EFFECTS OF ARSENIC SPECIATION AND PHOSPHATE CONCENTRATION ON ARSENIC INHIBITION OF SKELETONEMA COSTATUM (BACILLARIOPHYCEAE)1

ABSTRACT Arsenate is taken up readily by Skeletonema costatum (Greville) Cleve due to its chemical similarity to phosphate, and it inhibits primary productivity at concentrations as low as 67 nM when the phosphate concentration is low. A phosphate enrichment of greater than 0.3 μM alleviates this inhibition; however, the arsenate stress causes an increase in the cell's requirement for phosphorus. Arsenite is also toxic to Skeletonema at similar concentrations. Methylated species, such as dimethylarsinic acid, did not affect cell productivity at the levels examined. Thus, the reduction and methylation of arsenate to dimethylarsinic acid by the cell produces a stable, non‐toxic compound.