Diversity of green algae associated with littoral amphibious lichens

The littoral fluctuation zone of seashores worldwide is inhabited by lichens yet the diversity and specificity of their green algal photobionts remain poorly understood. We characterized the photobiont diversity of the amphibious lichens Verrucaria ceuthocarpa and V. ditmarsica sampled across 13 coastal sites in Sweden and Germany using phylogenetic analyses of nrSSU and nrITS DNA sequences. Most photobionts belonged to Kornmanniaceae (Ulvales), confirming their fundamental role in these habitats. In addition to the commonly reported algae Pseudendoclonium commune and P. submarinum, we detected four previously unknown lineages: two novel Pseudendoclonium species, a lineage related to Kornmannia leptoderma and a putative undescribed genus within the family. We also confirm the occurrence of the algal genus Urospora (Ulotrichales) as a lichen photobiont, representing its first record in Europe. It clustered with previously reported lichen-associated strains from Chile, suggesting a broader ecological relevance of this genus in seashore lichens. Importantly, we show that Urospora can be readily distinguished microscopically within lichen thalli by iodine staining, which reveals up to three pyrenoids per cell. Together, these findings expand current knowledge of algae essential for seashore lichen symbioses, revealing substantial hidden diversity and providing a framework for future taxonomic and ecological studies of littoral green algae.