Adverse effects of organic arsenical compounds towards Vibrio fischeri bacteria

The most frequently encountered forms of organic arsenic, namely, dimethylarsinic acid, monomethylarsonic acid, arsenobetaine, arsenocholine and Roxarsone (4-hydroxy-3-nitrobenzene arsonic acid) were tested for toxicity either by using the Microtox bioassay, based on the rapid (within 15 min) fading of luminescence emitted by Vibrio fischeri marine bacteria, or by monitoring growth rate of the same bacteria for 3 days. Organic arsenic was generally found to be less toxic to these biological models than inorganic arsenic. In many cases, EC50 values for DMA, MMA or HNAA when using luminescence or growth inhibition assays could not be determined because of the low toxicity of the compounds. Nevertheless, results from the luminescence inhibition assay, which was found to be more sensitive than the growth inhibition assay, allowed to rank toxicity as follows: arsenate at pH 8>HNAA at pH 5>arsenate at pH 5>MMA at pH 5>HNAA at pH 8>DMA at pH 5. Arsenobetaine and monomethylarsonic acid were unexpectedly found to stimulate bacterial growth (hormesis effect). pH was found to have a strong influence on the observed toxicity as a consequence of the pH-induced changes in the chemical speciation of the tested molecules. In most cases it appeared that negatively charged forms were less toxic than the uncharged ones.