The degradation of arsenobetaine to inorganic arsenic by sedimentary microorganisms

Two growth media containing arsenobetaine [(CH₃)₃ As⁺ CH₂COO^–] were mixed with coastal marine sediments, the latter providing a source of microorganisms. The mixtures were kept at 25 °C in the dark and shaken for several weeks under an atmosphere of air. The disappearance of arsenobetaine and the appearance of two metabolites were followed by HPLC. The HPLC-retention time of the first metabolite agreed with that of trimethylarsine oxide [(CH₃)₃AsO]. The second metabolite was identified as arsenate (As(V)) using hydride generation/cold trap/GC MS analysis and thin layer chromatography. This is the first scientific evidence showing that arsenobetaine is degraded by microorganisms to inorganic arsenic via trimethylarsine oxide. The degradation of arsenobetaine to inorganic arsenic completes the marine arsenic cycle that begins with the methylation of inorganic arsenic on the way to arsenobetaine.     

Degradation of arsenobetaine to inorganic arsenic by bacteria in seawater

The substances suspended in seawater were fractionated by membrane filtration into three fractions. Fraction 1 was collected on a membrane filter of 0.22 µm pore-size, fraction 2 on a 5 µm pore-size and fraction 3 on 0.22 µm pore-size from the filtrate passed through the 5 µm 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.     

Arsenic accumulation in three species of sea turtles

Arsenic in the liver, kidney and muscle of three species of sea turtles, e.g., green turtles (Chelonia mydas), loggerhead turtles (Caretta caretta) and hawksbill turtles (Eretmochelys imbricata), were determined using HG-AAS, followed by arsenic speciation analysis using HPLC-ICP-MS. The order of arsenic concentration in tissues was muscle > kidney > liver. Unexpectedly, the arsenic concentrations in the hawksbill turtles feeding mainly on sponges were higher than the two other turtles primarily eating algae and mollusk which accumulate a large amount of arsenic. Especially, the muscles of the hawksbill turtles contained remarkably high arsenic concentrations averaging 153 mg kg^–1 dry weight with the range of 23.1–205 mg kg^–1 (n=4), even in comparison with the data from other organisms. The arsenic concentrations in the tissues of the green turtles were significantly decreased with standard carapace length as an indicator of growth. In arsenic compounds, arsenobetaine was mostly detected in the tissues of all the turtles. Besides arsenobetaine, a small amount of dimethylarsinic acid was also observed in the hawksbill turtles.     

Biotransformation of arsenobetaine by microorganisms from the human gastrointestinal tract

Abstract

The consumption of fish and shellfish is a major route of human exposure to arsenic (As), because they contain relatively large concentrations of organoarsenicals, in particular arsenobetaine (AB). AB is considered non-toxic because of its rapid excretion from the human body. However, previous studies on human metabolism and excretion of AB have used the compound in solution rather than considering the effects that occur during the digestion of food in the gastrointestinal tract. In this preliminary study, we used microcosms inoculated with human faecal matter to investigate the aerobic and anaerobic degradation of AB by microorganisms associated with the large intestine. Samples were recovered over 30 days, centrifuged, filtered and the supernatant analysed for total As content and As speciation, using ICP–MS and HPLC–ICP–MS respectively. After 7 days the total As in the supernatants from the aerobic experiment fell to a minimum of 65% of the total added, recovering to 15% less than added after 30 days. By using anion and cation exchange chromatography coupled to ICP–MS detection, arsenobetaine (AB), dimethylarsinic acid (DMA), dimethylarsinoylacetic acid (DMAA) and trimethylarsine oxide (TMAO) were identified as degradation products. Results from the aerobic system showed that after 7 days incubation the AB had been degraded to DMA, DMAA and TMAO and after 30 days the degraded AB reappeared in the samples. The results for the anaerobic system showed no degradation of AB over the 30 day course of the experiment. These findings demonstrate for the first time that biocatalytic capability for AB degradation exists within the human gastrointestinal tract.