Although common knowledge dictates that the lichen thallus is formed solely by a fungus (mycobiont) that develops a symbiotic relationship with an alga and/or cyanobacterium (photobiont), the non-photoautotrophic bacteria found in lichen microbiomes are increasingly regarded as integral components of lichen thalli. For this study, comparative analyses were conducted on lichen-associated bacterial communities to test for effects of photobiont-types (i.e. green algal vs. cyanobacterial), mycobiont-types and large-scale spatial distances (from tropical to arctic latitudes). Amplicons of the 16S (SSU) rRNA gene were examined using both Sanger sequencing of cloned fragments and barcoded pyrosequencing. Rhizobiales is typically the most abundant and taxonomically diverse order in lichen microbiomes; however, overall bacterial diversity in lichens is shown to be much higher than previously reported. Members of Acidobacteriaceae, Acetobacteraceae, Brucellaceae and sequence group LAR1 are the most commonly found groups across the phylogenetically and geographically broad array of lichens examined here. Major bacterial community trends are significantly correlated with differences in large-scale geography, photobiont-type and mycobiont-type. The lichen as a microcosm represents a structured, unique microbial habitat with greater ecological complexity and bacterial diversity than previously appreciated and can serve as a model system for studying larger ecological and evolutionary principles.
Figure 1. Non-metric multidimensional scaling plots
produced from OTU-based Bray-Curtis dissimilarities. The first plot (A)
shows results obtained from clone library data of 16S sequences from the
order Rhizobiales, while the second plot (B) was produced from 454
barcoded 16S amplicon data representing approximately half the number of
samples but ∼100 times as many sequences from a much wider range of
bacterial diversity. Continuous lines act as visual aids to delimit
communities associated with the two major photobiont-types, whereas
dashed lines delimit communities associated with chlorolichens from
northern versus southern sites.
Figure 3. Relative abundances of bacterial taxa
recovered from each lichen sample analysed with 454 sequencing. Taxa
included are the classes with > 0.1% of 16S sequences in the full 454
data set, orders with > 0.5% of sequences, and families with
> 1% of sequences; at each rank, all sequences that did not fit into
one of these categories were classified as other. Photobiont is
indicated by the bar across the top of the figure (light green = green
algae; dark blue = Cyanobacteria), while the site is indicated with
symbols immediately below the bar ( = Eagle Summit, AK; + = Nome, AK;
x = Highlands, NC, and * = Cerro de la Muerte, CR). Mycobionts are
indicated by name along the base of the figure.
Hodkinson, B. P., Gottel, N. R., Schadt, C. W. and Lutzoni, F. (2012),
Photoautotrophic symbiont and geography are major factors affecting
highly structured and diverse bacterial communities in the lichen
microbiome. Environmental Microbiology, 14: 147–161.