PHOTOBIONT-RELATED DIFFERENCES IN CARBON ACQUISITION AMONG GREEN-ALGAL LICHENS

The photosynthetic properties of a range of lichens (eight species) co ntaining green algal primary photobionts of either the genus Coccomyxa , Dictyochloropsis or Trebouxia were examined with the aim of obtainin g 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) withi n the photobiont. It was found that lichens with Trebouxia photobionts (four species) were able to accumulate a small pool of inorganic carb on (DIC; 70-140 nmol per mg chlorophyll (Chl)), in the light, which th eoretically may result in, at least, a two- to threefold increase in t he stromal CO2 concentration, as compared to that in equilibrium with ambient air. The other lichens (four species), which were tripartite a ssociations between a fungus, a cyanobacterium (Nostoc) and a green al ga (Coccomyxa or Dictyochloropsis) accumulated a much smaller pool of DIC (10-30 nmol.(mg Chl)(-1)). This pool is most probably associated w ith the previously documented CCM of Nostoc, inferred from the finding that free-living cells of Coccomyxa did not show any signs of DIC acc umulation. In addition, the kinetics of fast CO2 exchange for free-liv ing Nostoc were similar to those of intact tripartite lichens, especia lly in their responses to the CCM and the carbonic anhydrase (CA) inhi bitor ethoxyzolamide. Trebouxia lichens had a higher photosynthetic ca pacity at low and limiting external CO2 concentrations, with an initia l slope of the CO2-response curve of 2.6-3.9 mu mol.(mg Chl)-1.h(-1).P a-1, compared to the tripartite lichens which had an initial slope of 0.5-1.1 mu mol.(mg Chl)(-1).h(-1).Pa-1, suggesting that the presence o f a CCM in the photobiont affects the photosynthetic performance of th e whole lichen. Regardless of these indications for the presence or ab sence of a CCM, ethoxyzolamide inhibited the steady-state rate of phot osynthesis at low CO2 in all lichens, indicating a role of CA in the p hotosynthetic process within all of the photobionts. Measurements of C A activity in photobiont-enriched homogenates of the lichens showed th at Coccomyxa had by far the highest activity, while the other photobio nts displayed only traces or no activity at all. As the CCM is apparen tly absent in Coccomyxa, it is speculated that this alga compensates f or this absence with high internal CA activity, which may function to reduce the CO2-diffusion resistance through the cell.