Selective reorganization of DOM and lifestyle-dependent microbe-DOM coupling along a coastal seasonal oxygen gradient

Seasonal oxygen depletion in stratified coastal bays drives shifts in dissolved organic matter (DOM) composition and microbial community structure, yet how DOM composition and microbial assemblages co-reorganize during the subsequent recovery phase remains poorly understood. We examined bottom-water DOM and prokaryotic assemblages across four cruises (August-October 2019) in a seasonally low-oxygen bay of the North Yellow Sea, where stratification breakdown generated a pronounced dissolved oxygen (DO) recovery gradient (2.5-7.2 mg L^-1). Integrating bulk and fluorescent DOM characterization, FT-ICR MS molecular profiling, and 16S rRNA sequencing of free-living (FL) and particle-attached (PA) fractions, we show that DOM reorganization along this gradient was selective rather than uniform. DOC concentrations and a blue-shifted marine humic-like component increased with rising DO, while SUVA₂₅₄ and red-shifted humic-like components declined. FT-ICR MS identified 1919 redox-sensitive formulae (RSFs) partitioned into DO-negative (sulfur-enriched) and DO-positive (CHON-enriched) pools, indicating systematic molecular-level compositional shifts during reoxygenation. Prokaryotic community composition co-varied significantly with DO independently of temperature and differed markedly between FL and PA lifestyles, although the magnitude of compositional turnover along the gradient was comparable in the two fractions (DO × lifestyle interaction, p = 0.575). Bipartite co-occurrence networks nevertheless revealed that FL communities maintained significantly denser associations with RSFs than PA communities. Together, these findings indicate that the seasonal oxygen-recovery transition acts as a selective geochemical filter, restructuring both DOM molecular composition and DOM-microbe associations in a lifestyle-dependent manner in organically enriched coastal systems.