Acute toxicity of arsenic to Aliivibrio fischeri (Microtox super( registered ) bioassay) as influenced by potential competitive-protective agents

In this study, we investigated the effect of some potential alleviative compounds against the acute toxicity of arsenic (As super(V), As super(III) and DMA super(V)) on Aliivibrio fischeri (formerly Vibrio fischeri), a bioluminescent model bacterium, through the Microtox registered bioassay. The compounds studied differed in their mechanism of action, and they included the following: phosphate and glycerol, as chemical analogues (and potential competitors) of As super(V) or As super(III), respectively; citrate, a weak natural organic ligand; and the antioxidant ascorbic acid. Special attention was paid to phosphate effects, a widespread pollutant in natural environments. As super(V) was found to be more acutely toxic than As super(III) to A. fischeri, in accordance with its higher interaction with the bacteria. Both As super(V) and As super(III) were found to be much more acutely toxic than DMA super(V), which was essentially non-acutely toxic even at very high concentrations. Phosphate presence (at equimolar P/As ratios or higher) resulted in the almost total suppression of bioluminescence inhibition, suggesting it exerts an alleviative effect against As super(V) acute toxicity on A. fischeri. Interestingly, the uptake and the percentage of extracellular As super(V) were not affected by the addition of phosphate, suggesting that such protective effect does not result from the competition for their common transporters. In contrast, the acute toxicity of As super(III) was essentially unaffected by phosphate. Glycerol did not decrease the acute toxicity or the uptake of As super(III) by A. fischeri, denoting the likely occurrence of an additional mechanism for As super(III) uptake in such bacteria. Similarly, citrate and ascorbic acid essentially did not caused alleviation of As super(V) or As super(III) acute toxicity. As for environmental and operational implications, P could beneficially protect aquatic microorganisms against acute detrimental effects of As super(V), whilst its presence could mask the toxicity due to As super(V) when assessed using the Microtox registered bioassay, thus leading to seriously underestimate the actual ecological and health risks.