Three different techniques were used to measure the toxicity of each of these three compounds. The first method, called growth inhibition, merely determined the change in the rate of growth of bacteria (of a specific organism, Pseudomonas fluorescens K27) that had been exposed to known concentrations of a particular compound. Effective concentration 50 or EC50 for each compound was determined as the concentration of a particular toxicant that cause a 50% decrease in relative growth rate as measured by the culture's cell population, which in turn was measure by optical density.
The second toxicity assessment technique used was growth rate inhibition. Roughly speaking this method calculates the specific growth rate of Pseudomonas fluorescens K27 as determined by the slope of the growth curve determined in the logarithmic phase of growth. The Ec50 for this method was calculated as the concentration of toxicant that caused a 50% decrease in the specific growth rate.
The last method of toxicity assay involved a commercial bioassay method called Microtox® which determines the decrease in intensity of light output from live cultures of bioluminescent bacteria (Vibrio fisheri ). As more toxicant is added-or as an identical amount of a more toxic toxicant is added-the bacterial bioluminescence decreases (as measured after a specific time after the addition of toxicant).
The results of this research were that the effective concentrations of the three compounds varied among the three different methods and two different organisms used (Pseudomonas fluorescens K27 or Vibrio fisheri ). When using the first organism and growth and growth rate inhibition parameters dimethyl selenone was the least toxic and selenite the most with selenate lying in between.
However, using Microtox bioassay the order of relative order of toxicty was different with selenate the most and dimethyl selenone of intermediate toxicity and selenite anion the least toxic of the three.
Given these result it is possible that for the K27 organism, different biological means of reduction may be occuring for the two Se oxyanions since Se has a different oxidation state in each: Se(VI) in selenate and Se(IV) in selenite.
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