Intermittent cyanobacterial accumulation reshapes water column sulfur cycling and favors sulfur-oxidizing denitrifiers in freshwater lake mesocosms

Recent studies have established that cyanobacterial bloom biomass (CBB) decomposition critically drives sulfur cycling in the water column of shallow lakes. While CBB can accumulate via single pulses or intermittent inputs depending on hydroclimatic conditions, how these distinct patterns affect sulfur cycling remains poorly understood. Here, using a 37-day mesocosm experiment, we show that single-pulse (SP) and intermittent (IP) inputs drive divergent sulfur-nitrogen coupling dynamics, as revealed by biogeochemical monitoring, fluorescence spectroscopy, 16S rRNA sequencing, and real-time quantitative PCR of functional genes ( soxB , a marker for sulfur oxidation, and nirS , a marker for denitrification). SP inputs trigger rapid degradation of labile tyrosine/tryptophan-like organics and a transient sulfide peak. This is followed by a successional community shift from sulfate-reducing bacteria to microaerophilic sulfur oxidizers (e.g., Thiovirga ), coinciding with substantial nitrate accumulation (> 20 mg L⁻¹ on day 24). Conversely, IP inputs sustain prolonged anoxia and continuous sulfide production, selectively enriching Sulfurovum (53.96 ± 15.68%), a chemolithoautotroph capable of coupling sulfur oxidation with denitrification. Concurrently, soxB and nirS gene abundances are markedly elevated on day 37 (6.00 × 10 4 and 6.41 × 10 5 copies mL⁻ 1 , respectively), with nitrate remaining persistently low (< 10 mg L⁻ 1 ) throughout the incubation. These findings indicate that intermittent CBB inputs promote sustained sulfide generation and increase microbial potential for sulfur oxidizing denitrification. This study provides new insight into coupled sulfur and nitrogen cycling during cyanobacterial blooms in freshwater lakes.