Arsenic biotransformation by the brown macroalga, Fucus serratus

The brown alga Fucus serratus was maintained in aquaria with added arsenate (0, 20, 50, and 100 microg As/L, four individuals per treatment) for up to 19 weeks. Biotransformation of arsenic by Fucus was monitored by high-performance liquid chromatography/inductively coupled plasma mass spectrometry and liquid chromatography/electrospray mass spectrometry analysis of aqueous extracts of algal frond tips removed periodically throughout the experiment. Major arsenic species monitored were arsenate, arsenite, methylarsonate, dimethylarsinate, and the four arsenosugars 1 to 4 found naturally in Fucus. Algae accumulated arsenate readily and transformed it into several arsenic compounds depending on the exposure concentration. At 100 microg As/L, the major metabolite was arsenite with smaller quantities of methylarsonate and dimethylarsinate, but only traces of arsenosugars were formed. In contrast, the 20-microg-As/L group accumulated only small quantities of arsenite and methylarsonate, while dimethylarsinate and arsenosugars were major arsenic metabolites. At 50 microg As/L exposure, algae had significant quantities of all arsenic metabolites monitored. Arsenate was toxic to the algae at 100 microg As/L but had no obvious detrimental effect at 20 microg As/L. The data are consistent with a process of arsenate detoxification by reduction and alkylation; at higher exposures, however, the alkylation processes become saturated, leading to an accumulation of arsenite and subsequent toxicity.