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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.

Vibrio fischeri bacteria, used as a biological target in either acute or chronic toxicity tests, display a low sensitivity to Cr(VI). This phenomenon could be due to the capacity of these bacteria to reduce Cr(VI) into Cr(III). This reducing capacity was found to depend on culture medium composition, pH value, incubation time and the presence of a carbon source. It also depends on the nature of the carbon source, glucose being more efficient than glycerol. This is probably related to differences in bacterial metabolism when given either glucose or glycerol. The thermostable Cr(VI)-reducing activity found in the supernatants of V. fischeri cultures grown on glucose suggests that, under these conditions, the bacteria release non-proteic reducing substances which have not been identified yet.

Results from two mathematical approaches to predict the toxicity of all the possible binary equitoxic mixtures of Co, Cd, Cu, Zn and Pb were compared to the observed toxicity of these mixtures to Vibrio fischeri bacteria. Combined effect of the metals was found to be antagonistic for Co-Cd, Cd-Zn, Cd-Pb and Cu-Pb, synergistic for Co-Cu and Zn-Pb and merely additive in other cases, revealing a complex pattern of possible interactions. Besides, Cd appears much less toxic to the bacterial model than to animal cells. The synergistic effect of the Co-Cu combination and the strong lowering of Pb toxicity in the presence of Cd deserve much attention when establishing environmental safety regulations.