PHOTOSYNTHETIC CAPACITY IN RELATION TO NITROGEN-CONTENT AND ITS PARTITIONING IN LICHENS WITH DIFFERENT PHOTOBIONTS

We tested the hypothesis that lichen species with a photosynthetic CO2 -concentrating mechanism (CCM) use nitrogen more efficiently in photos ynthesis than species without this mechanism, Total ribulose bisphosph ate carboxylase-oxygenase (Rubisco; EC 4.1.1.39) and chitin (the nitro genous component of fungal cell walls), were quantified and related to photosynthetic capacity in eight lichens. The species represented thr ee modes of CO, acquisition and two modes of nitrogen acquisition, and included one cyanobacterial (Nostoc) lichen with a CCM and N-2 fixati on, four green algal (Trebouxia) lichens with a CCM but without N-2 fi xation and three lichens with green algal primary photobionts (Coccomy xa or Dictyochloropsis) lacking a CCM, The latter have N-2-fixing Nost oc in cephalodia, When related to thallus dry weight, total thallus ni trogen varied 20-fold, chitin 40-fold, Chi a 5-fold and Rubisco 4-fold among the species. Total nitrogen was lowest in three of the four Tre bouxia lichens and highest in the bipartite cyanobacterial lichen, Lic hens with the lowest nitrogen invested a larger proportion of this int o photosynthetic components, while the species with high nitrogen made relatively more chitin, As a result, the potential photosynthetic nit rogen use efficiency was negatively correlated to total thallus nitrog en for this range of species, The cyanobacterial lichen had a higher p hotosynthetic capacity in relation to both Chi a and Rubisco compared with the green algal lichens. For the range of green algal lichens bot h Chi a and Rubisco contents were linearly related to photosynthetic c apacity, so the data did not support the hypothesis of an enhanced pho tosynthetic nitrogen use efficiency in green-algal lichens with a CCM.