High Phosphate Concentrations Accelerate Bacterial Peptide Degradation in Hypoxic Bottom Waters of the Northern Gulf of Mexico

Understanding mechanisms of organic matter decomposition in hypoxic waters becomes more important with increasing coastal regions affected by hypoxia. The development and dynamics of hypoxia in coastal oceans have been studied quite extensively, but the factors controlling decomposition rates and pathways of labile organic matter, keys to understanding hypoxia formation, are poorly understood. We investigated peptide degradation in a stratified water column in the hypoxic region of northern Gulf of Mexico by on-deck incubation experiments. Our results showed that decomposition efficiency of small peptides may be limited by the availability of soluble reactive phosphorus (P_i) in the surface water. In contrast, peptide degradation rate in the subsurface water was twice as high as that in the surface water. Once peptides were amended for incubation, high concentrations of P_i in subsurface waters led to the rapid growth of bacterial strains with high RNA contents, resulting in efficient peptide degradation. Consistent with the growth rate hypothesis, these results indicate that a high level of P_i is crucial in stimulating the growth of bacterial strains with high RNA contents and further organic matter decomposition in marine environments. This high decomposition potential of organic matter in subsurface hypoxic waters presents a positive feedback on the formation of hypoxia in coastal ocean, as a higher decomposition rate of organic matter leads to faster consumption of dissolved oxygen.