See those spots on the rock face? Those are lichens! These are plants made of algae and fungi living and working together to maintain the health of the whole organism. Frequently, however, lichens contain many different kinds of bacteria that can perform different metabolic processes that can help the lichen thrive. We don’t know a lot about the bacteria that live with lichen, or how and why they associate with lichen. Bates et al. explores the diversity of the bacterial community living with lichen, and their potential contributions to the overall health of a lichen community.
The researchers primarily looked at the bacterial communities to see if they vary based on the species of lichen. Do certain species benefit from certain additional metabolic process? In order to do this, the researchers used both traditional culture based methods – a method which grows the bacteria in a lab – and modern genetic techniques, which can more accurately describe bacterial communities than culturing.
After collecting and analyzing samples from 4 different species of lichens, the researchers compared the results and discovered that, while some bacterial species were present in all the lichens sampled, there were also certain bacterial species that associated only with certain species of lichen. Bacterial species associated with all the lichens were more likely to be nitrogen fixing bacteria, which provide crucial nitrogen to plants. This research allows us to fill in one small piece of the puzzle that is symbiotic lichens. Do these unique bacterial species contribute to the specific lichen species they associate with? Did they coevolve with the lichen, as the human gut microbiome has coevolved with our species? All these questions remain to be answered by microbiologists in the future.
In-depth, technical summary:
While we traditionally understand lichens to be symbiotic communities of algae and fungi or cyanobacteria, we don’t think of them as organisms having significant microbiomes. This, as it turns out, is false. Bates et al. (2011) set out to characterize the bacterial microbiomes of 4 species of lichens on 3 different rock outcroppings.
Using both traditional culture based methods, and 16S rRNA gene amplicon pyrosequencing, Bates et al. (2011) analyzed the lichen microbiome. They found no trace of archeal 16S rRNA genes, and their sequencing results yielded much more diversity than their culturing results, as can be expected. Using Bray-Curtis NMDS, they discovered that the bacterial communities cluster based on lichen species, rather than outcropping, suggesting that certain bacterial species do associate with certain lichen species. Among the bacteria ubiquitous in the lichen samples, most were potential nitrogen fixing bacteria of the Alphaproteobacteria. This suggests that lichen associate with nitrogen fixing bacteria, however the results are not definitive due to the low accuracy of their species identification, as well as the fact that they did not target functional genes in the lichen microbiome.
This research shows that, while there are certain bacteria present in all lichens just as there are in humans, there are species specific associations between lichen species and the lichen microbiome. Further research into the coevolution and specialized roles of bacteria in the lichen microbiome is required to understand the lichen microbiome, and to understand the full extent of their symbiotic relationships.