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The arsenate-induced synthesis of phytochelatins (PC), intracellular cysteine-rich metal-binding peptides, and its relationship with toxicity and with As accumulation in the cell have been studied in laboratory cultures of the marine microalga Phaeodactylum tricornutum. The time course of cellular PC and As in short-term exposures showed that the involvement of PC in the As detoxification as well as the pathway of cellular As depend on the extent of As accumulation and on the rate of PC synthesis. At arsenate concentrations causing As accumulation at a rate exceeding that of PC synthesis, cells seem to activate a mechanism of release of As mainly in a chemical form not complexed with PC. At arsenate concentrations at which the synthesis of PC occurs at a rate sufficient to allow a significant portion of As accumulated in the cell to be bound, the fate of cellular As seems to be mainly controlled by PC. The occurrence of these different pathways of As detoxification was discussed to explain the pattern of cellular As and PC in cells grown for three days at growth-inhibitory and at no growth-inhibitory concentration of arsenate.

The complexlng properties of ethylenediamine (en) were Investigated as a means of performing the analysis of ligandexchangeable and labile (I.e., directly reducible at pH 8) Cu In seawater at trace levels. Stripping polarographic or pseudopolarographlc determinations show that the Cu-en complex behaves reversibly In seawater, exchanging two electrons at the mercury drop electrode (HMDE). The role of chloride Ions In competitive reactions with en for copper during the stripping step was also studied. In seawater made 2 X 10'3 M In en, Cu(II) can be detected at the HMDE by differential pulse anodic stripping voltammetry (DPASV) at the 5 X 10"1°M level with a deposition time of 10 min. A new procedure for measuring pH 8 labile copper in seawater Is obtained by coupling DPASV with a medium alteration method. Addition of en at the end of the electrolysis increases peak height by more than twice by doubling the current yield per mole of Cu and by removing Interferences associated with the oxidation of Cu In Cl" media. This procedure facilitates the voltammetrlc study of Cu In seawater under natural conditions.