Abstract
In the surface ocean, microorganisms are both a source of extracellular H2O2 and, via the production of H2O2 destroying enzymes, also one of the main H2O2 sinks. Within microbial communities, H2O2 sources and sinks may be unevenly distributed and thus microbial community structure could influence ambient extracellular H2O2 concentrations. Yet the biogeochemical cycling of H2O2 and other reactive oxygen species (ROS) is rarely investigated at the community level. Here, we present a time series of H2O2 concentrations during a 28-day mesocosm experiment where a pCO2 gradient (400–1,450 μatm) was applied to subtropical North Atlantic waters. Pronounced changes in H2O2 concentration were observed over the duration of the experiment. Initially H2O2 concentrations in all mesocosms were strongly correlated with surface H2O2 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, p-values 300 nM in some mesocosms (2–6 fold higher than ambient seawaters). The correlation with ambient H2O2 was then no longer significant (p > 0.05) in all treatments. Furthermore, changes in H2O2 could not be correlated with inter-day changes in integrated irradiance. Yet H2O2 concentrations in most mesocosms were inversely correlated with bacterial abundance (negative Spearman Rank Coefficients ranging 0.59–0.94, p-values < 0.001–0.03). Our results therefore suggest that ambient H2O2 concentration can be influenced by microbial community structure with shifts toward high bacterial abundance correlated with low extracellular H2O2 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 H2O2 concentrations. This has potential chemical and biological implications during incubation experiments due to the role of H2O2 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.
Highlights
Reactive oxygen species (ROS) are ubiquitous in sunlit natural surface waters (Van Baalen and Marler, 1966; Moore et al, 1993; Miller and Kester, 1994)
Differences between the eight enclosures were small compared to the detection limit of the analytical method (
Photochemical processes are thought to be the dominant influence on ambient H2O2 concentrations in the surface ocean (O’Sullivan et al, 2005; Steigenberger and Croot, 2008), with large scale spatial variations typically explained in terms of latitudinal changes in light and ocean temperature (Yocis et al, 2000; Yuan and Shiller, 2001)
Summary
Reactive oxygen species (ROS) are ubiquitous in sunlit natural surface waters (Van Baalen and Marler, 1966; Moore et al, 1993; Miller and Kester, 1994). The most extensively measured ROS in the marine environment, H2O2, is present in the surface mixed layer at concentrations on the order of 10–100 nM (Price et al, 1998; Yuan and Shiller, 2001; Gerringa et al, 2004). Extracellular H2O2 is not generally considered to be a major constraint on cellular growth under natural conditions in the marine environment because most microorganisms are thought to produce catalase and peroxidase enzymes which control H2O2 decomposition rates in the surface ocean (Moffett and Zafiriou, 1990; Petasne and Zika, 1997). It has been suggested that microbes sensitive to H2O2 may not be cultivable under normal laboratory conditions (Morris et al, 2008, 2011), which may have severely biased our historical understanding of ROS interactions with marine microorganisms
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