Long-term sulfide input enhances chemoautotrophic denitrification rather than DNRA in freshwater lake sediments

Partitioning between nitrate reduction pathways, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) determines the fate of nitrate removal and thus it is of great ecological importance. Sulfide (S²⁻) is a potentially important factor that influences the role of denitrification and DNRA. However, information on the impact of microbial mechanisms for S²⁻ on the partitioning of nitrate reduction pathways in freshwater environments is still lacking. This study investigated the effects of long-term (108 d) S²⁻ addition on nitrate reduction pathways and microbial communities in the sediments of two different freshwater lakes. The results show that the increasing S²⁻ addition enhanced the coupling of S²⁻ oxidation with denitrification instead of DNRA. The sulfide-oxidizing denitrifier, Thiobacillus, was significantly enriched in the incubations of both lake samples with S²⁻ addition, which indicates that it may be the key genus driving sulfide-oxidizing denitrification in the lake sediments. During S²⁻ incubation of the Hongze Lake sample, which had lower inherent organic carbon (C) and sulfate (SO₄²⁻), Thiobacillus was more enriched and played a dominant role in the microbial community; while during that of the Nansi Lake sample, which had higher inherent organic C and SO₄²⁻, Thiobacillus was less enriched, but increasing abundances of sulfate reducing bacteria (Desulfomicrobium, Desulfatitalea and Geothermobacter) were observed. Moreover, sulfide-oxidizing denitrifiers and sulfate reducers were enriched in the Nansi Lake control treatment without external S²⁻ input, which suggests that internal sulfate release may promote the cooperation between sulfide-oxidizing denitrifiers and sulfate reducers. This study highlights the importance of sulfide-driven denitrification and the close coupling between the N and S cycles in freshwater environments, which are factors that have often been overlooked.