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Biogeosciences An interactive open-access journal of the European Geosciences Union
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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 20 Jun 2018

Submitted as: research article | 20 Jun 2018

Review status
This discussion paper is a preprint. A revision of the manuscript was accepted for the journal Biogeosciences (BG).

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

Mark J. Hopwood1, Nicolas Sanchez2, Despo Polyviou3, Øystein Leiknes2, Julian Gallego-Urrea4, Eric P. Achterberg1, Murat V. Ardelan2, Javier Aristegui5, Lennart Bach1, Sengul Besiktepe6, Yohann Heriot1, Ioanna Kalantzi7, Tuba Terbıyık Kurt8, Ioulia Santi7, Tatiana M. Tsagaraki9, and David Turner4 Mark J. Hopwood et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany
  • 2Norwegian University of Science and Technology, Trondheim, Norway
  • 3Ocean and Earth Science, National Oceanography Centre Southampton, UK
  • 4Marine Sciences, University of Gothenburg, Sweden
  • 5Instituto de Oceanografía y Cambio Global, IOCAG, Universidad de Las Palmas de Gran Canaria, ULPGC, Las Palmas, Spain
  • 6The Institute of Marine Sciences and Technology, Dokuz Eylul University, Turkey
  • 7Institute of Oceanography, Hellenic Centre for Marine Research, Heraklion, Greece
  • 8Marine Biology, Çukurova University, Turkey
  • 9Department of Biological Sciences, University of Bergen, Norway

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 (< 1 % change in [H2O2] comparing copepod densities from 1–10 L−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.

Mark J. Hopwood et al.
Interactive discussion
Status: final response (author comments only)
Status: final response (author comments only)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Mark J. Hopwood et al.
Mark J. Hopwood et al.
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Short summary
Hydrogen peroxide, H2O2, is formed naturally in sunlight exposed water by photochemistry. At high concentrations it is undesirable to biological cells because it is a stressor. Here, across a range of incubation experiments in diverse marine environments (Gran Canaria, the Mediterranean, Patagonia and Svalbard), we determine that two factors consistently affect the H2O2 concentrations irrespective of geographical location; bacteria abundance and experiment design.
Hydrogen peroxide, H2O2, is formed naturally in sunlight exposed water by photochemistry. At...