Evidence for the translocation of fixed N in the N2-fixing lichen Stereocaulon vesuvianum.

The fruticose lichen Stereocaulon vesuvianum is among the most abundant and widespread lichens in upland Britain. It typically produces cephalodia (nodules) that contain the cyanobacterium Stigonema, which can fix atmospheric nitrogen. However, over much of England, Wales, and southern Scotland S. vesuvianum no longer produces cephalodia and does not fix nitrogen, a morphological change linked to elevated atmospheric nitrogen deposition. This provided a unique opportunity to compare the ¹⁵N natural abundance signatures in N₂-fixing and non-N₂-fixing lichen populations, keeping in mind that fixed nitrogen has a ¹⁵N content close to that of atmospheric N₂ while, in comparison, several components of atmospheric combined N (e.g. nitrate and ammonium in precipitation) tend to be ¹⁵N depleted. We found that in N₂-fixing samples, there was a steep gradient in ¹⁵N relative abundance in the terminal 15 mm of thallus branches (pseudopodetia), changing from ¹⁵N depleted tissues at 10-15 mm below the tips to values close to that of atmospheric N₂ at the apices while in non-N₂-fixing samples thallus branches were uniformly ¹⁵N depleted. The ¹⁵N gradient in N₂-fixing material could not be explained by the presence of cephalodia since these are more abundant towards branch bases. The data provide the first evidence in lichens of translocation of recently fixed N to sink regions of active growth and production of asexual reproductive propagules, bringing lichens into line with N source-sink relationships in N₂-fixing plant symbioses.