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. Eutrophication-driven coastal hypoxia has been of great interest fordecades, though its mechanisms remain not fully understood. Here, we showedelevated concentrations of particulate and dissolved polyunsaturatedaldehydes (PUAs) associated with the hypoxic waters in the bottom layer of asalt-wedge estuary. Bacterial respiration within the hypoxic waters wasmainly contributed by particle-attached bacteria (PAB) (&gt; 0.8 µm), with free-living bacteria (0.2–0.8 µm) only accounting for25 %–30 % of the total rate. The concentrations of particle-adsorbed PUAs(∼ 10 µmol L−1) in the hypoxic waters were directlyquantified for the first time based on large-volume filtration andsubsequent on-site PUA derivation and extraction. PUA-amended incubationexperiments for PAB (&gt; 25 µm) associated with sinking orsuspended particles retrieved from the low-oxygen waters were also performedto explore the impacts of PUAs on the growth and metabolism of PAB andassociated oxygen utilization. We found an increase in cell growth of PAB inresponse to low-dose PUAs (1 µmol L−1) but an enhancedcell-specific bacterial respiration and production in response to high-dosePUAs (100 µmol L−1). Improved cell-specific metabolism of PAB inresponse to high-dose PUAs was also accompanied by a shift of PAB communitystructure with increased dominance of the genus Alteromonas within the Gammaproteobacteria.We thus conclude that a high PUA concentration associated with aggregateparticles within the bottom layer may be crucial for some species withinAlteromonas to regulate PAB community structure. The change in bacteria community couldlead to an enhancement of oxygen utilization during the degradation ofparticulate organic matter and thus likely contribute to the formation ofcoastal hypoxia. These findings are potentially important for coastalsystems with large river inputs, intense phytoplankton blooms driven byeutrophication, and strong hypoxia developed below the salt-wedgefront.

Impacts of biogenic polyunsaturated aldehydes on metabolism and community composition of particle-attached bacteria in coastal hypoxia