Microbial-driven transformation of dissolved organic nitrogen in estuarine waters: mechanisms and environmental...

Understanding dissolved organic nitrogen (DON) transformation is critical for estuarine nitrogen dynamics, yet microbial contribution and mechanism remain poorly constrained under high terrestrial input. This study reveals that DON in Pearl River Estuary (PRE) exhibits non-conservative mixing jointly regulated by terrestrial inputs, autochthonous production and microbial processes. By integrating field observations, model simulations, and incubation experiments, this study identifies the mid-salinity mixing zone (10-25 PSU) as a transformation hotspot. Based on dark in vitro incubations, the microbial transformation rate was approximately 0.67 ± 0.23 μmol L^-1 h^-1. This process accounted for 77-87% of the total potential DON pool (defined as the sum of measured concentration and transformed DON) in mixing zone. Molecular analysis indicates that while physical dilution primarily drives the decline in bulk aromaticity (AI_mod, DBE), microbial processing qualitatively reshapes the DON pool. Specifically, microbial metabolism promotes the relative accumulation of recalcitrant lignin-like compounds by selectively consuming labile components. Microbial metabolism (particularly Candidate_Actinomarina) is the core driver of DON transformation. FAPROTAX analysis suggests a functional transition from methyl-oxidation-associated N transformations (heterotrophic nitrification and denitrification) to sulfur-oxidation-associated N transformations underlies the DON molecular variations along the salinity gradient. Terrestrial input accelerates biochemical reactions such as demethylation and deamination through the priming effect. Furthermore, mineralization of labile terrestrial DON potentially supplies key substrates that are closely linked to N₂O production. These findings highlight the pivotal role of microbial transformation in regulating DON fate, providing new insights into estuarine nitrogen cycling under strong terrestrial inputs.