Photobiont-related differences in carbon acquisition among green-algal lichens

The photosynthetic properties of a range of lichens (eight species) containing green algal primary photobionts of either the genus Coccomyxa, Dictyochloropsis or Trebouxia were examined with the aim of obtaining a better understanding for the different CO₂ acquisition strategies of lichenized green algae. Fast transients of light/dark-dependent CO₂ uptake and release were measured in order to screen for the presence or absence of a photosynthetic CO₂-concentrating mechanism (CCM) within the photobiont. It was found that lichens with Trebouxia photobionts (four species) were able to accumulate a small pool of inorganic carbon (DIC; 70–140 nmol per mg chlorophyll (Chl)), in the light, which theoretically may result in, at least, a two to threefold increase in the stromal CO₂ concentration, as compared to that in equilibrium with ambient air. The other lichens (four species), which were tripartite associations between a fungus, a cyanobacterium (Nostoc) and a green alga (Coccomyxa or Dictyochloropsis) accumulated a much smaller pool of DIC (10–30 nmol·(mg Chl)^–1). This pool is most probably associated with the previously documented CCM of Nostoc, inferred from the finding that free-living cells of Coccomyxa did not show any signs of DIC accumulation. In addition, the kinetics of fast CO₂ exchange for free-living Nostoc were similar to those of intact tripartite lichens, especially in their responses to the CCM and the carbonic anhydrase (CA) inhibitor ethoxyzolamide. Trebouxia lichens had a higher photosynthetic capacity at low and limiting external CO₂ concentrations, with an initial slope of the CO₂-response curve of 2.6–3.9 mol·(mg Chl)^–1·h^–1·Pa^–1, compared to the tripartite lichens which had an initial slope of 0.5–1.1 mol-(mg Chl)^–1·h^–1·-Pa^–1, suggesting that the presence of a CCM in the photobiont affects the photosynthetic performance of the whole lichen. Regardless of these indications for the presence or absence of a CCM, ethoxyzolamide inhibited the steady-state rate of photosynthesis at low CO₂ in all lichens, indicating a role of CA in the photosynthetic process within all of the photobionts. Measurements of CA activity in photobiont-enriched homogenates of the lichens showed that Coccomyxa had by far the highest activity, while the other photobionts displayed only traces or no activity at all. As the CCM is apparently absent in Coccomyxa, it is speculated that this alga compensates for this absence with high internal CA activity, which may function to reduce the CO₂-diffusion resistance through the cell.Abbreviations CA carbonic anhydrase (EC 4.2.1.1) - CCM CO₂-concentrating mechanism - Chl chlorophyll - DIC dissolved inorganic carbon - EZ ethoxyzolamide or 6-ethoxy-2-benzo-thiazole-2-sulfonamide - GA glycolaldehyde - Hepps 4-(2-hydroxyethyl)-l-piperazinepropanesulfonic acid - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39) This research was supported by a grant from the Swedish Natural Sciences Resource Council to K.P.