Effects of inorganic nitrogen on C2H2 reduction and CO2 exchange in the Peltigera praetextata-Nostoc and Peltigera aphthosa-Coccomyxa-Nostoc symbioses

Uptake of NH ₄ ⁺ and NO ₃ ^- by the N₂-fixing lichens Peltigera praetextata (two-component lichen) and P. aphthosa (three-component lichen) was studied. In addition, the effects of these ions, separately and in combination, on C₂H₂ reduction and CO₂ exchange were examined. Both NH ₄ ⁺ and NO ₃ ^- were utilized by the lichens. NH₄NO₃ caused an increased liberation of NO ₃ ^- from the lichens as compared to the release observed in untreated lichen thalli. NH ₄ ⁺ and NO ₃ ^- led to reduced C₂H₂ reduction by P. praetextata, which, however, was less pronounced than when the two ions were given in combination. In P. aphthosa the C₂H₂ reduction was inhibited by NH ₄ ⁺ and NH₄NO₃, but not by NO ₃ ^- alone. NH ₄ ⁺ and NO ₃ ^- had no effect on the net photosynthesis of P. praetextata, while, in combination, they led to inhibition, although only at a concentration higher than that inhibitory to the C₂H₂ reduction of P. aphthosa. The photsynthesis was inhibited by all salts, but only initially, probably a salt effect. Effects of NH ₄ ⁺ on the membrane potential of the cyanobiont are suggested as an important factor causing the depression of net photosynthesis.