Environmental Science and Engineering Seminar

The distribution of manganese (Mn) oxide deposits is variable throughout the geologic record and in modern marine sediments. The formation of Mn oxides via the oxidation of Mn(II) by molecular oxygen is constrained owing to a reactivity barrier imposed by the first electron transfer step. Despite a wide diversity of aerobic microorganisms that can precipitate Mn oxides, the mechanisms of formation are poorly understood. Further, the physiological basis for microbial Mn(II) oxidation remains an enigma. We have recently revealed that Mn oxide formation by some fungal and bacterial species is a consequence of Mn(II) oxidation by the reactive oxygen species (ROS) superoxide (O₂^-). This superoxide production occurs extracellularly, in some cases through the activity of transmembrane and secreted proteins (exoproteins). While we have also shown that extracellular superoxide production is widespread in bacteria, this production does not in fact confer the ability to produce Mn oxides.  Indeed, back-reaction between the products, Mn(III) and hydrogen peroxide, formed upon Mn(II) and superoxide reaction inhibits Mn oxide formation. These findings indicate that at least two processes – the generation of superoxide and consumption of hydrogen peroxide – are requisite for Mn oxide formation. In fact, the enzymes we have identified in superoxide production possess a peroxidative-oxidative oscillatory behaviour as is widely observed in eukaryotic peroxidases, but not yet appreciated in prokaryotic systems. Together, these findings indicate that ROS and Mn cycling are tightly coupled in natural systems, wherein the production and consumption of superoxide and hydrogen peroxide could be mediated by the recycling of catalytic levels of aqueous Mn – a so-called cryptic Mn cycle.  In this way, Mn may in fact be the antioxidant of the ocean – and quite possibly the primordial superoxide dismutase.