Thallus heterogeneity of the cyanolichen Pseudocyphellaria crocata

A lichen is a symbiotic association between a fungus (the mycobiont) and one or more photosynthetic partners (the photobiont) which may be a green alga, a cyanobacterium (blue-green algae) or both a green alga and a cyanobacterium. This is a complex association where the fungus produces a thallus distinct from its free- living form, and this thallus “houses” the photobiont. Pseudocyphellaria crocata is a cyanolichen, with the cyanobacterium Nostoc providing both photosynthate and fixed nitrogen to the fungal partner. Based upon previous data collected from other Nostoc symbioses, a proposed symbiotic association model was created by Rai et al. (2000). This model suggests that there are some substantial structural and functional changes between differing parts of the symbiotic tissue, i.e. the further from the growing margin the cyanobacterium are, the higher the level of heterocyst frequency and nitrogen fixation. The aims of this project were to both (i) look at the differing patterns of gene expression for the photobiont across different areas of the thallus, and (ii) to examine any physiological changes in intrathalline heterogeneity. The main results demonstrate that there are key differences in both gene expression and the physiological measurements that are involved in cell specific functions in the continuous cyanobacterial layer across the thallus, suggesting some form of thallus heterogeneity in Pseudocyphellaria crocata. The gene expression data showed an increase in nitrogen fixation and heterocyst frequency in the thallus centre and conversely an increase in the photosynthetic capability in the growing margins, as is also seen in the proposed symbiotic association model. Whereas the physiological measurements, being chlorophyll fluorescence and oxygen evolution, were too highly variable and were therefore too difficult to draw any solid conclusions from. However, between the two data sets, the overall results imply that there is some form of thallus heterogeneity occurring in Pseudocyphellaria crocata. Further research is needed to provide insight into the communication and co-ordinated growth of the respective symbionts, in order to gain a deeper understanding of this symbiosis