Effects of inorganic nitrogen on C2H 2 reduction and CO 2 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.