15N – nitrate uptake and nitrogen exchange in the bionts of the lichen Parmelia sulcata

Abstract

Nitrate uptake and nitrogen inclusion into amino acids were studied in the intact thallus and isolated bionts of the lichen Parmelia sulcata with the aid of mass spectroscopic tracing of heavy isotope 15N. The isolated photobiont, the green algae Trebouxia sp. did not take up nitrate, whereas the mycobiont and intact thalli were enriched in 15N when incubated with Na15NO3. Pulse feeding experiments with intact thalli followed by separation of photobiont showed that the labelled nitrate was originally assimilated by the mycobiont and only after that was detected in the photobiont. The isolated mycobiont after pulse labeling excreted labeled compounds into the incubation medium. Amino acids were detected in the exudate. The quantities of two amino acids considerably exceeded those of the others. One was identified as alanine, the other could not yet be identified with certainty. Both of these high-quantity compounds were also much more enriched in 15N than the others. These two compounds are proposed to be the transport forms of nitrogen within the Parmelia sulcata thallus.