-honmachi 2-7-1 , Shimonoseki 759-65, Japan, and tKanagawa Prefectural Public Health Laboratories, Nakao-cho, Asahi-ku, Yokohama 241, JapanMicrobial degradation experiments were performed with each standard arsenical [arsenobetaine, trimethylarsine oxide, dimethylarsinic acid, methanearsonic acid, inorganic arsenic(V) and inorganic arsenic(III)]. As typical origins for marine micro-organisms, sediments, macro-algae, mollusc intestine and suspended substances were used. The results were from these experiments led us to the following conclusions: (1) there is an arsenic cycle which begins with the methylation of inorganic arsenic on the route to arsenobetaine and terminates with the complete degradation of arsenobetaine to inorganic arsenic; (2) all the organoarsenic compounds which are derived from inorganic arsenic in seawater, through the food chains, have the fate that they, at least in part, finally return to the original inorganic arsenic.

Arsenic compounds were extracted with chloroform/methanol/water from tissues of marine animals (four carnivores, five herbivores, five plankton feeders). The extracts were purified by cation and anion exchange chromatography. Arseno-,AsO] and arsenite, arsenate, and methylarsonic acid [(CH,AsO(OH),] as a group with the same retention time were identified by high-pressure liquid chromatography. Arsenic was determined in the collected fractions by graphite furnace atomic absorption spectrometry. Arsenobetaine found in all the animals was almost always the most abundant arsenic compound in the extracts. These results show that arsenobetaine is present in marine animals independently of their feeding habits and trophic levels.Arsenobetaine-containing growth media (ZoBell 22163; solution of inorganic salts) were mixed with coastal marine sediments as the source of microorganisms. Arsenobetaine was converted in both media to trimethylarsine oxide and trimethylarsine oxide was converted to arsenite, arsenate or methylarsonic acid but not to dimethylarsinic acid. The conversion rates in the inorganic medium were faster than in the ZoBell medium. Two dominant bacterial strains isolated from the inorganic medium and identified as members of the Vibro-Aeromonas group were incapable of degrading arsenobetaine.

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The substances suspended in seawater were fractionated by membrane filtration into three fractions . Fraction 1 was collected on a membrane filter of 0.22 jm pore-size, fraction 2 on a 5 am pore-size and fraction 3 on 0 .22 µm pore-size from the filtrate passed through the 5 jim membrane filter. Arsenobetaine was incubated with each of these fractions in two media (ZoBell 2216E and a solution of inorganic salts) at 25 °C in the dark under aerobic conditions . The mixture added with fraction 3 was considered to contain only bacteria. In every case, in the inorganic salt medium, inorganic arsenic(V) was derived from arsenobetaine via trimethylarsine oxide . In the ZoBell medium, arsenobetaine was not degraded to inorganic arsenic, although trimethylarsine oxide was derived in every case . We conclude that the degradation of arsenobetaine to trimethylarsine oxide or inorganic arsenic can be accomplished in seawater by bacteria alone . 75

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