Symbiosis between river and dry lands: Phycobiont dynamics on river gravel bars.

River gravel bars are dynamic and heterogeneous habitats straddling the transition between aquatic and terrestrial environments. Periodic flooding, low nutrient concentrations, frost, lack of stable sites, drought, and ground surface heat significantly influence the biota of these habitats. Mutualistic symbiosis may be a successful strategy for organisms to survive and proliferate under such harsh conditions. The lichen genus Stereocaulon was selected as a model symbiotic system from among the organisms living on river gravel bars. The goal of the current study was to determine the effect of this dynamic environment on phycobiont (i.e., green eukaryotic photobiont) community structure. We analyzed 147 Stereocaulon specimens collected in the Swiss Alps using Sanger sequencing (fungal internal transcribed spacer (ITS) rDNA, algal ITS rDNA, and algal actin type I gene) and analyzed 8 selected thalli and 12 soil samples using Illumina metabarcoding (ITS2 rDNA). Phytosociological sampling was performed for all 13 study plots. Our analyses of communities of phycobionts, lichens, bryophytes, and vascular plants indicated a gradual change in the phycobiont community along a successional gradient. The particularly large phycobiont diversity associated with Stereocaulon mycobionts included algae, here reported as phycobionts for the first time. Each of the two Stereocaulon mycobiont operational taxonomic units had a distinct pool of predominant phycobionts. The thalli selected for Illumina metabarcoding contained a wide range of additional algae, i.e., they showed algal plurality.