Oxic-anoxic regime shifts mediated by feedbacks between biogeochemical processes and microbial community dynamics

Timothy Bush,Jef Huisman,Rosalind J Allen,Gerard Muyzer,Muhe Diao,Ruben Sinnige

doi:10.1038/s41467-017-00912-x

Timothy Bush, Jef Huisman + Show 4 more

Open Access

https://doi.org/10.1038/s41467-017-00912-x

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Abstract

Although regime shifts are known from various ecosystems, the involvement of microbial communities is poorly understood. Here we show that gradual environmental changes induced by, for example, eutrophication or global warming can induce major oxic-anoxic regime shifts. We first investigate a mathematical model describing interactions between microbial communities and biogeochemical oxidation-reduction reactions. In response to gradual changes in oxygen influx, this model abruptly transitions between an oxic state dominated by cyanobacteria and an anoxic state with sulfate-reducing bacteria and phototrophic sulfur bacteria. The model predictions are consistent with observations from a seasonally stratified lake, which shows hysteresis in the transition between oxic and anoxic states with similar changes in microbial community composition. Our results suggest that hysteresis loops and tipping points are a common feature of oxic-anoxic transitions, causing rapid drops in oxygen levels that are not easily reversed, at scales ranging from small ponds to global oceanic anoxic events.

Highlights

Regime shifts are known from various ecosystems, the involvement of microbial communities is poorly understood
The microbial community consists of three functional groups: oxygenproducing cyanobacteria (CB), phototrophic sulfur bacteria (PB) such as purple or green sulfur bacteria, and sulfate-reducing bacteria (SB) (Fig. 1)
Oxygen (O) produced by cyanobacteria is assumed to be inhibitory to both sulfate-reducing bacteria[22] and phototrophic sulfur bacteria[23]

Summary

Introduction

Regime shifts are known from various ecosystems, the involvement of microbial communities is poorly understood. In productive waters, microbial degradation of organic matter can create anoxic conditions in the deeper layers (hypolimnion), which may shift the microbial community to heterotrophic bacteria utilizing nitrate or sulfate as an electron acceptor In between these layers, in the metalimnion, oxygen diffusing down from the epilimnion meets sulfide diffusing up from the hypolimnion, providing a niche for photosynthetic and non-photosynthetic sulfur-oxidizing bacteria[18, 19]. We first present a simple mathematical model of a microbial ecosystem containing cyanobacteria, sulfate-reducing bacteria and phototrophic sulfur bacteria We show that this model can undergo regime shifts between oxic and anoxic states in response to gradual parameter variations that mimic changes in vertical stratification and oxygen diffusivity across the thermocline. We compare the model predictions with data from a seasonally stratified lake with an anoxic hypolimnion during summer, and discuss the wider implications of oxic-anoxic regime shifts for other ecosystems

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