Particulate thiols in coastal waters: The effect of light and nutrients on their planktonic production

Reduced sulfur compounds of low molecular weight, mainly particulate glutathione, were present in concentrations up to 600 pM in Saanich Inlet and off southern California. The vertical distributions of particulate glutathione (PGSH) in Saanich Inlet showed a subsurface maximum and decreased with depth in close correspondence with chlorophyll profiles. As with other measures of phytoplankton standing stock, PGSH decreased offshore in the Southern California Bight. In response to exposure to lower than ambient light levels, PGSH decreased in samples after 24‐h, on‐deck incubations except when the ambient nitrate concentration was high. Nitrate enrichment experiments, however, did not lead to enhanced PGSH production in offshore samples that were initially low in nitrate. Diel changes in PGSH were calculated to represent 0.5–4.5% of the corresponding calculated change in total particulate sulfur.