Biochemical Barriers on the Path to Ocean Anoxia?

The kinetics of microbial respiration suggests that, if excess organic matter is present, oxygen should fall to nanomolar levels in the range of the Michaelis-Menten constants (K(m)). Yet even in many biologically productive coastal regions, lowest observed O(2) concentrations often remain several orders of magnitude higher than respiratory K(m) values. We propose the hypoxic barrier hypothesis (HBH) to explain this apparent discrepancy. The HBH postulates that oxidative enzymes involved in organic matter catabolism are kinetically limited by O(2) at concentrations far higher than the thresholds for respiration. We found support for the HBH in a meta-analysis of 1,137 O(2) K(m) values reported in the literature: the median value for terminal respiratory oxidases was 350 nM, but for other oxidase types, the median value was 67 μM. The HBH directs our attention to the kinetic properties of an important class of oxygen-dependent reactions that could help explain the trajectories of ocean ecosystems experiencing O(2) stress.