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.