THE EFFECT OF ARSENATE AND ARSENITE ON PHOTOSYNTHESIS IN SCENEDESMUS-OBLIQUUS - A POTENTIOMETRIC STUDY IN A CLOSED CO2-SYSTEM

A potentiometric titration method was used to study the adverse effect of arsenate (As(V) and arsenite (As(III) on inorganic carbon uptake i n suspensions of the green alga Scenedesmus obliquus. The measurements were performed in a closed CO2-system with diluted synthetic seawater (1 parts per thousand salinity) as ionic medium. Usually, the algal c hlorophyll concentration was 0.4 mg dm-3, while the arsenate- and arse nite-concentrations were varied within the limits 0.1 to 200 mumol dm- 3. In some experiments arsenate toxicity was studied in the presence o f 1 to 100 mumol dm-3 of phosphate (P(V)). With concentrations of arse nate or arsenite less than 0.1 mumol dm-3 no toxic effects were observ ed. However, at As-concentrations of 200 mumol dm-3, the algal carbon uptake was reduced by 41% with arsenate and 29% with arsenite, i.e., a rsenate is more toxic to Scenedesmus obliquus than arsenite. The toxic ity of arsenate was negligible in the presence of a ten fold excess of phosphate. This is probably due to chemical similarities between arse nate and phosphate causing competition between the ions for the bindin g sites. The importance of taking the speciation as well as the buffer capacity of the algal system into account, when calculating the carbo n uptake, is also discussed.