Abstract
Redox conditions in natural waters are a fundamental control on biogeochemical processes and ultimately many ecosystem functions. While the dioxygen/water redox couple controls redox thermodynamics in oxygenated aquatic environments on geological timescales, it is kinetically inert in the extracellular environment on the much shorter timescales on which many biogeochemical processes occur. Instead, electron transfer processes on these timescales are primarily mediated by a relatively small group of trace metals and stable radicals, including the reactive oxygen species superoxide. Such processes are of critical biogeochemical importance because many of these chemical species are scarce nutrients, but may also be toxic at high concentrations. Furthermore, their bioavailability and potentially toxicity is typically strongly influenced by their redox state. In this paper, I examine to what extent redox conditions in oxygenated natural waters are expected to be reflected in the redox states of labile redox-active compounds that readily exchange electrons with the dioxygen/superoxide redox couple, and potentially with each other. Additionally, I present the hypothesis that that the relative importance of the dioxygen/superoxide and superoxide/hydrogen peroxide redox couples exerts a governing control on local redox conditions in oxygenated natural waters on biogeochemically important timescales. Given the recent discovery of widespread extracellular superoxide production by a diverse range of organisms, this suggests the existence of a fundamental mechanism for organisms to tightly regulate local redox conditions in their extracellular environment in oxygenated natural waters.
Highlights
Electron transfer and transport in natural waters is a fundamental control on chemical conditions, biological processes, and ecosystem structure
Other labile redox active compounds (LRACs) in natural waters may include certain reduced sulfur species such as thiols and metal sulfide clusters, which may Possible stable redox states that are accessible by one-electron transfers
The input of electrons into the LRAC pool in oxygenated natural waters is primarily driven through the slow oxidation of kinetically inert reduced compounds and/or physical transport of reduced LRACs from more reducing environments such as sediments or rainwater (Figure 3)
Summary
Electron transfer processes on these timescales are primarily mediated by a relatively small group of trace metals and stable radicals, including the reactive oxygen species superoxide. Such processes are of critical biogeochemical importance because many of these chemical species are scarce nutrients, but may be toxic at high concentrations. Their bioavailability and potentially toxicity is typically strongly influenced by their redox state. I present the hypothesis that the relative importance of the dioxygen/superoxide and superoxide/hydrogen peroxide redox couples exerts a governing control on local redox conditions in oxygenated natural waters on biogeochemically important timescales.
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