The role of secondary metabolites in protecting lichens from climate change induced stress.

Abstract

Lichen secondary metabolites play a great diversity of roles in lichen biology by acting as UV screens, antimicrobials, herbivore deterrents or allelopathic compounds. The main aim of the work described in this thesis was to test the potential roles of secondary metabolites in the tolerance of lichens (twelve species) to a variety of abiotic stresses. The first stress considered was that of tolerance to high levels of photosynthetically active radiation (PAR). Photosynthetic organisms possess a great diversity of mechanisms to protect themselves from the potentially stressful effects of high PAR. In addition to non-photochemical quenching and antioxidant mechanisms, it has been suggested that lichens use secondary metabolites for photoprotection. A few studies have shown that even faintly pigmented or unpigmented lichen substances can reduce photoinhibition. Here, the acetone rinsing technique was used to harmlessly remove secondary metabolites from all lichen thalli and then the chlorophyll fluorescence technique was used to measure their tolerance to photoinhibition in desiccated and hydrated states. Results showed that colorless lichen substances can increase the tolerance of lichen photobionts to photoinhibition when thalli are hydrated, apparently by increasing reflectance. Interestingly, substances can also photoprotect lichens in the dry state, while having no effect on reflectance. The acetone rinsing technique was also used to compare the relative importance of lichen substances in photoprotection in sun and shade collections of four species of Afromontane lichens. Results showed that lichens collected from sunny microhabitats have higher tolerance to photoinhibition than those from shaded locations. Furthermore, removal of lichen substances increases sensitivity to photoinhibition much more in sun than shade collections. Results further emphasized the importance of lichen secondary metabolites in photoprotection. This study also considered whether melanins, a pigmented secondary metabolite can cause problems with using fluorimetry techniques for e.g., to measure NPQ. A dissecting technique was used to remove the lower cortices and medullas of two lichen species so that NPQ could be measured from the underside of the thallus with an imaging PAM. Results confirmed that NPQ can be satisfactorily assessed with a standard fluorimeter by taking measurement from above using intact thalli. However, interestingly, photobionts from the bottom of the photobiont layer tended to have slightly lower rates of photosynthetic activity and lower NPQ than those at the top, i.e., display mild “shade” characteristics. The results presented in the final chapter looked at the protective role of secondary metabolites against desiccation-induced stress. It was hypothesized that secondary metabolites may act as antioxidants that protect lichens from desiccation-induced stress. Unfortunately, the findings of this part of the work did not present a consistent story, but rather indicated that according to species, the secondary metabolites can act as antioxidants or prooxidants. Nevertheless, taken together, the work presented in this thesis clearly shows that lichen secondary metabolites, whether pigmented or unpigmented, play important roles in photobiont photoprotection.