Kinetics of biological hydrogen peroxide production

Abstract

Reactive oxygen species (ROS) such as hydrogen peroxide (H2O2), superoxide (O2-), and hydroxyl radical (HO•) are important to the biogeochemical cycling of trace metals and carbon and could potentially have significant effects on marine processes. This study includes projects that examine both biological production and decay rates of H2O2 (and, to a lesser extent, O2-) at both the macroscopic (ecosystem) and microscopic (cellular) level. Dark production rates (PH2O2) and decay rate coefficients (kloss,H2O2) were determined by spiked batch incubations in the Prochlorococcus-dominated waters of Station ALOHA. Both parameters were low but exhibited considerable day-to-day variability, ranging from 0.0 to 2.9 nM hr-1 for PH2O2 and from 0.010 to 0.048 hr-1 for kloss,H2O2. A strong correlation was found between PH2O2 and kloss,H2O2 in the mixed layer. Concentration of biota yielded higher values for PH2O2 (4.5 to 8.4 nM hr-1) and enrichment incubations increased values of kloss,H2O2 (0.199 to 0.293 hr-1). Overall, biological production of H2O2 was expected to be significant compared to photochemical production. Production rates for both H2O2 and O2- of five species of marine diatoms were measured side by side by loading cells onto filters and using chemiluminescence detection of the ROS. In addition, the ability of these organisms to break down these ROS was examined by measuring recovery of O2- and H2O2 added to the analytical medium. O2- production rates ranged from zero to 7.3 x 10-16 mol cell-1 hr 1, while H2O2 production rates ranged from zero to 3.4 x 10-16 mol cell-1 hr-1. Results suggest that extracellular ROS production occurs through a variety of mechanisms even amongst similar organisms. In all organisms, recovery rates for killed cultures (94-100% H2O2; 10-80% O2-) were consistently higher than those for live cultures (65-95% H2O; 10-50% O2-). While recovery rates for killed cultures in H2O2 indicate that nearly all H2O2 is degraded by active cell processes, O2- decay appears to occur via a combination of active and passive processes. Overall, rates and pathways for ROS production and decay were shown to vary greatly from species to species, even among those that are closely related. Finally, twelve species of bacteria were evaluated for H2O2 production and decay both by the filter-loading method used for bacteria as well as the spiked batch incubation method used at Station ALOHA. Cell-normalized production rates (PH2O2,cell) ranged from zero to 3.81 amol cell-1 hr-1 and cell-density normalized decay rates (kH2O2,cell) ranged from 2.18 to 212 x 10-8 hr-1 (cell mL-1)-1. Values for kH2O2,cell and PH2O2,cell from the two methods generally agreed with each other, but in a few organisms there was a significant difference that indicated that H2O2 production might vary depending on an organism’s surroundings. Comparison of PH2O2,cell with previously published O2- production rates suggests that, as with diatoms, a variety of pathways are likely to be responsible for H2O2 production. Rates for kH2O2,cell and PH2O2,cell suggest that bacteria may be primary contributors to H2O2 decay in the ocean and may be one of the primary contributors to biological H2O2 production.