Experiment design and bacterial abundance control extracellular H 2 O 2 concentrations during four series of mesocosm experiments

• Published in: Biogeosciences
• Main Authors: Hopwood, MJ, Sanchez, N, Polyviou, D, Leiknes, O, Gallego-Urrea, JA, Achterberg, EP, Ardelan, MV, Aristegui, J, Bach, L, Besiktepe, S, Herlot, Y, Kalantzi, I, Kurt, TT, Santi, I, Tsagaraki, TM, Turner, D
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus GmbH 2020
• Subjects: Biological Sciences; Other biological sciences; Global change biology; Patagonia; Svalbard; Calanus finmarchicus
• Online Access: https://doi.org/10.5194/bg-17-1309-2020; http://ecite.utas.edu.au/139162

The extracellular concentration of H 2 O 2 in surface aquaticenvironments is controlled by a balance between photochemical production and the microbial synthesis of catalase and peroxidase enzymes to remove H 2 O 2 from solution. In any kind of incubation experiment, theformation rates and equilibrium concentrations of reactive oxygen species(ROSs) such as H 2 O 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 2 O 2 production and catalaseperoxidase enzyme production rates differ between species. Whilst there are extensive measurements of photochemical H 2 O 2 formation rates and the distribution of H 2 O 2 in the marine environment, it is poorly constrained how different microbial groups affect extracellular H 2 O 2 concentrations, how comparable extracellular H 2 O 2 concentrations within large-scale incubation experiments are to thoseobserved in the surface-mixed layer, and to what extent a mismatch withenvironmentally relevant concentrations of ROS in incubations couldinfluence biological processes differently to what would be observed innature. Here we show that both experiment design and bacterial abundanceconsistently exert control on extracellular H 2 O 2 concentrations across a range of incubation experiments in diverse marine environments. During fourlarge-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 2 O 2 concentrations. For example, in threeof fourexperiments where pH was manipulatedto0.40.5 below ambientpH, no significant change was evident inextracellular H 2 O 2 concentrations relative to controls. Aninfluence was sometimes inferred from zooplankton density, but notconsistently between different incubation experiments, and no change in H 2 O 2 was evident in controlled experiments using differentdensities of the copepod Calanus finmarchicus grazing on the diatom Skeletonema costatum ( <1 % change in[ H 2 O 2 ] comparing copepod densities from 1 to 10 L −1 ). Instead, the changes in H 2 O 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 2 O 2 was stronger in some incubations than others ( R 2 range0.09 to0.55), yet high bacterial densities were consistently associated with low H 2 O 2 . Nonetheless, the main control on H 2 O 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 2 O 2 was persistently elevated (26-fold) above ambient concentrations; whereas using closed high-density polyethylene mesocosms in Crete, Svalbard and Patagonia H 2 O 2 within incubations was always reduced (median 10 %) relative to ambient waters.