Climate-Driven Harmful Algal Blooms Impair the Coastal Nitrogen Filter and Shift Denitrification Pathways toward N2O Accumulation

Climate change is expanding harmful algal blooms (HABs) beyond nutrient-driven paradigms, yet their effects on coastal nitrogen cycling remain poorly understood. Here, we investigated a climate-driven dry-season Phaeocystis globosa bloom in Xiamen Bay, a subtropical coastal embayment, using field observations, isotopic incubations, and metagenomics. Contrary to the conventional view that HABs stimulate denitrification, the bloom suppressed sedimentary denitrification by ∼70% and reduced total dissolved excess gaseous nitrogen (ΔN2 + ΔN2rO) by ∼50% relative to the pre-bloom period. Despite this decline in nitrogen removal, N2O yield (ΔN2O/(ΔN2 + ΔN2O)) increased by approximately an order of magnitude from 0.04% to 0.30%, indicating a shift toward incomplete denitrification. This shift was linked to a sulfur-mediated microbial reorganization. In seawater, sulfur-metabolizing denitrifiers, particularly Roseobacter, capable of utilizing algal-derived sulfur compounds (e.g., DMSP), were enriched and became dominant. These taxa harbored clade I nosZ, whose sensitivity to oxygen and pH likely constrained N2O reduction. In sediments, chemolithoautotrophic sulfur-oxidizing denitrifiers, particularly Sulfurovum, became dominant and were associated with reduced N2 production. Together, these compartment-specific responses weakened denitrification and shifted its end-product composition toward a higher N2O share, revealing a sulfur-coupled microbial mechanism by which climate-driven HABs impair the coastal nitrogen filter.