The Loroxanthin cycle: A new type of xanthophyll cycle in green algae (Chlorophyta)

Xanthophyll cycles have proven to be major contributors to photoacclimation for many organisms. This work describes a light-driven xanthophyll cycle operating in the chlorophyte Chlamydomonas reinhardtii and involving the xanthophylls Lutein (L) and Loroxanthin (Lo). Pigments were quantified during a switch from high to low light and at different time points from cells grown in Day/night cycle. Trimeric LHCII was purified from cells acclimated to high or low light and their pigment content and spectroscopic properties were characterized. The Lo/(L+Lo) ratio in the cells varies by a factor of 10 between cells grown in low or high light leading to a change in the Lo/(L+Lo) ratio in trimeric LHCII from 0.5 in low light to 0.07 in high light. Trimeric LhcbMs binding Loroxanthin have 5+/-1% higher excitation energy transfer from carotenoid to Chlorophyll as well as higher thermo- and photostability than trimeric LhcbMs that only bind Lutein. The Loroxanthin cycle operates on long time scales (hours to days) and likely evolved as a shade adaptation. It has many similarities with the Lutein-epoxide - Lutein cycle of plants.

Preference of lichens for shady habitats is correlated with intolerance to high nitrogen levels

Based on findings in vascular plants showing that the capacity to provide enough carbon skeletons for rapid ammonium assimilation into amino acids is a prerequisite for tolerance to eutrophication, we tested the hypothesis that lichens from shady habitats are particularly sensitive to nitrogen pollution. We tested this hypothesis using published ecological indicator values (estimates of eutrophication tolerance and light preferences on an ordinal scale) for more than 500 central European lichen species. Our results show that shade-adapted lichens are indeed at the same time intolerant to eutrophication. However, not all eutrophication-sensitive lichens inhabit shady environments, suggesting the existence of several independent mechanisms causing intolerance of high nitrogen levels in lichens. The correlation of shade adaptation with nitrogen intolerance is limited to epiphytic and saxicolous species, since terricolous lichens are out-competed by vascular plants in dense vegetation. Our results suggest that lichen communities of shady bark, wood and rock are particularly sensitive to eutrophication.