Rapid breakdown of exogenous extracellular hydrogen peroxide by lichens

All organisms, even highly stress-tolerant lichens, produce a variety of reactive oxygen species (ROS) during and after stress. Furthermore, the cell walls of some lichens in Suborder Peltigerineae contain laccases, and therefore can produce quinone radicals that can break down to yield ROS. While the extracellular ROS produced by these enzymes probably play important roles in the biology of these lichens, they may also be potentially harmful and need to be rapidly broken down. To test this, rates of breakdown of exogenously supplied H2O2 were measured in a range of lichen species. Considerable diversity existed in rates of H2O2 breakdown but rates were on average almost double in members of Suborder Peltigerineae. While all lichens tested appeared to lack extracellular peroxidases and catalases, enzymes normally involved in breaking down H2O2, extracellular tyrosinase activity could be readily detected in the Peltigerineae. A role for tyrosinases in H2O2 breakdown was supported by the results from experiments involving inhibitors, and demonstration of the simultaneous release into an incubation solution of tyrosinase activity and the ability to breakdown H2O2. Rates of breakdown were very high, and tyrosinase appeared to break down H2O2 by a catalase-like mechanism. However, significant rates of breakdown of H2O2 also occurred in species that did not possess cell wall redox enzymes. These species probably took up the exogenously supplied H2O2 intracellularly and then broke it down by the usual catalases and peroxidases. The importance of H2O2 degradation is discussed in terms of its possible role in defence against the harmful effects of ROS.