Occurrence and Speciation of Arsenic in Common Australian Coastal Polychaete Species

Environmental Context. In well-oxygenated water and sediments, nearly all arsenic is present as arsenate (AsO₄³⁻). As arsenate is a phosphate (PO₄³⁻) analogue, organisms living in arsenate-rich environments must acquire the nutrient phosphorus yet avoid arsenic toxicity. Organisms take in and transform arsenic compounds by many means. Three major modes of arsenic biotransformation have been found to occur in the environment—redox transformation between arsenate and arsenite (AsO₂⁻), the reduction and methylation of arsenic, and the biosynthesis of organoarsenic compounds such as arsenobetaine. These biotransformations lead to biogeochemical cycling of arsenic compounds and bioconcentration of arsenic in aquatic organisms and thence into the food web.

Abstract. The paper reports the whole-tissue total arsenic concentrations and water-soluble arsenic species in eight common coastal Australian polychaete species.

Laboratory experiments showed the period of depuration did not significantly alter the whole-tissue total arsenic concentrations in the two estuarine polychaete species tested.

Significant differences were found between the whole-tissue total arsenic concentrations of the eight polychaete species (mean arsenic concentrations ranged from 18 to 101 µg g⁻¹ dry mass). Total arsenic concentrations in polychaete species, grouped on the basis of a combination of their feeding guild and habitat type, were also significantly different with a significant interaction between these factors indicating that both factors simultaneously influence arsenic concentration in polychaetes.

A large number of polychaete species contained similar arsenic species with high proportions of arsenobetaine (AB; 57–88%) and relatively low proportions of As³⁺, As⁵⁺, methyarsonic acid, dimethylarsinic acid, arsenocholine, trimethylarsoniopropionate, and tetramethylarsonium ion (not detected to 12%). All polychaete species contained arsenoribosides (5–30%). This study identified two Australian polychaete species with particularly unusual whole-tissue water-soluble arsenic species proportions: Australonuphis parateres contained a very high proportion of trimethylarsoniopropionate (~33%), while Notomastus estuarius had a very low proportion of arsenobetaine (9%) and high proportions of As³⁺ (~30%), As⁵⁺ (~8%), arsenoribosides (30%), and an unknown anionic arsenic species (~4%).

Most polychaetes accumulate arsenobetaine, except deposit feeders inhabiting estuarine mud habitats. Thus most polychaetes, which are prey for higher organisms, are a source of arsenobetaine in benthic food webs. Deposit feeders inhabiting estuarine muddy substrates contain appreciable quantities of inorganic arsenic and arsenoribosides that may be metabolized to different end products in higher organisms.