Concentrations and speciation of arsenic in New England seaweed species harvested for food and agriculture

A survey of arsenic (As) concentrations and speciation was conducted on 10 species of seaweed from commercial harvesters and from collection at two sites in New England. Concentrations of As ranged from 4 to 106 mg/kg, mostly in the form of arsenosugars, with the distribution of arsenosugar analogs varying between taxa. In brown algae, As levels were correlated with phosphate concentrations, and arsenosugar speciation reflected differences in sulfur and phosphate concentrations between taxa. Several samples of the brown algae species Laminaria digitata contained significant levels of inorganic As (2.8-20 mg/kg), the most toxic form of As. A weak acid extraction with microwave heating was compared with a weaker methanol: water extraction method, and found to give slightly higher extraction efficiency with comparable relative concentrations of inorganic As, supporting the use of this faster and simpler extraction method for monitoring. Seaweed is a niche dietary item in the U.S. but its popularity is increasing; it is also used in agriculture and livestock farming which provide potential indirect routes for human exposure. The presence of occasional high concentrations of iAs, as well as the lack of toxicity studies on organic As species, suggest that monitoring of these high As foods is warranted.