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Ecology

Invisible communities

The surface of a single leaf can house more than 600 species of bacteria

eduardo cesarAtlantic Rainforest in the São Paulo interior: groups of bacteria typical for each tree specieseduardo cesar

The leafage of trees form an immense reservoir, unknown and extremely diversified, of microorganisms. A team from the University of São Paulo (USP) reached this conclusion after having verified that on the surface of a simple leaf of an Atlantic Rainforest tree hundreds of species of bacteria, organized into communities, can live. A preliminary projection suggests that a whole tree could house a number of species of bacteria millions of times greater that the human organism: in the intestine millions of bacteria live that represent from 300 to 1,000 species. An estimate carried out starting from this study suggests that there could be something between 2 million and 13 million in total of new species of bacteria living on the surface of the leaves of around 20,000 species of Atlantic Rainforest plants, without considering their roots, stalks and other parts of the vegetable. To know this diversity with precision would be an advance and more for the studies about this group of organisms, since it is the largest and most diversified of all, where 1 ton of soil can contain 4 million species, whilst in the oceans a further 2 million live.

But this study, published in the June 30th edition of Science, does not just outline the dimension of a category of organisms that were not taken into account during the surveys into the biological richness of an environment – normally they consider only the animals and vegetables. The study, coordinated by Márcio Lambais, with participation by Juliano Cury, Ricardo Büll and Ricardo Rodrigues, all from USP’s Luiz de Queiroz Higher School of Agriculture (Esalq), as well as David Crowley, from the University of California, United States, also calls the attention for the perspective of an interaction between plants and the communities of bacteria – a community is a group of organism populations whatever that integrate among themselves and with the environment. “Various attributes of the plant could in truth be a consequence of an interaction with microorganisms” explains  Rodrigues. In simpler terms: a chemical compound that helps the plant to defend itself from a pest attack could be the result of this living together with the millions of hosts invisible to the naked eye.

It was already known that the leaves house an high variety of microorganisms, but the researchers never imagined that they would find amounts so surprising when they began to study the microbial diversity of the surface of the leaves of nine species of trees at the Caetetus Ecology Station, in Gália, in the interior of the state of São Paulo. Once the panorama of diversity had been done, by means of molecular analysis, they amplified the results by comparing three species of plants: the catuaba or catigua (Trichilia catigua), from whose bark a tincture used as an aphrodisiac and against rheumatism is extracted, the red catuaba (Trichilia clausenii) and the gabiroba (Campomanesia xanthocarpa).

It was then that they verified that on each leaf a minimum of 95 and a maximum of 671 bacteria species could live. Another impressive piece of data: there were almost no species in common between the plants. “Apparently communities of bacteria exist typical of each species of tree” comments Lambais. From this survey, developed within the Permanent Small Parts program, linked to the Biota-FAPESP program, a new and immense front for studies has opened up. Now the researchers are putting out questions about how plants and bacteria can interact, what types of mutual benefits can spring up from this interaction and if the same species of plant, in different locations or environments, can house the same communities of bacteria. The replies should take up another good few years of work.

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