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 in 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 chlorophyll concentration was 0.4 mg dm-3, while the arsenate- and arsenite-concentrations were varied within the limits 0.1 to 200 mumol dm-3. In some experiments arsenate toxicity was studied in the presence of 1 to 100 mumol dm-3 of phosphate (P(V)). With concentrations of arsenate or arsenite less than 0.1 mumol dm-3 no toxic effects were observed. 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., arsenate is more toxic to Scenedesmus obliquus than arsenite. The toxicity of arsenate was negligible in the presence of a ten fold excess of phosphate. This is probably due to chemical similarities between arsenate and phosphate causing competition between the ions for the binding sites. The importance of taking the speciation as well as the buffer capacity of the algal system into account, when calculating the carbon uptake, is also discussed.

Nyckelord

ARSENATE

ARSENITE

PHOTOSYNTHESIS

SCENEDESMUS-OBLIQUUS

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