The role of secondary metabolites in protecting lichens from climate change induced stress.
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Date
2022
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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.
Description
Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.