NADPH-dependent extracellular superoxide production is vital to photophysiology in the marine diatom Thalassiosira oceanica

Abstract

Reactive oxygen species (ROS) like superoxide drive rapid transformations of carbon and metals in aquatic systems and play dynamic roles in biological health, signaling, and defense across a diversity of cell types. In phytoplankton, however, the ecophysiological role(s) of extracellular superoxide production has remained elusive. Here, the mechanism and function of extracellular superoxide production by the marine diatom Thalassiosira oceanica are described. Extracellular superoxide production in T. oceanica exudates was coupled to the oxidation of NADPH. A putative NADPH-oxidizing flavoenzyme with predicted transmembrane domains and high sequence similarity to glutathione reductase (GR) was implicated in this process. GR was also linked to extracellular superoxide production by whole cells via quenching by the flavoenzyme inhibitor diphenylene iodonium (DPI) and oxidized glutathione, the preferred electron acceptor of GR. Extracellular superoxide production followed a typical photosynthesis-irradiance curve and increased by 30% above the saturation irradiance of photosynthesis, while DPI significantly impaired the efficiency of photosystem II under a wide range of light levels. Together, these results suggest that extracellular superoxide production is a byproduct of a transplasma membrane electron transport system that serves to balance the cellular redox state through the recycling of photosynthetic NADPH. This photoprotective function may be widespread, consistent with the presence of putative homologs to T. oceanica GR in other representative marine phytoplankton and ocean metagenomes. Given predicted climate-driven shifts in global surface ocean light regimes and phytoplankton community-level photoacclimation, these results provide implications for future ocean redox balance, ecological functioning, and coupled biogeochemical transformations of carbon and metals.

Siderophores provoke extracellular superoxide production by carbon-starving Arthrobacter strains when carbon sources recover

“…Our data showed that DTPA induced A. QXT-31 to produce more extracellular superoxide, even at concentrations (5–10 μM) (Fig. 4a) lower than used in previous research (40–170 µM) 7-10,22 . Similar phenomena were also observed when DTPA was replaced with DFO, EDTA, and acetohydroxamic acid (Supplementary Fig.…”

“…The above findings prompted a rethink of the methodology of cellular superoxide quantification, where a metal-chelator, DTPA, was widely used 7-10,22,23 . DTPA was initially exploited in a superoxide producing system (xanthine-xanthine oxidase system; used to generate superoxide at an expected rate) to maintain superoxide signals by suppressing interference from metal ions 23,24 .…”

Chemical Diversity and Biochemical Transformation of Biogenic Organic Sulfur in the Ocean

“…Glutathione disulfide (GSSG) is also present in cells in millimolar concentration (Bennett et al, 2009), which is formed as a result of GSH oxidation. In conjunction with GSH, GSSG helps to prevent oxidative damage and detoxify harmful substances (Diaz et al, 2019).…”

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

“…Addition of superoxide dismutase (SOD), an enzyme that eliminates superoxide, inhibited growth, suggesting that superoxide is involved in growth and proliferation as previously suggested for pathogenic strains of Escherichia coli and Chattonella marina (29)(30)(31)(32). Further, recent evidence indicates that extracellular superoxide production by the diatom Thalassiosira oceanica may play a critical role in maintaining the internal redox conditions in photosynthesizing cells (21). This suite of beneficial physiological processes all result from or result in the reduction of molecular oxygen that is not otherwise considered in biogeochemical cycles of oxygen and related elements.…”

“…Dark, extracellular superoxide production is in fact prolific among marine heterotrophic bacteria, cyanobacteria, and eukaryotes (9,(11)(12)(13)(14)(15)(16)(17)(18). Production of extracellular superoxide proceeds via a one-electron transfer initiated by transmembrane, outer membrane-bound, or soluble extracellular enzymes thought to belong generally to NAD(P)H oxidoreductases (19), and more recently to heme peroxidases and glutathione reductases (9,20,21). At circumneutral pH, the superoxide anion (O 2…”