The present study focuses on the biouptake, biotransformations, and toxicity of arsenic species on the marine green alga Chlorella salina in seawater from the Bulgarian Black Sea coast. Exposure to equal concentrations of As(III) or As(V) led to equivalent levels of toxicity and total intracellular arsenic content. Biouptake and toxicity of methylated arsenic species, monomethylarsonate (MMA) and dimethylarsinate (DMA), were approximately three orders of magnitude lower than those for inorganic arsenic species. Seawater enrichment with phosphate (up to 1.3mg P L−1) resulted in a significant reduction of both intracellular As content and toxicity due to As(III) and As(V). In contrast, the toxicity and intracellular content ofMMAandDMAwere unaffected by the presence of phosphate.We measured the distribution and excretion of intracellular arsenic species, and demonstrated that the release of As(V) and/or As(III), together with the bio-reduction of As(V) and the subsequent methylation of As(III) may be a detoxification mechanism for these algae. The implications of the results with respect to arsenic species bioavailability and toxicity in marine water are further discussed.