Comparative biochemistry of oxygen toxicity in lactic acid-forming aquatic fungi

Abstract

Taxonomically diverse aquatic fungi ranging in oxidative capabilities from obligate aerobes to aerotolerant anaerobes were examined for growth under hyperbaric (0.9 atm) O2, and for the ability to degrade H2O2 and O 2 - . The results support the presumption that several Oomycetes and Chytridiomycetes are biochemically adapted to environments low in O2. Results further indicate significant differences between Oomycetes and Chytridiomycetes in the enzymatic means of dealing with O 2 - and H2O2, supporting the recent concept of a great evolutionary divergence between the groups. In general, facultative anaerobes and aerotolerant anaerobes were more severely inhibited by hyperbaric O2, and they exhibited lower superoxide dismutase (SOD), catalase and peroxidase activities than did strongly-oxidative species. SOD activity, which was detected in all isolates, was insensitive to cyanide in Oomycetes but cyanide sensitive in the Blastocladiales (Chytridiomycetes). All strongly-oxidative Oomycetes exhibited readily-detectable catalase and peroxidase activities, while activities were very low or absent in strongly-fermentative species. As with the Oomycetes, peroxidase activities among the Blastocladiales were high in aerobes and low in strong fermenters. Surprisingly, however, none of the Blastocladiales, including strongly-oxidative species, exhibited substantial catalase activity. Catalase and SOD activities in faculatively anaerobic Oomycetes increased with increasing O2 concentration; but even in hyperbaric (0.5 atm) O2, activities for both enzymes in the aerotolerant anaerobe Aqualinderella fermentans were very low compared with activities in aerobes.