Abstract
Lichens are commonly recognized as a symbiotic association of a fungus and a chlorophyll containing partner, either green algae or cyanobacteria, or both. The fungus provides a suitable habitat for the partner, which provides photosynthetically fixed carbon as energy source for the system. The evolutionary result of the self-sustaining partnership is a unique joint structure, the lichen thallus, which is indispensable for fungal sexual reproduction. The classical view of a dual symbiosis has been challenged by recent microbiome research, which revealed host-specific bacterial microbiomes. The recent results about bacterial associations with lichens symbioses corroborate their notion as a multi-species symbiosis. Multi-omics approaches have provided evidence for functional contribution by the bacterial microbiome to the entire lichen meta-organism while various abiotic and biotic factors can additionally influence the bacterial community structure. Results of current research also suggest that neighboring ecological niches influence the composition of the lichen bacterial microbiome. Specificity and functions are here reviewed based on these recent findings, converging to a holistic view of bacterial roles in lichens. Finally we propose that the lichen thallus has also evolved to function as a smart harvester of bacterial symbionts. We suggest that lichens represent an ideal model to study multi-species symbiosis, using the recently available omics tools and other cutting edge methods.
Highlights
Twenty years after the theory of evolution by natural selection started to revolutionize biology, the German mycologist Anton de Bary introduced the term symbiosis to the broader scientific community as a living together of dissimilar organisms (de Bary, 1879)
The observed compositional differences within the same lichen species can be attributed to various reasons such as metagenomic sequencing approach, utilized databases, or activity of the bacteria in case of metatranscriptomic analysis (Aschenbrenner, 2015) since less than 10% of a microbial community is metabolically active at one time (Locey, 2010)
While the predominance of Alphaproteobacteria was reported in other studies (Bates et al, 2011; Hodkinson et al, 2012), bacterial community composition in general differed among lichen species
Summary
Twenty years after the theory of evolution by natural selection started to revolutionize biology, the German mycologist Anton de Bary introduced the term symbiosis to the broader scientific community as a living together of dissimilar organisms (de Bary, 1879). While the predominance of Alphaproteobacteria was reported in other studies (Bates et al, 2011; Hodkinson et al, 2012), bacterial community composition in general differed among lichen species. Hodkinson et al (2012) who thoroughly studied the bacterial communities associated with various lichen species comprising 24 mycobiont types with all photobiont combinations of different sampling locations (tropical and arctic regions) highlighted the photobiont type (chlorolichens vs cyanolichens) and large-scale geography as the main driving forces. Chlorolichens would preferably enrich species capable of nitrogen fixation rather than cyanolichens Another suggestion was that green algae release different types of fixed carbon (sugar alcohols: ribitol, erythritol, or sorbitol) than cyanobacteria (glucose; Elix and StockerWörgötter, 2008), thereby shaping the bacterial community with respect to carbon utilization. This agrees with previous results of Cardinale et al (2012b) who showed that growth types do not affect the main bacterial community structure
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