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Trace chemical species in marine incubation experiments, part A. Experiment design and bacterial abundance control extracellular H2O2 concentrations

Journal article
Authors Mark J. Hopwood
Nicolas Sanchez
Despo Polyviou
Øystein Leiknes
Julian A. Gallego-Urrea
Eric P. Achterberg
Murat V. Ardelan
Javier Aristegui
Lennart Bach
Sengul Besiktepe
Yohann Heriot
Ioanna Kalantzi
Tuba Terbıyık Kurt
Ioulia Santi
Tatiana M. Tsagaraki
David R. Turner
Published in Biogeosciences Discussions
ISSN 1810-6277
Publication year 2018
Published at Department of marine sciences
Language en
Links https://www.biogeosciences-discuss....
Subject categories Geochemistry, Water in nature and society, Environmental chemistry, Climate Research, Marine ecology

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 (>1000L) 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 (<1% change in [H2O2] comparing copepod densities from 1–10L−1). Instead, the changes in H2O2 concentration contrasting high/low zooplankton incubations appeared to arise from the resulting changes in bacterial activity. The correlation between bacterial abundance and extracellular H2O2 was stronger in some incubations than others (R2 range 0.09 to 0.55), yet high bacterial densities were consistently associated with low H2O2. Nonetheless, the main control on H2O2 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, H2O2 was persistently elevated (2–6 fold) above ambient concentrations; whereas using closed high density polyethylene mesocosms in Crete, Svalbard and Patagonia H2O2 within incubations was always reduced (median 10–90%) relative to ambient waters.

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