Free radicals and chemiluminescence as products of the spontaneous oxidation of sulfide in seawater, and their biological implications

The discovery of symbioses between marine invertebrates and sulfide-oxidizing bacteria at deep-sea hydrothermal vents and in other high-sulfide marine environments has stimulated reseal ch into the adaptations of metazoans to potentially toxic concentrations of sulfide. Most of these studies have focused on a particular action of sulfide-its disruption of aerobic metabolism by the inhibition of mitochondrial respiration-and on the adaptations of sulfide-tolerant animals to avoid this toxic effect (1). We propose that sulfidic environments impose another hitherto overlooked type of toxicity: exposure to free radicals of oxygen, which may be produced during the spontaneous oxidation of sulfide, thus imposing an oxidative stress. Here we present evidence that oxygen- and sulfur-centered free radicals are produced during the oxidation of sulfide in seawater, and we propose a reaction pathway for sulfide oxidation that is consistent with our observations. We also show that chemiluminescence at visible wavelengths occurs during sulfide oxidation, providing a possible mechanism for the unexplained light emission from hydrothermal vents (2, 3).