Evidence for ethanolic fermentation in lichens during periods of high thallus water content

Direct gas exchange between 'landplants' and the atmosphere represents a co ntinuos process of vital importance and it involves more gas species than C O2, O-2, and water vapour. Within these exchange processes emissions of vol atile organic compounds (VOC) contribute significantly to the oxidation cap acity and hence the O-3 level of the atmosphere. In order to investigate th e VOC exchange of lichens, some abundant boreal macrolichens were enclosed in a dynamic cuvette system and measured under climate chamber conditions. Air samples were simultaneously collected from a cuvette with lichens enclo sed and from an empty reference cuvette. The results obtained from lichens measured under prepurified air conditions pointed to a temporary emission o f acetaldehyde occurring mainly within the range of higher thallus water co ntents (TWC) when diffusion resistances are high and the CO2 and O-2 exchan ge is impeded. We supposed the acetaldehyde emission being caused by a temp orary O-2 deprivation. Therefore subsequent VOC measurements were conducted under synthetic air conditions with and without O-2 and with increased CO2 content. The switch from oxigenated to anoxic conditions resulted in an in crease of the acetaldehyde emission of about 100%. A simultaneous emission of ethanol could, be monitored by the operation of a proton-transfer-reacti on mass spectrometer (PTR-MS). The acetaldehyde/ethanol co-emission confirm ed the occurrence of the ethanolic fermentation within lichens, As ethanoli c fermentation enables the lichens to bypass intrathalline oxygen deficienc y at high TWC levels, it represents both a source of VOC emissions to the a tmosphere and a so far unconsidered carbon loss for lichens.