Dynamic Regulation of Extracellular Superoxide Production by the Coccolithophore Emiliania huxleyi (CCMP 374)

Production of extracellular superoxide radical in microorganisms and its environmental implications: A review

Extracellular superoxide radical (O2•-) is ubiquitous in microbial environments and has significant implications for pollutant transformation. Microbial extracellular O2•- can be produced through multiple pathways, including electron leakage from the respiratory electron transport chain (ETC), NADPH oxidation by the transmembrane NADPH oxidase (NOX), and extracellular reactions. Extracellular O2•- significantly influences the geochemical processes of various substances, including toxic metals and refractory organic pollutants. On one hand, extracellular O2•- can react with variable-valence metals and detoxify certain highly toxic metals, such as As(III), Cr(VI), and Hg(II). On the other hand, extracellular O2•- can directly or indirectly (via Bio-Fenton) degrade many organic pollutants, including a variety of emerging contaminants. In this work, we summarize the production mechanisms of microbial extracellular O2•-, review its roles in the transformation of environmental pollutants, and discuss the potential applications, limiting factors, and future research directions in this field.

Production of Extracellular Reactive Oxygen Species by Marine Biota

Reactive oxygen species (ROS) are produced ubiquitously across the tree of life. Far from being synonymous with toxicity and harm, biological ROS production is increasingly recognized for its essential functions in signaling, growth, biological interactions, and physiochemical defense systems in a diversity of organisms, spanning microbes to mammals. Part of this shift in thinking can be attributed to the wide phylogenetic distribution of specialized mechanisms for ROS production, such as NADPH oxidases, which decouple intracellular and extracellular ROS pools by directly catalyzing the reduction of oxygen in the surrounding aqueous environment. Furthermore, biological ROS production contributes substantially to natural fluxes of ROS in the ocean, thereby influencing the fate of carbon, metals, oxygen, and climate-relevant gases. Here, we review the taxonomic diversity, mechanisms, and roles of extracellular ROS production in marine bacteria, phytoplankton, seaweeds, and corals, highlighting the ecological and biogeochemical influences of this fundamental and remarkably widespread process.

Spatial Heterogeneity in Particle-Associated, Light-Independent Superoxide Production Within Productive Coastal Waters

In the marine environment, the reactive oxygen species (ROS) superoxide is produced through a diverse array of light-dependent and light-independent reactions, the latter of which is thought to be primarily controlled by microorganisms. Marine superoxide production influences organic matter remineralization, metal redox cycling, and dissolved oxygen concentrations, yet the relative contributions of different sources to total superoxide production remain poorly constrained. Here we investigate the production, steady-state concentration, and particle-associated nature of light-independent superoxide in productive waters off the northeast coast of North America. We find exceptionally high levels of light-independent superoxide in the marine water column, with concentrations ranging from 10 pM to in excess of 2,000 pM. The highest superoxide concentrations were particle associated in surface seawater and in aphotic seawater collected meters off the seafloor. Filtration of seawater overlying the continental shelf lowered the light-independent, steady-state superoxide concentration by an average of 84%. We identify eukaryotic phytoplankton as the dominant particle-associated source of superoxide to these coastal waters. We contrast these measurements with those collected at an off-shelf station, where superoxide concentrations did not exceed 100 pM, and particles account for an average of 40% of the steady-state superoxide concentration. This study demonstrates the primary role of particles in the production of superoxide in seawater overlying the continental shelf and highlights the importance of light-independent, dissolved-phase reactions in marine ROS production.

Dark biological superoxide production as a significant flux and sink of marine dissolved oxygen

Extracellular production of the reactive oxygen species (ROS) superoxide results from the one-electron reduction of O₂. Nearly all major groups of marine microbes produce extracellular superoxide. In this global estimate of marine microbial superoxide production we determine that dark extracellular superoxide production is ultimately a net sink of dissolved oxygen comparable in magnitude to other major terms in the marine oxygen cycle. This abundant source of superoxide to the marine water column provides evidence that extracellular ROS play a significant role in carbon oxidation and the redox cycling of metals in marine environments. Consideration of this significant reductive flux of dissolved oxygen is essential for field, laboratory, and modeling techniques for determining productivity and oxygen utilization in marine systems.

The balance between sources and sinks of molecular oxygen in the oceans has greatly impacted the composition of Earth’s atmosphere since the evolution of oxygenic photosynthesis, thereby exerting key influence on Earth’s climate and the redox state of (sub)surface Earth. The canonical source and sink terms of the marine oxygen budget include photosynthesis, respiration, photorespiration, the Mehler reaction, and other smaller terms. However, recent advances in understanding cryptic oxygen cycling, namely the ubiquitous one-electron reduction of O₂ to superoxide by microorganisms outside the cell, remains unexplored as a potential player in global oxygen dynamics. Here we show that dark extracellular superoxide production by marine microbes represents a previously unconsidered global oxygen flux and sink comparable in magnitude to other key terms. We estimate that extracellular superoxide production represents a gross oxygen sink comprising about a third of marine gross oxygen production, and a net oxygen sink amounting to 15 to 50% of that. We further demonstrate that this total marine dark extracellular superoxide flux is consistent with concentrations of superoxide in marine environments. These findings underscore prolific marine sources of reactive oxygen species and a complex and dynamic oxygen cycle in which oxygen consumption and corresponding carbon oxidation are not necessarily confined to cell membranes or exclusively related to respiration. This revised model of the marine oxygen cycle will ultimately allow for greater reconciliation among estimates of primary production and respiration and a greater mechanistic understanding of redox cycling in the ocean.