Carbonic anhydrase(s) associated with lichens: in vivo activities, possible locations and putative roles

The activity of the enzyme carbonic anhydrase (CA; E.C. 4.2.1.1) was examined in eight species of lichen in vivo by measurements of ¹⁸ O exchange from doubly labelled CO₂ (¹³ C¹⁸ O¹⁸ O) to water, with the aim of determining whether this enzyme is present in lichens and if so to what extent it participates in physiological processes. The lichens were chosen to represent different mycobiont (Heterodera, Leptogium, Lobaria, Peltigera, Pseudocyphellaria, Xantoparmelia) and photobiont (Coccomyxa, Dictyochloropsis, Myrmecia, Nostoc, Trebouxia) genera as well as varying morphology. All lichens were found to possess considerable activity of CA, which varied by a factor of about four between the species when compared on a chlorophyl basis, and by u factor of about 10 when related to the weight. Incubation of lichen tholli with ethoxyzolamide (EZA) and acetazolamide (AZA), which inhibit intracellular and extracellular CA respectively, showed that intracellular CA was present in all of the species. Incubation with EZA also resulted in inhibition of photosynthetic CO₂ , uptake in all species suggesting a role of internal CA in the CO₂ , acquisition process in all the investigated photobiont genera. A part of the CA activity was inhibited by AZA in all except two of the four cyanobacterial Nostoc lichens, indicating the possible presence of extracellular forms of CA. This CA also appeared to be involved in the acquisition of CO₂ , as AZA inhibited photosynthetic CO₂ uptake in these lichens. The CA inhibitors also affected CO₂ efflux in the dark, i.e. fungal respiration, which was inhibited by both AZA and EZA in six of the species, suggesting that a proportion of the CA activity might be located in the mycobiont.