Differences in peptide decomposition rates and pathways between hypoxic and oxic coastal environments

Understanding the mechanisms of organic matter decomposition in hypoxic waters becomes more important globally with increasing hypoxia in coastal regions. Peptide decomposition rates were measured in low-oxygen bottom water and compared to those in oxygenated surface water of a stratified 18-m water column at a hypoxic northern Gulf of Mexico station. Water from each depth was spiked with small peptides alanine–valine–phenylalanine–alanine (AVFA) and VFA and incubated onboard for ca. 80 h to determine peptide decomposition rates, decomposition products, and bacterial community structure shifts. Decomposition rates of amended peptides were twice as high in the bottom water as in the surface water, and the pathways of peptide decomposition differed. In surface waters, the small peptides were hydrolyzed into individual amino acids extracellularly by aminopeptidase, based on the analysis of hydrolyzed fragments; hydrolysis rates remained constant throughout the incubation period. In contrast, in bottom waters the amended peptides were metabolized rapidly into ammonium by bacteria through either direct uptake or tight coupling of extracellular hydrolysis and subsequent uptake after an initial adaptation period (~ 22 h). Dissolved oxygen level did not affect peptide decomposition patterns significantly in surface or bottom water, when it was manipulated by sparging with air or nitrogen gas. The high efficiency of peptide decomposition in bottom waters corresponded to the rapid growth of several genera of Alpha- and Gammaproteobacteria. These bacterial species may have caused rapid peptide decomposition, but the exact mechanisms remain unclear. The high efficiency of peptide decomposition in hypoxic regions illustrates biogeochemical feedback to hypoxia formation in stratified coastal regions.