Does anyone know what is the maximal concentration of oxygen that can be tolerated by the parasite Giardia intestinalis?
Short answer: 50-60 µM O2 is ok; 80 µM is toxic.
Giardia lamblia (aka intestinalis) is a microaerophilic intestinal parasite.
Lloyd et al. (2000) The microaerophilic flagellate Giardia intestinalis: oxygen and its reaction products collapse membrane potential and cause cytotoxicity. Microbiology 146: 3109 - 3118
Giardia colonises the upper intestine, and the oxygen concentration there has been measured at 60 µM. At oxygen concentrations up to 50 µM the organism can scavenge oxygen; above 80 µM oxygen inhibits the oxygen-scavenging process. In this paper the authors used 10% air (equivalent to 25 µM oxygen) to characterise oxygen toxicity.
The source of oxygen sensitivity was investigated by:
Li and Wang (2006) A likely molecular basis of the susceptibility of Giardia lamblia towards oxygen Molecular Microbiology 59: 202 - 211
These authors propose that the enzyme DT-diaphorase, previously suggested as a component of the oxygen detoxification system promotes the formation of reactive oxygen species which lead to death. Thus they found that overproduction increased sensitivity to oxygen, whereas a knock-down decreased sensitivity. This work does not, however, use controlled microaerophilic conditions.