Imagine all the leaves of all the plants currently living on planet Earth.
Now, add up all the surface areas of all of those leaves.
And your answer is?… No idea?….
Luckily, some microbiologists have made an estimate, and it’s an astounding number.
According to a current review (see ref 1 below), the terrestrial leaf surface area that might be colonized by microbes is approximately 640,000,000 square kilometers or about 250 million square miles! (For perspective, consider that the total surface area of the Earth is 197 million square miles.)
From this, microbiologists have estimated that the planetary leaf-surface bacterial population on Earth may be as large as 1026 cells! (Yes, that’s the numeral 1 with 26 zeros behind it.)
“Clearly, in aggregate, these bacteria are sufficiently numerous to contribute in many processes of importance to global processes, as well as to the behavior of the individual plants on which they live.”-(from ref 1 below).
In other words, the bacteria that live on the leaf surface are so numerous that they not only likely affect the plant on which they dwell but, collectively – on a planetary scale – they are so numerous as to significantly affect the global carbon and nitrogen cycles on Earth.
The term “phyllosphere” was first published by Dr. Jakoba Ruinen in 1961 (see ref 2 below), who studied tropical ecology beginning in the 1950’s. She called the interface between leaves and air the “phyllosphere”, and said that this was a much neglected milieu, compared to studies of the rhizosphere.
“Under the microscope, aerial plant leaves resemble eerie landscapes, with deep gorges, tall peaks and gaping pits that riddle the waxy surface. [e.g., see photo below] Add to this scenery a climate that features temperature highs of 50 °C [122 °F] or more, exposure to harmful ultraviolet rays, erratic periods of drought and limited access to nutrients, and one gets the picture that this is a hostile environment. Still, many bacteria, fungi, yeast and other microorganisms dwell in great abundance in this ‘phyllosphere’…” (from ref 3 below).
“The microbial communities of leaves are diverse and include many different genera of bacteria, filamentous fungi, yeasts, algae, and, less frequently, protozoa and nematodes.” (from ref 1 below)
Most of what we know about these leaf “epiphytes” come from culturing (or trying to culture) representative isolates in the lab.
In a recent report (ref 4 below), the investigators have taken a 21st-century approach to the question: “What bacteria live in the phyllosphere?”.
Using a new “metaproteogenomic” approach, they analyzed the bacteria associated with leaves of field-grown soybean and clover plants and wild populations of Arabidopsis thaliana plants. (This technique is related to proteogenomics.)
This study not only revealed “who” was there, but also provided clues to how such bacteria survive in the phyllosphere. Briefly, they discovered “…a high consistency of the communities on the 3 different plant species, both with respect to the predominant community members…” and with respect to the proteins that these bacteria apparently use to survive such a relatively hostile environment.
Bottom Line: Using new molecular biology techniques, scientists have discovered important new information regarding not only what bacteria live in the phyllosphere but also how they do it.
1. Lindow, S. and Brandl, M.T. (2006) “Microbiology of the Phyllosphere”, Applied and Environmental Microbiology vol. 69, pp. 1875-1883. (full text)
2. Ruinen, J. (1961) “The Phylloshere. I. An Ecologically Neglected Mileau”, Plant and Soil vol. 15, pp. 81-106. (preview PDF)
3. Leveau, J. (2009) “Life on leaves.” Nature vol. 461, p. 741. (Abstract)
4. Delmotte, N., et al. (2009) “Community proteogenomics reveals insights into the physiology of phyllosphere bacteria” Proceedings of the National Academy of Sciences (USA) vol. 106, pp. 16428-16433. (full text).
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