Estrutura e dinâmica das comunidades microbianas (bactérias e protozoários) no plâncton de ambientes tropicais de água doce : padrões espaciais, temporais e fatores intervenientes.

Abstract

Microorganisms play a key role in aquatic biogeochemical cycles and in the flow of matter and energy through planktonic food webs. The first two chapters of this thesis aimed to investigate the main factors affecting bacterial abundances in tropical freshwater environments, due to an apparent paradox which emerged from previous studies: despite the higher bacterial production in the tropics, the bacterial abundance found in those regions is lower, compared to temperate environments. This fact raised the hypothesis that bacterial abundance could be controlled by heterotrophic flagellates in the lower latitudes. By analyzing a large data set, we showed that both bacterial and heterotrophic flagellate abundances are consistently lower in tropical when compared to temperate regions, resulting in a similar coupling between those communities. A stronger top-down control on both microbial communities exerted by ciliates and cladocerans, may explain those patterns. The experimental approach allowed us to demonstrate that protists, mainly ciliates, are likely the main responsible for bacterial loss in a tropical lake and that predation pressure also influences bacterial community size-structure. In the third chapter, we aimed to comprehend the role of temporal and spatial scales in structuring the planktonic ciliate communities from Neotropical floodplains, through a metacommunity approach. Contrasting results have been reported in the literature: while some authors advocated that microorganisms have a cosmopolitan distribution, others argued that they displayed biogeographic patters similar to those found for larger sized organisms. Taking into account both spatial and temporal scales, our analysis allowed us to demonstrate a clear influence of the temporal variation in structuring the planktonic ciliate communities, and with the analysis of a great spatial extent, it was possible to verify that biogeographic patterns also occur for those small-sized organisms.