The beauty and the yeast: can the microalgae Dunaliella form a borderline lichen with Hortaea werneckii?

Abstract

Lichenized fungi usually develop complex, stratified morphologies through an intricately balanced living together with their algal partners, but several species are known to form only more or less loose associations with algae. These borderline lichens are still little explored although they could inform us about early stages of lichen evolution. We studied the association of the extremely halotolerant fungus Hortaea werneckii with the alga Dunaliella atacamensis, discovered in a cave in the Atacama Desert (Chile), and with D. salina, common inhabitant of saltern brines. D. atacamensis forms small colonies, in which cells of H. werneckii can be frequently observed, while such interaction has not been observed with D. salina. As symbiotic interactions between Dunaliella and Hortaea have not been reported, we performed a series of co-cultivation experiments to inspect whether these species could interact and develop more distinct lichen-like symbiotic structures. We set up co-cultures between axenic strains of Hortaea werneckii (isolated both from Mediterranean salterns and from the Atacama cave) and isolates of D. atacamensis (from the Atacama cave) and D. salina (isolated from Mediterranean salterns). Although we used different growth media and cultivation approaches, bright field and SEM microscopy analyses did not indicate any mutual effects in these experiments. We discuss the implications for fungal algal interactions along the transition from algal exploiters to lichen symbioses.

Antarctolichenia onofrii gen. nov. sp. nov. from Antarctic Endolithic Communities Untangles the Evolution of Rock-Inhabiting and Lichenized Fungi in Arthoniomycetes

“…However, with their phenotypically more plastic mycelia (which develop either filamentous or short yeast-like cells), they are unable to form any clear thallus structures [33,71]. The same behavior was also observed in the halophilic black yeast Hortaea werneckii, which was found to co-grow with the algae Dunaliella atacamensis on spiderwebs [75]; nevertheless, attempts to co-grow and study this association in vitro culture failed [74]. A certain degree of interaction was also observed by co-culturing some Coniosporium or Knufia RIF species, where a slight increase in hyphal branching near algal cells was reported [76].…”

“…Their rather primitive thallus, relatively simple in structure, is made by a tight fungal coat of one cell layer around the filamentous photobiont, a thread of Trentepohlia, which does not differ much from the free-living forms of the individual symbionts [72]. Other melanized, filamentous, and yeast-like fungi discovered to co-grow in nature in a tight association with algae were used to attempt in vitro co-culture experiments to study their interaction [71,73,74]. The two RIF genera, Lichenothelia and Saxomyces, are usually found on rocks together with coccoid Trebouxiophyceae algae and can be co-grown with them in culture [71].…”

Understanding Fungi in Glacial and Hypersaline Environments

“…The role of fungi as ecological drivers in hypersaline habitats has not yet been investigated. Open questions include the following: (a) the influence of available nutrients, such as phosphorus and nitrogen, on fungal abundance; (b) possible correlations and interactions of fungi with the alga D. salina (81); and (c) the role of fungi in the microbial mats that form the floor of the precipitation basins of salterns (which in some ancient salterns may have been continuously cultivated for hundreds of years) (20). Other important research topics are fungal degradation of wood immersed in hypersaline water (144) and the role of hypersaline water as a reservoir of species pathogenic to corals and other marine animals (e.g., A. sydowii), a role possibly exacerbated by global warming (108).…”