Experiment design and bacterial abundance control extracellular H2O2 concentrations during four series of mesocosm experiments

Abstract

The extracellular concentration of H ₂O ₂ in surface aquatic environments is controlled by a balance between photochemical production and the microbial synthesis of catalase and peroxidase enzymes to remove H ₂O ₂ from solution. In any kind of incubation experiment, the formation rates and equilibrium concentrations of reactive oxygen species (ROSs) such as H ₂O ₂ 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 H ₂O ₂ production and catalase-peroxidase enzyme production rates differ between species. Whilst there are extensive measurements of photochemical H ₂O ₂ formation rates and the distribution of H ₂O ₂ in the marine environment, it is poorly constrained how different microbial groups affect extracellular H ₂O ₂ concentrations, how comparable extracellular H ₂O ₂ concentrations within large-scale incubation experiments are to those observed in the surface-mixed layer, and to what extent a mismatch 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 H ₂O ₂ concentrations across a range of incubation experiments in diverse marine environments. During four large-scale (> 1000 L) mesocosm experiments (in Gran Canaria, the Mediterranean, Patagonia and Svalbard) most experimental factors appeared to exert only minor, or no, direct effect on H ₂O ₂ concentrations. For example, in three of four experiments where pH was manipulated to 0.4-0.5 below ambient pH, no significant change was evident in extracellular H ₂O ₂ concentrations relative to controls. An influence was sometimes inferred from zooplankton density, but not consistently between different incubation experiments, and no change in H ₂O ₂ was evident in controlled experiments using different densities of the copepod Calanus finmarchicus grazing on the diatom Skeletonema costatum (< 1 % change in [H ₂O ₂] comparing copepod densities from 1 to 10 L-1). Instead, the changes in H ₂O ₂ concentration contrasting high-and low-zooplankton incubations appeared to arise from the resulting changes in bacterial activity. The correlation between bacterial abundance and extracellular H ₂O ₂ was stronger in some incubations than others (R ² range 0.09 to 0.55), yet high bacterial densities were consistently associated with low H ₂O ₂. Nonetheless, the main control on H ₂O ₂ concentrations during incubation experiments relative to those in ambient, unenclosed waters was the regulation of incident light. In an open (lidless) mesocosm experiment in Gran Canaria, H ₂O ₂ was persistently elevated (2-6-fold) above ambient concentrations; whereas using closed high-density polyethylene mesocosms in Crete, Svalbard and Patagonia H ₂O ₂ within incubations was always reduced (median 10 %-90 %) relative to ambient waters.

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