Acute toxicity of arsenic to Aliivibrio fischeri (Microtox 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(V), As(III) and DMA(V)) on Aliivibrio fischeri (formerly Vibrio fischeri), a bioluminescent model bacterium, through the Microtox 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(V) or As(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(V) was found to be more acutely toxic than As(III) to A. fischeri, in accordance with its higher interaction with the bacteria. Both As(V) and As(III) were found to be much more acutely toxic than DMA(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(V) acute toxicity on A. fischeri. Interestingly, the uptake and the percentage of extracellular As(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(III) was essentially unaffected by phosphate. Glycerol did not decrease the acute toxicity or the uptake of As(III) by A. fischeri, denoting the likely occurrence of an additional mechanism for As(III) uptake in such bacteria. Similarly, citrate and ascorbic acid essentially did not caused alleviation of As(V) or As(III) acute toxicity. As for environmental and operational implications, P could beneficially protect aquatic microorganisms against acute detrimental effects of As(V), whilst its presence could mask the toxicity due to As(V) when assessed using the Microtox bioassay, thus leading to seriously underestimate the actual ecological and health risks.