Melanisation in the old forest lichen Lobaria pulmonaria reduces the efficiency of photosynthesis

The old forest lichen Lobaria pulmonaria synthesizes melanic pigments when exposed to ultraviolet light and high solar radiation. Here, we tested the effect of melanisation on photosynthetic efficiency. Melanisation effectively reduces high-light stress in lichen photobionts, as the photobionts of melanised thalli are healthy, based on chlorophyll contents and maximum rates of photosynthesis. However, the quantum yields of both photosynthetic CO2 uptake and O2 evolution were more than 40 percent lower in melanised thalli compared with control thalli. While chlorophyll fluorescence measurements suggested that melanised and pale thalli had similar apparent electron transport rates, this result was probably an artefact caused by screening reducing the light reaching the photobionts. Melanic thalli also had a higher chlorophyll a/b ratio and more xanthophyll cycle pigments, suggesting that the photosynthetic apparatus had adapted to high light. In conclusion, while protecting photobionts from high light, melanisation clearly reduced photosynthetic efficiency. Melanised thalli will be significantly disadvantaged if light levels return to lower values, more typical for those habitats in which this shade adapted lichen is most abundant.