Trace chemical species in marine incubation experiments, part A. Experiment design and bacterial abundance control extracellular H<sub>2</sub>O<sub>2</sub> concentrations

Abstract. The extracellular concentration of H2O2 in surface aquatic environments is controlled by a balance between photochemical production and the microbial synthesis of catalase and peroxidase enzymes to remove H2O2 from solution. In any kind of incubation experiment, the formation rates and equilibrium concentrations of ROS may be sensitive to both the experiment design (particularly to the regulation of incident light) and the abundance of different microbial groups (as both cellular H2O2 production and catalase/peroxidase enzyme production rates differ between species). Whilst there are extensive measurements of photochemical H2O2 formation rates and the distribution of H2O2 in the marine environment, it is poorly constrained how different microbial groups affect extracellular H2O2 concentrations, how comparable extracellular H2O2 concentrations within large scale incubation experiments are to those observed in the surface-mixed layer, and to what extent a miss-match with environmentally relevant concentrations of ROS in incubations could influence biological processes differently to what would be observed in nature. Here we show that both experiment design and bacterial abundance consistently exert control on extracellular H2O2 concentrations across a range of incubation experiments in diverse marine environments. During 4 large scale (> 1000 L) mesocosm experiments (in Gran Canaria, the Mediteranean, Patagonia and Svalbard) most experimental factors appeared to exert only minor, or no, direct effect on H2O2 concentrations. For example, in 3 of 4 experiments where pH was manipulated (to 0.4–0.5 below ambient pH) no significant change was evident in extracellular H2O2 concentrations relative to controls. An influence was sometimes inferred from zooplankton density, but not consistently between different incubation experiments and no change in H2O2 was evident in controlled experiments using different densities of the copepod Calanus finmarchichus grazing on the diatom Skeletonema costatum (