Abstract
The last decades of research led to a change in understanding of lichens that are now seen as self-sustaining micro-ecosystems, harboring diverse microbial organisms in tight but yet not fully understood relationships. Among the diverse interdependencies, the relationship between the myco- and photobiont is the most crucial, determining the shape, and ecophysiological properties of the symbiotic consortium. Roughly 10% of lichens associate with cyanobacteria as their primary photobiont, termed cyanolichens. Up to now, the diversity of cyanobionts of bipartite lichens resolved by modern phylogenetic approaches is restricted to the filamentous and heterocytous genera of the order Nostocales. Unicellular photobionts were placed in the orders Chroococcales, Pleurocapsales, and Chroococcidiopsidales. However, especially the phylogeny and taxonomy of the Chroococcidiopsidales genera remained rather unclear. Here we present new data on the identity and phylogeny of photobionts from cyanolichens of the genera Gonohymenia, Lichinella, Peccania, and Peltula from a broad geographical range. A polyphasic approach was used, combining morphological and cultivation-depending characteristics (microscopy, staining techniques, life cycle observation, baeocyte motility, and nitrogen fixation test) with phylogenetic analyses of the 16S rRNA and 16S–23S ITS gene region. We found an unexpectedly high cyanobiont diversity in the cyanobacterial lichens of the order Lichinales, including two new genera and seven new species, all of which were not previously perceived as lichen symbionts. As a result, we describe the novel unicellular Chroococcidiopsidales genera Pseudocyanosarcina gen. nov. with the species Pseudocyanosarcina phycocyania sp. nov. (from Peltula clavata, Australia) and Compactococcus gen. nov. with the species Compactococcus sarcinoides sp. nov. (from Gonohymenia sp., Australia) and the new Chroococcidiopsidales species Aliterella compacta sp. nov. (from Peltula clavata, Australia), Aliterella gigantea sp. nov. (from Peltula capensis; South Africa), Sinocapsa ellipsoidea sp. nov. (from Peccania cerebriformis, Austria), as well as the two new Nostocales species Komarekiella gloeocapsoidea sp. nov. (from Gonohymenia sp., Czechia) and Komarekiella globosa sp. nov. (from Lichinella cribellifera, Canary Islands, Spain). Our study highlights the role of cyanolichens acting as a key in untangling cyanobacterial taxonomy and diversity. With this study, we hope to stimulate further research on photobionts, especially of rare cyanolichens.
Highlights
Understanding microbial interactions especially those of symbiotic character — has been and is still a flowering topic across scientific disciplines
An evaluation of the isolated cyanobiont strains in the context of the polyphasic approach (Komárek et al, 2014) and additional ecologically based analyses indicated the establishment of the new species Aliterella gigantea, A. compacta, Sinocapsa ellipsoidea, Komarekiella gloeocapsoidea, and K. globosa and led to the description of the two novel genera Compactococcus with C. sarcinoides and Pseudocyanosarcina with Pseudocyanosarcina phycocyania
The finding of new species and even two new genera in close vicinity of the traditional genus Chroococcidiopsis based on isolates from cyanolichens was surprising
Summary
Understanding microbial interactions especially those of symbiotic character — has been and is still a flowering topic across scientific disciplines In this context, lichens represent an extraordinary example where at least one fungus lives in an intimate association with at least one photosynthetic active alga or cyanobacterium. Eukaryotic green algae, such as members of the green algal family Trebouxiophyceae, are frequent partners of lichen mycobionts (chlorolichens), while only 10% of all known lichens have prokaryotic cyanobacteria as their primary partner of choice, called cyanolichens In terms of their mycobiont–photobiont relationship, cyanolichens can be classified into two functional groups (Rikkinen, 2017): (i) bipartite lichens with one mycobiont and one cyanobacterial photobiont and (ii) tripartite lichens, where one mycobiont associates simultaneously with a green algal and a cyanobacterial photobiont. The cyanobiont is predominantly responsible for nitrogen fixation (Millbank and Kershaw, 1969)
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