High Potential Microbial Denitrification in Non-hypoxic Intermediate Waters of the South China Sea Basin

Jian Zeng^1*Baohong Chen²Guozong Shi^1,3Yanyun Guan^1,4Zesheng Zhuo¹

• ¹Xiamen Ocean Vocational College, Xiamen, China
• ²Third Institute of Oceanography Ministry of Natural Resources, Xiamen, China
• ³Applied Technology Engineering Center of Fujian Provincial Higher Education for Marine Resource Protection and Ecological Governance, Xiammen, China
• ⁴Xiamen Key Laboratory of Intelligent Fishery, Xiammen, China

The discovery of microbial denitrification in non-extreme oxygen-deficient environments has drawn growing attention. At the same time, it is reshaping previous understanding of the spatial pattern of marine nitrogen (N) sinks. In this study, we investigated the potential microbial denitrification in non-hypoxic subsurface and intermediate waters of the South China Sea (SCS) basin. A series of 15N-isotope tracers incubation experiments, combined with functional gene characterization and hydro-chemical parameters analysis, were conducted during cruise. The results revealed that representative denitrification functional genes (narG and nirS) are ubiquitously presented at moderate abundances (0.1×104~12×105 copies/L) across the water columns. In the intermediate waters (600~1500 m) with low dissolved oxygen (DO) saturation (20%~30%), weak in situ denitrification rates (0.2~1.1 nmol N₂/L/d) were detected. However, under simulated anoxic conditions, active denitrification was detected in most sampling layers, with potential rates (0.2~33 nmol N₂/L/d) comparable to those in typical oxygen-deficient zones (ODZs). This suggests that denitrification potential in the SCS basin has long been overlooked. Furthermore, significant correlations between suspended particulate matter (SPM) and particulate organic carbon (POC), contents with both denitrification rates and functional gene abundances were also observed. It is inferred that low ambient DO levels, as well as hypoxic micro-niches in particulate matter, may together drive denitrification - 2 - occurrence in the basin waters. Besides, particulate matter plays a critical role in influencing metabolic activity and spatial variability of denitrification in the basin. Since the mid-water of the SCS basin sustains a large particulate loading from terrestrial input and hydrodynamics, it is likely to maintain strong denitrification potential in the water body. We further propose a preliminary framework of coupling between particle transport driven by complex environmental dynamics and microbial N removal. Our study not only provides a potential implication for the need to re-evaluate the N budgets in the SCS basin, but also offers a new perspective of mechanism interpretation for microbial N removal in non-hypoxic marine environments.