AUTHOR=Hopwood Mark J. , Riebesell Ulf , Arístegui Javier , Ludwig Andrea , Achterberg Eric P. , Hernández Nauzet 

TITLE=Photochemical vs. Bacterial Control of H2O2 Concentration Across a pCO2 Gradient Mesocosm Experiment in the Subtropical North Atlantic

JOURNAL=Frontiers in Marine Science

VOLUME=5

YEAR=2018

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2018.00105

DOI=10.3389/fmars.2018.00105

ISSN=2296-7745

ABSTRACT=<p>In the surface ocean, microorganisms are both a source of extracellular H<sub>2</sub>O<sub>2</sub> and, via the production of H<sub>2</sub>O<sub>2</sub> destroying enzymes, also one of the main H<sub>2</sub>O<sub>2</sub> sinks. Within microbial communities, H<sub>2</sub>O<sub>2</sub> sources and sinks may be unevenly distributed and thus microbial community structure could influence ambient extracellular H<sub>2</sub>O<sub>2</sub> concentrations. Yet the biogeochemical cycling of H<sub>2</sub>O<sub>2</sub> and other reactive oxygen species (ROS) is rarely investigated at the community level. Here, we present a time series of H<sub>2</sub>O<sub>2</sub> concentrations during a 28-day mesocosm experiment where a pCO<sub>2</sub> gradient (400–1,450 μatm) was applied to subtropical North Atlantic waters. Pronounced changes in H<sub>2</sub>O<sub>2</sub> concentration were observed over the duration of the experiment. Initially H<sub>2</sub>O<sub>2</sub> concentrations in all mesocosms were strongly correlated with surface H<sub>2</sub>O<sub>2</sub> concentrations in ambient seawaters outside the mesocosms which ranged from 20 to 92 nM over the experiment duration (Spearman Rank Coefficients 0.79–0.93, <italic>p</italic>-values &lt; 0.001–0.015). After approximately 9 days of incubation however, H<sub>2</sub>O<sub>2</sub> concentrations had increased across all mesocosms, later reaching &gt;300 nM in some mesocosms (2–6 fold higher than ambient seawaters). The correlation with ambient H<sub>2</sub>O<sub>2</sub> was then no longer significant (<italic>p</italic> &gt; 0.05) in all treatments. Furthermore, changes in H<sub>2</sub>O<sub>2</sub> could not be correlated with inter-day changes in integrated irradiance. Yet H<sub>2</sub>O<sub>2</sub> concentrations in most mesocosms were inversely correlated with bacterial abundance (negative Spearman Rank Coefficients ranging 0.59–0.94, <italic>p</italic>-values &lt; 0.001–0.03). Our results therefore suggest that ambient H<sub>2</sub>O<sub>2</sub> concentration can be influenced by microbial community structure with shifts toward high bacterial abundance correlated with low extracellular H<sub>2</sub>O<sub>2</sub> concentrations. We also infer that the nature of mesocosm experiment design, i.e., the enclosure of water within open containers at the ocean surface, can strongly influence extracellular H<sub>2</sub>O<sub>2</sub> concentrations. This has potential chemical and biological implications during incubation experiments due to the role of H<sub>2</sub>O<sub>2</sub> as both a stressor to microbial functioning and a reactive component involved in the cycling of numerous chemical species including, for example, trace metals and haloalkanes.</p>