Loss of nitrogen fixing capacity in a montane lichen is linked to increased nitrogen deposition

Abstract

1. The circumboreal/circumpolar N 2 -fixing lichen Stereocaulon vesuvianum is among the most widespread and abundant fruticose species in montane Britain but has lost the capacity to fix N 2 over large areas of the country. 2. To investigate whether loss of N 2 -fixation in S. vesuvianum is linked to increased N deposition, we examined thallus morphology, physiology and chemistry at twelve locations representing an N deposition gradient of 3-40 kg ha −1 year −1 .Measurements were made in parallel on a non-N 2 -fixing reference species (Parmelia saxatilis). The presence or absence of cephalodia (N 2 -fixing nodules containing the cyanobacterium Stigonema sp) was recorded in over 500 herbarium specimens of S. vesuvianum dating back to 1820. 3. Cephalodium abundance in S. vesuvianum, and 15 N concentration in S. vesuvianum and P. saxatilis, were strongly negatively correlated with N deposition and particularly with dry deposited N; cephalodia do not form at total N deposition rates ≥8-9 kg ha −1 year −1 . Other morphological oddities in S. vesuvianum at N-polluted sites include increased apothecium (fungal reproductive structure) production and green algal biofilm development. Biofilm covered thalli without cephalodia lacked nitrogenase activity and cephalodia at sites where they rarely develop had nitrogenase activities typical for this species. The presence or absence of cephalodia in herbarium specimens of S. vesuvianum suggest that the present-day N-deposition linked gradient in N 2 -fixing capacity did not exist in the 19th century and largely developed between 1900-1940. 4. Synthesis. We provide clear evidence that N 2 -fixing capacity in S. vesuvianum has been lost in regions subjected to many decades of enhanced atmospheric N deposition. This loss is consistent with established models of diazotrophy, which identify supply of combined N as an inhibitor of N 2 -fixation. Progressive depletion of thallus 15 N with increasing N deposition is in line with available data indicating that much atmospheric N pollution is 15 N-depleted. Rates of nitrogenase activity in S. vesuvianum are low compared to other symbiotic systems and perhaps more likely supplanted by elevated N deposition. We suggest that | 281

The Tripartite Lichen Ricasolia virens: Involvement of Cyanobacteria and Bacteria in Its Morphogenesis

“…They depend on an efficient nitrogenase activity system that occurs in cyanobiont heterocysts [ 108 , 109 ]. Kershaw and Millbank [ 110 ] studied the nitrogen metabolism of Nostoc of cephalodia in Peltigera and L. pulmonaria [ 111 ], and they proved that practically all of the nitrogen fixed by Nostoc is used by the mycobiont, which was confirmed more recently in Lobaria oregana [ 112 ], L. pulmonaria [ 113 ] and Stereocaulon vesuvianum [ 114 ]. These results could be extrapolated to the thalli of R. virens , which could explain the large size of its thalli.…”

Leveraging natural history collections to understand the impacts of global change

“…Crittenden et al (2023), in Journal of Ecology , used contemporary surveys along a nitrogen deposition gradient in Wales, and herbarium collections dating back to 1820, to show how nitrogen deposition has led to the loss of the ability to fix nitrogen in montane lichens.…”

“…They found that fluctuating asymmetry generally increased over the course of the 20th century, which could have implications for the health of bumblebee populations. Crittenden et al (2023) (2023) examine whether published studies on changes in mammalian body sizes through time are repeatable (they mostly are not) and suggest a number of ways these kinds of studies could be improved, not just for mammals, but for other taxa as well. Wilson et al (2023) introduce Mothra, a computer vision pipeline to phenotype specimens.…”