The Oxygen Tension Within Lichen Thalli

SUMMARY A technique is described for estimating the internal pO2 of moisture saturated lichen thalli under conditions of known light intensity and temperature. The method is polarographic, using unprotected microelectrodes, with brief experimental periods. In lichens with the phycobiont forming a zone of the thallus (Peltigera canina, P. poly‐dactyla and P. aphthosa) the pO2 in the algal zone is normally slightly hypoaerobic rising to aerobic in bright sunny conditions, and becoming substantially hypoaerobic (pO2= 1–1.5 × 104 Pa) in darkness. The pO2 within external cephalodia of lichens which have them is more affected by light. P. aphthosa and Placopsis gelida were examined and showed large and abrupt responses to changes in light intensity, the pO2 ranging from 3 × 104 Pa to 104 Pa in Peltigera aphthosa; 2.1 × 104 Pa to 104 Pa in Placopsis gelida. The nitrogenase activity shown by these lichens is probably due to the high proportion of heterocysts, conferring some immunity to high pO2 conditions.