Distribution of functional microorganisms and its significance for iron, sulphur, and nitrogen cycles in reservoir sediments

The biogeochemical cycles of sulphur (S), iron (Fe) and nitrogen (N) elements play a key role in the reservoir ecosystem. However, the spatial positioning and interrelationship of S, Fe and N cycles in the reservoir sediment profile have not been explored to a greater extent. Here, we measure the gradients of Fe²⁺, SO₄²⁻, NO₃⁻, NH₄⁺, DOC, TC and TN in the pore water of the sediment, and combining the vertical distribution of the functional microorganisms involved in S, Fe and N cyclings in the sediments to determine the redox stratification in the sediment. It is found that the geochemical gradient of S, Fe and N of the reservoir sedimentary column is mainly defined by the redox process involved in the related functional microorganisms. According to the type of electron acceptor, the sediment profile is divided into 3 redox intervals, namely aerobic respiration (0–10 cm), denitrification/iron reduction (10–28 cm) and sulfate reduction (28–32 cm). In the aerobic respiration zone, NH₄⁺ is oxidized by aerobic AOB to NO₃⁻ (0–5 cm), and Fe²⁺ is oxidized by microaerobic FeRB to Fe³⁺ (3–10 cm). In the denitrification/iron reduction zone, Acinetobacter and Pseudomonas, as the dominant NRB genera, may use nitrate as an electron acceptor to oxidize Fe²⁺ (11–16 cm). The dominant genera in SOB, such as Sulfururvum, Thiobacillus and Thioalkalispira, may use nitrate as an electron acceptor to oxidize sulfide, leading to SO₄²⁻ accumulation (14–24 cm). In the sulfate reduction zone, SO₄²⁻ is reduced by SRB. This study found that functional microorganisms forming comprehensive local ecological structures to adapt to changing geochemical conditions, and which would be potentially important for the degradation and preservation of C and the fate of many nutrients and contaminants in reservoirs.