Abstract
ABSTRACTSulfate-reducing bacteria (SRB) and sulfur-oxidizing bacteria drive major transformations in the sulfur cycle, and play vital roles in oxic--anoxic transitions in lakes and coastal waters. However, information on the succession of these sulfur bacteria in seasonally stratified lakes using molecular biological techniques is scarce. Here, we used 16S rRNA gene amplicon sequencing to study the spatio-temporal dynamics of sulfur bacteria during oxic--anoxic regime shifts in Lake Vechten. Oxygen and sulfate were mixed throughout the water column in winter and early spring. Meanwhile, SRB, green sulfur bacteria (GSB), purple sulfur bacteria (PSB), and colorless sulfur bacteria (CSB) exclusively inhabited the sediment. After the water column stratified, oxygen and nitrate concentrations decreased in the hypolimnion and various SRB species expanded into the anoxic hypolimnion. Consequently, sulfate was reduced to sulfide, stimulating the growth of PSB and GSB in the metalimnion and hypolimnion during summer stratification. When hypoxia spread throughout the water column during fall turnover, SRB and GSB vanished from the water column, whereas CSB (mainly Arcobacter) and PSB (Lamprocystis) became dominant and oxidized the accumulated sulfide under micro-aerobic conditions. Our results support the view that, once ecosystems have become anoxic and sulfidic, a large oxygen influx is needed to overcome the anaerobic sulfur cycle and bring the ecosystems back into their oxic state.
Highlights
Oxygen depletion may lead to hypoxia and anoxia in lakes, coastal waters and the open ocean, which is detrimental to many aquatic organisms (Diaz and Rosenberg 1995; VaquerSunyer and Duarte 2008; Breitburg et al 2018)
When hypoxia spread throughout the water column during fall turnover, Sulfate-reducing bacteria (SRB) and green sulfur bacteria (GSB) vanished from the water column, whereas colorless sulfur bacteria (CSB) and purple sulfur bacteria (PSB) (Lamprocystis) became dominant and oxidized the accumulated sulfide under micro-aerobic conditions
Our results show that Lake Vechten is characterized by distinct seasonal changes in the oxygen, sulfate and sulfide concentrations and a marked seasonal succession of a wide diversity of sulfur bacteria
Summary
Oxygen depletion may lead to hypoxia and anoxia in lakes, coastal waters and the open ocean, which is detrimental to many aquatic organisms (Diaz and Rosenberg 1995; VaquerSunyer and Duarte 2008; Breitburg et al 2018). The model predicts that the transition from oxic to anoxic water may occur in the form of a regime shift. The SRB cause an increase in sulfide concentration, which suppresses cyanobacterial growth, and the lake rapidly shifts from an oxic into an anoxic state. A large oxygen influx is needed to bring the system back into its oxic state. These model predictions are supported by observations from a seasonally stratified lake, which shows hysteresis in the transition between oxic and anoxic states with similar changes in microbial community composition as predicted by the model (Bush et al 2017). Hysteresis effects have been reported for anoxia in coastal ecosystems (e.g. Conley et al 2009; Middelburg and Levin 2009; Zhang et al 2010)
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