Microgeographic distribution and mycobiont–photobiont associations in Arctic Umbilicaria lichens

Lichen symbioses are shaped by interactions between mycobionts and photobionts, yet how these relationships are structured at extremely small spatial scales in polar ecosystems remains poorly understood. Here, we investigated mycobiont–photobiont associations in Umbilicaria lichens collected from two isolated boulders in Ny-Ålesund, Svalbard, using ITS rDNA sequencing of both symbiotic partners and analyses of group I introns in mycobiont SSU rDNA. We identified five co-occurring Umbilicaria taxa across the two boulders, with four taxa occurring on each boulder. These taxa displayed pronounced horizontal and vertical segregation associated with rock orientation and surface position, revealing strong ecological structuring at the within-boulder scale. In contrast, all photobionts belonged to Trebouxia jamesii and showed low genetic differentiation, forming four weakly structured haplotype groups that were not correlated with host species or microhabitat. This pattern indicates weak sorting amongst local T. jamesii lineages and extensive photobiont sharing across mycobiont species and microhabitats. In addition, group I introns at the SSU rDNA 1769 insertion site were detected in two Umbilicaria lineages and occurred as short and long variants with highly conserved catalytic core structures. Their lineage-specific distribution and structural conservation provide additional information on mycobiont evolutionary history and may be relevant to stress responses, although their functional and adaptive significance remains to be experimentally evaluated. Together, our results demonstrate that Arctic Umbilicaria communities combine high fungal diversity and fine-scale spatial niche partitioning with broad photobiont sharing. This dual strategy of ecological differentiation and symbiotic flexibility may facilitate lichen persistence in extreme polar environments.