High rates of extracellular superoxide production by lichens in the suborder Peltigerineae correlate with indices of high metabolic activity

Rates of extracellular superoxide radical (O-2(.-)) formation were measured in 34 species of lichens from different taxonomic groupings and contrasting habitats before and after desiccation stress. All 21 species from the suborder Peltigerineae produce O-2(.-) extracellularly at high rates, even when they are not stressed. In addition, some species show a burst of O-2(.-) production during rehydration following desiccation. Extracellular production of O-2(.-) is almost absent in the species from other lichen groups. In general, production of high levels of O-2(.-) and the existence of an inducible oxidative burst are best developed in species growing in wet microhabitats. Rates of O-2(.-) production are also positively correlated to previously published indices of lichen metabolic activity. Preliminary studies on the identity of the O-2(.-) producing enzymes suggest that they do not possess the classical characteristics of those suggested to produce reactive oxygen species in higher plants. Patterns of O-2(.-) production are discussed in terms of the strategies used by different lichens groups in their defence against pathogenic fungi and bacteria.