Impact of orthophosphate on the light-independent production of reactive oxygen species during microbially-mediated Fe redox cycles

Abstract

Although light-independent production of reactive oxygen species (ROS) in subsurface environments has been increasingly documented, the processes controlling ROS transformations at redox interfaces remain poorly understood. The Fe(III)-reducing facultative anaerobe Shewanella oneidensis promotes light-independent ROS production in the presence of Fe(III) when alternately exposed to oxic and anoxic conditions and represents a good model of microbial processes at redox interfaces. In this study, S. oneidensis was incubated under three different orthophosphate concentrations from low natural conditions to high growth-promoting conditions to identify the processes regulating the light-independent production of ROS during redox oscillations and determine the impact of this important nutrient on ROS generation. Ferryl complexes were not significant intermediates in the redox cycling of Fe in this system. Instead, hydrogen peroxide (H₂O₂) and hydroxyl radicals (•OH) represented the main ROS produced in a process that is periodic during redox oscillations. Hydroxyl radicals were mainly produced during the aerated phases in concentrations inversely proportional to orthophosphate concentrations as a result of the complexation of Fe(III) and precipitation of oxidation-resistant vivianite. In relatively low orthophosphate concentration compared to the total reactive sites available on the Fe(III) oxides, •OH was produced at the mineral surface. Increased cell metabolic activities in the highest orthophosphate medium may have increased the •OH production efficiency during the aerated phases by enhancing O₂^•− formation and/or Fe(III) reduction. Unexpectedly, H₂O₂ was mainly detected under Fe(III) reduction conditions with net production rates that were proportional to orthophosphate concentrations, except when the solubility of vivianite was exceeded. Hydrogen peroxide is proposed to originate from the reduction of dissolved O₂ by Fe(II) produced during dissimilatory Fe(III) reduction in microaerobic conditions. Precipitation of vivianite likely affects the availability of Fe(II) for ROS reactions during redox cycles, thus decreasing overall H₂O₂ production rates. The rise in metabolic activity, outcompetition of Fe(III) hydrolysis by orthophosphate complexation, and potential dimerization of surface generated •OH are proposed to result in the accumulation of H₂O₂ under suboxic conditions.