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 e xamined in eight species of lichen in vivo by measurements of O-18 exc hange from doubly labelled CO2 ((COO)-C-13-O-18-O-18) to water, with t he aim of determining whether this enzyme is present in lichens and if so to what extent it participates in physiological processes. The lic hens were chosen to represent different mycobiont (Heterodera, Leptogi um, Lobaria, Peltigera, Pseudocyphellaria, Xantoparmelia) and photobio nt (Coccomyxa, Dictyochloropsis, Myrmecia, Nostoc, Trebouxia) genera a s well as varying morphology. All lichens were found to possess consid erable activity of CA, which varied by a factor of about four between the species when compared on a chlorophyl basis, and by a factor of ab out 10 when related to the weight. Incubation of lichen thalli with et hoxyzolamide (EZA) and acetazolamide (AZA), which inhibit intracellula r and extracellular CA respectively, showed that intracellular CA was present in all of the species. Incubation with EZA also resulted in in hibition 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 t he possible presence of extracellular forms of CA. This CA also appear ed to be involved in the acquisition of CO2, as AZA inhibited photosyn thetic CO2 uptake in these lichens. The CA inhibitors also affected CO 2 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 o f the CA activity might be located in the mycobiont.