This study examines the effects of environmental factors on the surface phosphatase activities of the lichens Peltigera canina (L.) Willd., P. praetextata (Flōrke. ex Sommerf) Vain, and Cladonia arbuscula (Wallr.) Rabenh. Four plants of each species were collected in the U. K. from low (Middleton Common, Windy Nook) and high (Nenthead) Zn environments. The mean tissue (soil) Zn concentrations in P. canina, P. praetextata and C. arbuscula (high Zn) were 4.3 (18.9), 5.4 (25.4) and 4.1 (8.3) times higher respectively than the same species (low Zn).Phosphomonoesterase (PMEase) activity was measurable at all pH values in the range 3.0 - (10.3) 11.0 with 250 µM p-nitrophenyl phosphate (pNPP) and 250 µM 4-methylumbelliferyl phosphate (4-MUP) Both pNPP and 4-MUP showed maximum PMEase at pH: 10.3 in P. canina (low Zn), P. praetextata (low and high Zn); 6.0 in P. canina (high Zn), C. arbuscula (low Zn); 6.0-7.0 in C. arbuscula (high Zn). Maximum PMEase in P. canina and P. praetextata (high Zn) exceeded maximum PMEase in the same species (low Zn), while the converse was true for C. arbuscula. Fluorometric determination of PMEase with 4-MUP yielded lower maximum rates of activity in all species from low and high Zn environments than spectrophotometric methods (pNPP).PMEase activity in all species from low and high Zn environments exceeded phosphodiesterase (PDEase) activity between 1.5 and 5.1 times. P. canina and P. praetextata displayed a greater tolerance to Zn than C. arbuscula, with inhibitory effects on PMEase activity noticeable at 1 mM in P. canina, and P. praetextata (high Zn) and 10 µM Zn in all other plants. Storage for six weeks, rhizinal cellular damage and light all resulted in an increase in PMEase activity with 250 µM pNPP in P. canina (high Zn) of less than 4 %. The lower layer (rhizines and veins) of P. canina was shown to have greater PMEase activity at all pH values, with 1 µM 4-MUP, 250 µM 4-MUP and 250 µM pNPP. Staining techniques undertaken on P. canina (high Zn) suggest that phosphatase activity is probably due to the fungus, especially the rhizines and veins, and not the cyanobacterium or contaminant bacteria. PMEase and PDEase were associated with the cell wall, cytoplasm and most noticeably at hyphal junctions. All plants displayed maximum PMEase activities at pH values which did not correspond with any of the presumed soil pH values
