Re: What effect should ethidium bromide have on paramecium?

I am not sure that I completely understand your experimental set-up; I am 
unclear about the purpose of barium and calcium chloride. Following are 
some possible explanations for seeing only a transient effect of ethidium 
bromide on the culture:

1.  The mutagenic effect of ethidium bromide depends on the dose to which 
your Paramecium culture was exposed. Did you experiment with different 
concentations of the mutagen? Alternatively, Paramecium might have a very 
good DNA damage repair system.

2.  Another possibility is decomposition of ethidium bromide with time in 
the light (the compound is light sensitive)or degradation by 
microorganisms in the culture (I don't know if you used an axenic culture).

I am not sure that either of these possibilities applies to your 
experiment, but without some more information this is all I can think of.

Good luck! Be careful when handling this potent mutagen. Wear gloves, 
goggles and a protective lab coat at all times, and don't forget to 
properly and safely dispose of the ethidium bromide and all liquids 
containing this compound.