Microbial Taxa Exhibit Distinct Biogeographic Patterns Depending On How Rare They Are

In 1934, Baas Becking had already anticipated the fundamental role played by environmental conditions in shaping microbial communities, by stating that: “Everything is everywhere, but, the environment selects.”

Behind this principle is the idea that the small size and high abundance of microbes result in very high dispersal rates, virtually allowing them not to be restricted by geographical barriers. However, several studies now show that microorganisms also display biogeographic patterns, as usually found for macroorganisms.

Therefore, we currently aim to answer: at what spatial extent the patterns of microbial species distribution resemble those of larger organisms? By approaching multiple spatial scales, it is possible to disentangle the relative importance of niche and spatial processes, since dispersal limitation becomes increasingly conspicuous with the increase of the spatial extent. Moreover, temporal scales should also be considered, especially in ecosystems such as floodplains, where environmental conditions change seasonally due to periodic flood events.

Furthermore, not all microbial species show exceptionally high abundances. As in any other community, microbial communities are usually composed of few highly abundant species, while most of them are rare. Therefore, it is expected that common and rare species should be governed by different assembly mechanisms.

In our study, planktonic ciliates were sampled from shallow lakes of four Neotropical floodplains in Brazil (spatial extent of ca. 3000 km), during high and low water periods. Our work showed that the relative importance of the assembly processes of planktonic ciliate communities was dependent on the spatial extent, hydrological period and on patterns of commonness and rarity.

In these floodplain lakes, environmental conditions influenced the distribution of common ciliate species, whereas spatial processes were more associated with rare species. However, this pattern depended both on the temporal and spatial scales considered. During high waters, environmental gradients prevailed in explaining the distribution of both common and rare species. During low waters, when habitats were more isolated, common species remained to be mainly driven by environmental conditions, but rare species were solely influenced by the spatial component. At the broadest spatial extent, rare species showed clear biogeographic patterns.

The distinct patterns found for common and rare species may be accounted for by differences in dispersal rates, which are mainly attributable to the contrasting size of the populations. Rare species, having a small population size, show lower dispersal rates than the common ones, and would be more influenced by the spatial distance. Common species, in turn, should not be subject to strong dispersal limitation, and being better dispersers, would instead be more determined by the environmental conditions.

We also found that common and rare species likely have different ecological niches, since different sets of environmental variables influenced their distribution. In summary, ciliate metacommunities can be viewed as subcommunities of common and rare species which are structured by different assembly processes, due to differences in their dispersal ability and ecological niches. Our results somewhat agree with the perspective that common species are closer to being cosmopolitan, while rare species show more limited distributions because those patterns depend on the temporal and spatial scales analyzed.

These findings are described in the article entitled Common and Rare Taxa of Planktonic Ciliates: Influence of Flood Events and Biogeographic Patterns in Neotropical Floodplains, published in the journal Microbial Ecology. This work was led by Bianca Trevizan Segovia at the University of British Columbia with contribution from Juliana Déo Dias, Adalgisa Fernanda Cabral, Bianca Ramos Meira, Fernando Miranda Lansac-Tôha, Fabio Amodêo Lansac-Tôha, Luis Mauricio Bini, Luiz Felipe Machado Velho

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