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.