Carbon and nitrogen distribution in the green algal lichens Hypogymnia physodes and Platismatia glauca in relation to nutrient supply

With the aim of understanding how some lichens can survive intensive fertilization we investigated two green algal (Trebouxia) lichens, Hypogymnia physodes (L.) Nyl. and Platismatia glauca (L.) W. Culb., and compared control (Ctr), and intensively fertilized (F) thalli. We measured total N, proteins and amino acids to assess lichen N status. Chlorophyll a indicated photosynthetic capacity and photobiont mass, ergosterol the metabolic demands of the fungus, and chitin the fungal biomass. For carbon status we measured glucose, the photobiont (Trebouxia) export product ribitol, and the mycobiont-specific carbohydrates arabitol and mannitol. The F-thalli had 2-3 times higher protein and N concentrations, 5-10 times higher chlorophyll a concentrations, while ergosterol and chitin were doubled. The ribitol concentrations were 4-5 times higher in the F-thalli, while the fungal carbohydrates did not increase to the same extent. The amino acid arginine had increased 60-fold. The F-thalli also had a relatively higher N investment in the photobiont in relation to mycobiont tissue compared to the Ctr-thalli, probably resulting in an increased capacity for carbon assimilation, most possibly required for maintaining the higher nutrient status of the F-thalli. Arginine accumulation possibly avoided toxic effects of accumulated NH4+, albeit binding a significant fraction of assimilated carbon.