Differential responses of lichen symbionts to enhanced nitrogen and phosphorus availability:: An experiment with Cladina stellaris

Background and Aims Lichens can be both nitrogen- (N) and phosphorous- (P) limited and thus may be susceptible to nutrient enrichment. Nutrient enrichment with N and P may have differing impacts on the lichen structure because of different physiological responses of fungal and algal partners to these nutrients. The hypothesis was tested that the differential responses of lichen symbionts to enhanced availability of N and P is reflected in the lichen thallus structure and the wall-to-wall interface between the algal and fungal cells. Methods Lichen cushions of Cladonia stellaris were treated with one P and two N concentrations alone and in combination that yielded total depositions of approx. 300 (moderate) and 1000 (high) mg N m(-2) and 100 (high) mg P m(-2) over an experiment lasting 14 weeks. The effects of N and P inputs on the relative volumes of fungal and algal cell in the medullary tissue and on the thallus structure were studied using light microscopy. The interface between algal and fungal cell walls was examined using transmission electron microscopy. Key Results The influence of excess P on the lichen thallus structure was stronger than that of additional N. Addition of P reduced the N : P ratio in podetia, the proportion of the medullary layer volume occupied by the algal cells, the thallus volume occupied by the internal lumen, and the algal cell-wall area covered by fungal hyphae. Conclusions Ecologically realistic changes in the availability of key macronutrients can alter the growth of symbionts. Reduction in the proportion of photobiont cells indicates that the application of P either stimulates fungal hyphal growth in the medullary tissue or impairs the cell division of the algal cells. The results suggest that both the N and P availability and thallus N : P ratio affect the growth rates of lichen symbionts.