Diurnal cycling of glutathione in marine phytoplankton: Field and culture studies

Glutathione is an abundant intracellular thiol that is involved in the detoxification of reactive oxygen species generated in the presence of light. We used short-term continuous cultures of Emiliania huxleyi and Thalassiosira pseudonana to show that a diurnal light cycle causes intracellular glutathione to undergo at least twofold variations, with maximum concentrations occurring during light periods. Cysteine concentrations also appear to vary in E. huxleyi to a similar extent, with the highest concentrations occurring at night. A complementary field study conducted at Salt Pond in Falmouth, Massachusetts, yielded a similar diurnal cycle of glutathione concentrations in particulate samples. This cycling is important to consider when measuring particulate thiols in both culture and field studies and provides additional evidence that glutathione plays an important role as an antioxidant in eukaryotic marine phytoplankton. Glutathione (g-glutamylcysteinylglycine) is found in all eukaryotes and some prokaryotes, including cyanobacteria and purple bacteria (Newton et al. 1996). It is often the most abundant small peptide and intracellular thiol. Concentrations are typically in the millimolar range and can account for up to 90% of nonprotein thiols in plants and algae (Rennenberg 1982; Rennenberg and Lamoureux 1990; Ahner et al. 2002). Glutathione has several physiological roles (Fig. 1); therefore, several environmental variables might control glutathione levels. One of the major roles of glutathione is the detoxification of reactive oxygen species (ROS) in the chloroplast, where it acts as an intermediate in the removal of superoxide radicals that can be generated during light saturation of photosynthesis (Polle and Rennenberg 1992). Since this stress is dependent upon light, intracellular concentrations of glutathione might be expected to vary with light intensity and duration in marine phytoplankton. There are several lines of evidence in the literature on which we formed this hypothesis. Three species of mixotrophic algae, when grown in the presence of light, contained twofold to fivefold more glutathione than when grown heterotrophically in the dark (Fahey et al. 1987). Matrai and Vetter (1988) reported decreases in glutathione concentrations in natural assemblages of coastal phytoplankton following 24 h of dark incubation. Perhaps most importantly, Schupp and Rennenberg (1988) showed that glutathione in pine needles (Picea abies) followed a diurnal cycle with high concentrations during midday and low concentrations during the night. More recently, investigators interested in the potential role 1 Present address: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92037. 2 Corresponding author ([email protected]).