Spatial and temporal variability of widespread dark production and decay of hydrogen peroxide in freshwater

Abstract

Hydrogen peroxide (H2O2) is an oxidant and reductant of redox active metals and a potential source of strong oxidants such as the hydroxyl radical (·OH). H2O2 production in freshwater has been largely attributed to photo-oxidation of chromophoric dissolved organic matter, while its decay has been linked to enzymatic processes as well as to chemical reactions with metals. More recently, however, microorganisms were postulated as a significant source as well as a sink of H2O2 in freshwater. In this study, we examined the spatial and temporal variability of dark H2O2 production rates ( $${\text{P}}_{{{\text{H}}_{ 2} {\text{O}}_{ 2} }}$$ ) and pseudo-first order dark decay rate coefficients ( $${\text{k}}_{{{\text{loss,H}}_{ 2} {\text{O}}_{ 2} }}$$ ) in incubations of water samples from sites with a range of trophic states in Colorado (CO) and Massachusetts (MA). Observed values of $${\text{P}}_{{{\text{H}}_{ 2} {\text{O}}_{ 2} }}$$ and $${\text{k}}_{{{\text{loss,H}}_{ 2} {\text{O}}_{ 2} }}$$ ranged from 3 to 259 nM h−1 and 0.02 to 8.87 h−1, respectively. Microbial cell numbers and chlorophyll content correlated strongly with $${\text{k}}_{{{\text{loss,H}}_{ 2} {\text{O}}_{ 2} }}$$ while filtering the freshwater samples removed the majority of $${\text{k}}_{{{\text{loss,H}}_{ 2} {\text{O}}_{ 2} }}$$ , indicating breakdown by biota as the major sink of H2O2. Dark production of H2O2 was also ubiquitous, but $${\text{P}}_{{{\text{H}}_{ 2} {\text{O}}_{ 2} }}$$ was not well correlated with indicators of microbial abundance. For instance, several oligotrophic sites (with low $${\text{k}}_{{{\text{loss,H}}_{ 2} {\text{O}}_{ 2} }}$$ ) exhibited moderately high $${\text{P}}_{{{\text{H}}_{ 2} {\text{O}}_{ 2} }}$$ , while a sample with unusually high chlorophyll content (and a correspondingly high $${\text{k}}_{{{\text{loss,H}}_{ 2} {\text{O}}_{ 2} }}$$ ) had a relatively low $${\text{P}}_{{{\text{H}}_{ 2} {\text{O}}_{ 2} }}$$ . One possible explanation for this phenomenon is that the ability to break down H2O2 is similar among different microorganisms, but the ability to produce H2O2 differs with microbial composition.