In the turbulent phyllosphere, bacteria associate tightly with host plants

Large populations of bacteria  reside on leaf surfaces, in a zone known as the phyllosphere. Dr. Noah Fierer and colleagues at the University of Colorado, Boulder used high resolution DNA barcoding techniques to create a profile of the bacterial communities on 56 different tree species. They were interested in measuring 1) intraspecific variation, or the changes in phyllosphere communities across  different species and 2) interspecific variation, the changes in community composition within a species from tree to tree. Looking at the community variation within a single tree species allowed the researchers to isolate specific environmental variables that may be influencing community structure. To understand interspecific changes in diversity along environmental gradients, Fierer and colleagues barcoded Pinus ponderosa trees taken from several locations across the world.

Dr. Fierer found that phyllosphere communities are not randomly distributed, but organized in patterns that could be predicted given the relatedness of the tree species. In other words, related bacteria inhabit related tree leaves. Moreover, they found that the interspecific bacterial diversity- the difference between Pine and Oak tree phyllosphere communities, for instance- is greater than the intraspecific variability between distant individuals of the same species. Certain bacterial taxa were more common on gymnosperms (plants that reproduce by seed) than angiosperms (flowering plants). However, within a single species such as P. Pondersoa, bacterial communities could be highly similar across thousands of miles. Given that the bacterial communities in two handfuls of soil taken 10 meters apart are likely to look nothing alike, this result suggests a strong selection for particular bacterial species inhabiting particular leaves, regardless of environmental gradients.

Terrestrial leaf surface area is estimated to exceed 10^8 km^2, making the phyllosphere one of the largest microbial habitats on earth. Phyllosphere communities face the distinct challenge of extreme and unpredictable environmental variability. Imagine if your world could go from brilliantly sunny to dark in a manner of seconds, and that such events occurred randomly throughout the day. Rainstorms are not trivial events, but epic floods that pose imminent threat to all but the most resilient individuals. Temperature change is rapid and often extreme, and gusts of wind threaten to blow you upside down or off into oblivion. One may imagine that in such a turbulent world, bacteria with very specific properties would not only inhabit, but become expert dwellers on, very particular types of leaves. Dr. Fierer’s research supports this hypothesis by demonstrating a tight coupling between bacterial species and leaf properties associated with different tree species.

 

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