimental for most aerobic organisms. Oxygen consumption by microorganisms can facilitate the development of hypoxia and anoxia and, vice versa, microbial activity is also strongly affected by oxygen availability. To explore the interactions between microorganisms and oxic-anoxic transitions, this thesis investigates the dynamic changes in microbial community composition (especially of microorganisms involved in the sulfur and nitrogen cycle) in a seasonally stratified lake (Lake Vechten). Specifically, the following research questions have been addressed: 1. How do oxic-anoxic transitions affect bacterial community dynamics? 2. How are microbial and chemical feedbacks involved in oxic-anoxic transitions? 3. How do oxic-anoxic transitions affect the microbial sulfur and nitrogen cycle? In general, the bacterial community composition at different depths in the lake diverged when the bottom layers became anoxic during summer stratification and converged when the lake was mixed. Mathematical model and field observations show that gradual changes in oxygen influx can induce major oxic-anoxic regime shifts, associated with hysteresis loops and tipping points. The dynamics of various sulfur bacteria during oxic-anoxic transitions show that once ecosystems have become anoxic and sulfidic, a large oxygen influx is needed to bring the ecosystem back into the oxic state. Nitrogen microorganisms in the water column and sediment displayed a pronounced seasonal succession, which was closely linked to the oxic-anoxic transitions. In particular, our results show that microbial communities do not only track seasonal changes in environmental conditions, but also affect and modify the environment through their involvement in biogeochemical oxidation-reduction processes.