Liverwort-Fungal Associations: Novel Symbioses In Polar Regions?

Symbioses between plants and fungi are ubiquitous in terrestrial ecosystems, in which they play key roles in plant nutrient acquisition and the cycling of elements. The plant host provides carbon fixed in photosynthesis to the fungi, which in return provide the host with limiting nutrients such as nitrogen and phosphorus. These associations, referred to as mycorrhiza, are hypothesised to have been a key factor in the colonisation of the land by plants. However, we still know very little about fungal symbioses in one of the most ancient groups of land plants — the liverworts — and in the bryophyte enriched floras of the polar regions. This study provides the first large-scale survey of fungal colonisation of liverworts on Svalbard in the High Arctic and on South Georgia in sub-Antarctica, increasing current knowledge of these symbioses from three to 35 liverwort species across both polar regions. Fungal DNA was sequenced to establish the identity of the mycobionts present, with in planta FISH and light microscopy being used to determine the distribution of fungal structures. Furthermore, elemental and isotopic concentrations of liverwort tissues were measured to explore whether mycobionts might be associated with liverwort nutrition. To isolate the fungal contribution to liverwort N nutrition and test whether fungi receive C from their liverwort host, a dual 15N and 1 4C isotope labelling experiment was established, with NanoSIMS being utilized to visualise the flow of N into liverwort tissue. I found that fungal colonisation was frequent in the liverworts surveyed, with species from the Serendipitaceae being among the most abundant fungal taxa recorded. Interestingly, all of the mycobionts identified (apart from Rhizoscyphus ericae) were endemic to either the Arctic or Antarctic. Blue staining hyphal coils and dark septate endophytes were frequently observed, with the former correlating positively with plant N concentration, suggesting that mycobionts might enhance the acquisition of this element by their liverwort hosts. Stable isotope labelling combined with NanoSIMS demonstrated that 15N is transferred via mycobiont hyphae into liverwort tissues, where it is incorporated. However, I did not observe a reciprocal transfer of 14C from liverwort to mycobionts. This research suggests that the distribution and function of liverwort-fungal associations varies considerably with latitude. The findings are important for understanding C and N cycling in polar soils, how climate change might alter the magnitude of these nutrient fluxes and, more broadly, the evolution of liverwort-fungal associations.

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