Symbiosis between river and dry lands: phycobiont dynamics on river gravel bars

River gravel bars are dynamic and heterogeneous habitats standing on transition between aquatic and terrestrial environment. Periodical flooding, low nutrient content, frost, missing safe sites, drought, and heat on the ground surface significantly influence life in these habitats. Mutualistic symbiosis may be a successful strategy for organisms to survive and to proliferate under harsh conditions. The lichen genus Stereocaulon was selected as a model symbiotic system among the organisms living on river gravel bars. The aim of our work was to determine effect of this dynamic environment on a phycobiont (i.e., green eukaryotic photobiont) community structure. We analysed 147 Stereocaulon specimens collected in the Swiss Alps using Sanger sequencing (fungal ITS rDNA, algal ITS rDNA, algal actin type I gene) and 8 selected thalli and 12 soil samples using Illumina metabarcoding (ITS2 rDNA). We performed phytosociological sampling on each study plot (n=13). Our analyses of communities of phycobionts, lichens, bryophytes, and vascular plants indicated an ongoing colonisation by phycobionts and gradual change of phycobiont community along to the successional gradient. We recovered great phycobiont diversity associated with Stereocaulon mycobionts including algae reported as phycobionts for the first time. Each of two Stereocaulon mycobiont OTUs has distinct pool of predominant phycobionts in the study area. Finally, all thalli selected for Illumina metabarcoding contained a wide range of additional intrathalline algae, i.e., showed algal plurality. In general, succession process on newly emerged or recently disturbed localities also takes place within a community of microscopic symbiotic organisms, such as phycobionts.