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

Abstract

Eutrophication-driven coastal hypoxia has been of great interest for decades, though its mechanisms remain not fully understood. Here, we showed elevated concentrations of particulate and dissolved polyunsaturated aldehydes (PUAs) associated with the hypoxic waters in the bottom layer of a salt-wedge estuary. Bacterial respiration within the hypoxic waters was mainly contributed by particle-attached bacteria (PAB) (> 0.8 µm), with free-living bacteria (0.2–0.8 µm) only accounting for 25 %–30 % of the total rate. The concentrations of particle-adsorbed PUAs (∼ 10 µmol L−1) in the hypoxic waters were directly quantified for the first time based on large-volume filtration and subsequent on-site PUA derivation and extraction. PUA-amended incubation experiments for PAB (> 25 µm) associated with sinking or suspended particles retrieved from the low-oxygen waters were also performed to explore the impacts of PUAs on the growth and metabolism of PAB and associated oxygen utilization. We found an increase in cell growth of PAB in response to low-dose PUAs (1 µmol L−1) but an enhanced cell-specific bacterial respiration and production in response to high-dose PUAs (100 µmol L−1). Improved cell-specific metabolism of PAB in response to high-dose PUAs was also accompanied by a shift of PAB community structure with increased dominance of the genus Alteromonas within the Gammaproteobacteria. We thus conclude that a high PUA concentration associated with aggregate particles within the bottom layer may be crucial for some species within Alteromonas to regulate PAB community structure. The change in bacteria community could lead to an enhancement of oxygen utilization during the degradation of particulate organic matter and thus likely contribute to the formation of coastal hypoxia. These findings are potentially important for coastal systems with large river inputs, intense phytoplankton blooms driven by eutrophication, and strong hypoxia developed below the salt-wedge front.