Investigating the role of the bacterial endosymbiotic microbiome in the extremophilic fungi Pisolithus tinctorius

A PhD scholarship is available from the University of Canterbury to investigate why a extremophilic basidiomycete fungal species (Pisolithus tinctorius) found throughout geothermal areas in New Zealand and the US, hosts intra-tissue (endosymbiotic) bacterial communities populated with new candidate phyla and novel deeply-branching lineages. The role in which these endosymbiotic bacterial community play with respect to the fungal host is not known although we hypothesise that this relationship is mutualistic, permitting both cohorts to survive in highly challenging and nutrient-poor geothermal soils.

This doctoral research will focus on the use molecular, ecological, geochemical and cultivation techniques to test whether the determine the role that these endosymbiotic microbial communities play within the fungal host.

The current understanding of endosymbiotic microbiomes (microbial communities operating within host tissues) asserts that they are limited to microorganisms (e.g. bacteria, archaea, fungi, protists) interacting with their larger eukaryotic hosts (e.g. plants, animals, insects). Contrary to this, there are limited reports of endosymbiotic prokaryote-fungi interactions. Preliminary research [unpublished] on Pisolithus fungi growing in the acidic geothermal soils of Yellowstone National Park (YNP) and in New Zealand’s Taupō Volcanic Zone (TVZ) shows that the fruiting-bodies contain a highly unusual bacterial community with unknown function including at least three novel, deeply-diverging microbial; lineages that likely represent new candidate phyla or classes.

Fungi from the genus Pisolithus traditionally displays an ectomycorrhizal phenotype (they live ectosymbiotically on the exterior of plant-root tips), providing host plants with nitrogen and nutrients, while acquiring atmospherically-fixed carbon (carbohydrates) in return. However, Pisolithus specimens recently collected from YNP were not associated with any root systems contradicting the fungi’s usual phenotype, and raising questions on how carbon is procured. The discovery of a novel bacterial community and its association with an accumulation of sulphur and metal-based compounds in the Pisolithus fruiting-body may provide a solution to this conundrum. Preliminary molecular analysis has identified novel bacterial phylotypes that includes taxa associated with sulphur- and iron-cycling, and phototrophy. We hypothesise that this observed symbiotic relationship increases the bioenergetic potential of both the fungi and the microbiome; the bacterial function is autotrophic fixation of carbon dioxide (CO2) powered by chemotrophic growth on geothermal gases and metals adsorbed by the fungi. This relationship normally displayed by mycorrhizal association with plants facilitates the fungi to growth in geothermal soils depleted in organic material. As bacterial endosymbionts may also reside in the hyphae and could function to detoxify the metals and gases readily found in geothermal areas. Reciprocally, the internal microbiomes benefit through protection against desiccation and predation, gain access to nitrogen and micronutrients, and access an effective mechanism for dispersal via fungal sporulation.

Depending on the candidate, there is an opportunity to undertake fieldwork in Yellowstone National Park, Wyoming, USA. This project is part of a greater research project between researchers at the University of Canterbury, GNS Science, and NASA to understand the bacterial and archaeal interactions with the Pisolithus strains in the US and New Zealand.

The doctoral candidate will be supervised by Dr Matthew Stott ([Email Address Removed]) and Dr Ken Cullings ([Email Address Removed])