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.
Supplementary material to "Low molecular weight organic anions (carboxylates) increase microbial activity and alter microbial community composition in uncontaminated and diesel contaminated soil"