Uptake of Dissolved Organic Selenides by Marine Phytoplankton

Se is present in multiple oxidation states in nature, each of which has unique chemical and biological reactivities. As a consequence, the rate of Se incorporation into food webs or its role as either a limiting nutrient or a toxic substance is a function of complex biogeochemistry. In particular, little is understood about the accumulation of dissolved organic selenides by phytoor bacterioplankton. We assessed the bioavailability of dissolved organic selenides to marine and estuarine phytoplankton by presenting various algal species with filtered lysates of the diatom, Thalassiosira pseudonana, grown on media amended with radiolabeled selenite (75Se[IV]). Species known to accumulate selenite effectively also accumulated Se from radiolabeled cell lysates and to approximately the same extent. When exposed to a 4.5-nM solution of lysate Se, T. pseudonana, Heterocapsa triquetra (Dinophyceae), Tetraselmis levis (Prasinophyceae), Synechococcus bacillus (Cyanobacteria), and Dunaliella tertiolecta (Chlorophyceae) incorporated 42%–53%, 42%, 30%, 32%, and 4% of the dissolved Se, respectively. Se cell contents of T. pseudonana, T. levis, and D. tertiolecta grown in media containing 4.5 nM lysate Se were very similar to Se content of cells grown in 4.5 nM selenite. Our results suggest that recycling of Se(2II) may be more important than previously thought. Consequently, uptake of organic selenides by phytoplankton may need to be considered in models predicting Se incorporation into aquatic food webs, given that it is a significant fraction of dissolved Se in estuarine and oceanic waters. Se can act as a limiting nutrient for marine organisms at low concentrations or as a toxic contaminant when concentrations in organismal tissues are sufficiently elevated (Harrison et al. 1988; Lauchli 1993; Lemly 1996). The existence of multiple oxidation states of Se in natural waters complicates prediction of Se accumulation into plankton food webs, because the different forms have different biological reactivities (Wrench and Measures 1982; Cutter and Bruland 1984). Uptake of inorganic forms of dissolved Se, i.e., selenite (Se[IV]) and selenate (Se[VI]), by phytoplankton and bacteria is widely presumed to be the main pathway for Se into aquatic food webs (Luoma et al. 1992; Wang et al. 1996). Once incorporated by cells, the oxidized inorganic forms are reduced to organic selenides (Se[2II]) (Wrench and Campbell 1981; Bottino et al. 1984; Besser et al. 1994). These organic selenides, usually in the form of free and combined seleno-methionine and seleno-cysteine (Fisher and Reinfelder 1991), can then be released into the environment through excretion, cell lysis, or grazing activity (Cutter 1982; Bottino et al. 1984; Besser et al. 1994). Dissolved organic selenides may be an important source of Se for phytoplankton cells, because they can account for ;80% of the dissolved Se in open ocean surface waters and a significant fraction in many other environments as well (Wrench 1983; Cutter 1989; Cutter and Cutter 1995). Nev-