Relationships between acid-soluble thiol peptides and accumulated Pb in the green alga Stichococcus bacillaris

Stichococcus bacillaris, an ubiquitous green microalga accumulated inorganic lead (Pb) from aqueous solutions extra- and intracellularly. In response to Pb uptake acid-soluble thiol peptides (glutathione - GSH and phytochelatins - PC) were synthesized. The proportion of the intracellular Pb uptake by algal cells was low and comprised only 3-6% of the total metal sorption. The intracellular uptake was dependent on external Pb concentration, time of metal exposure and cell metabolism. Pb accumulation in alga was determined by means of 210Pb radiometry. Reduced GSH and PC were determined in algal cells using HPLC with the post-column derivatization with Ellman's reagent. Within the studied concentration range 0.1-20 µM, inorganic lead caused a significant production of induced thiol peptides: PC (n=2-4) and some other unidentified oligopeptides, probably (GluCys)n. The time of appearance and the concentration of individual oligomers of phytochelatins were dependent on the external Pb concentration and time of metal exposure. In algal cells exposed to Pb, significant changes in the GSH level accompanying the formation of the induced thiol peptides were also observed. The GSH level decreased in the cells exposed to the lower (up to 10 µM) studied Pb concentrations or increased in the cells treated with higher (20 µM) Pb concentrations. The thiol groups originated from induced peptides (mainly phytochelatins) followed a stoichiometric relationship 2:1 to the intracellular Pb amounts, however, only at the lowest studied external concentration (0.1 µM). At higher concentrations (up to 2.5 µM), intracellular Pb concentration was equal or even exceeded (at Pb>2.5 µM) two to three times the level of induced thiols. S. bacillaris accumulated intracellularly by 46% more Pb in light than in dark and the level of induced thiol peptides was significantly higher in the cells exposed to Pb under illumination. The rapid formation of these peptides in S. bacillaris in response to Pb, and their elimination (by about 90%) when algae were placed into the Pb-free solution reveal a tight regulation of GSH and phytochelatin pools in the algal cells exposed to toxic metals. The obtained results suggest that both PCs and GSH are the primary line of defence against the Pb toxicity. Additionally, the induced thiol peptides in S. bacillaris could be a good indicator of intracellular Pb availability and stress at the metal concentrations found in polluted fresh waters.