Photosynthetic capacity in relation to nitrogen content and its partitioning in lichens with different photobionts

Abstract

We tested the hypothesis that lichen species with a photosynthetic CO2‐concentrating mechanism (CCM) use nitrogen more efficiently in photosynthesis than species without this mechanism. Total ribulose bisphosphate carboxylase‐oxygenase (Rubisco; EC 4.1.1.39) and chitin (the nitrogenous component of fungal cell walls), were quantified and related to photosynthetic capacity in eight lichens. The species represented three modes of CO2 acquisition and two modes of nitrogen acquisition, and included one cyanobacterial (Nostoc) lichen with a CCM and N2 fixation, four green algal (Trebouxia) lichens with a CCM but without N2 fixation and three lichens with green algal primary photobionts (Coccomyxa or Dictyochloropsis) lacking a CCM. The latter have N2‐fixing Nostoc in cephalodia. When related to thallus dry weight, total thallus nitrogen varied 20‐fold, chitin 40‐fold, Chl a 5‐fold and Rubisco 4‐fold among the species. Total nitrogen was lowest in three of the four Trebouxia lichens and highest in the bipartite cyanobacterial lichen. Lichens with the lowest nitrogen invested a larger proportion of this into photosynthetic components, while the species with high nitrogen made relatively more chitin. As a result, the potential photosynthetic nitrogen use efficiency was negatively correlated to total thallus nitrogen for this range of species. The cyanobacterial lichen had a higher photosynthetic capacity in relation to both Chl a and Rubisco compared with the green algal lichens. For the range of green algal lichens both Chl a and Rubisco contents were linearly related to photosynthetic capacity, so the data did not support the hypothesis of an enhanced photosynthetic nitrogen use efficiency in green‐algal lichens with a CCM.