Biodiversidade e padrões de coocorrência entre fitoplâncton e bactéria na cascata trófica dos reservatórios do rio Tiête: uma abordagem com High Throughput Sequencing

Abstract

Microbial communities of aquatic environments make up a complex ecological network where organisms interact with each other and with the environment. The way these microbial communities are structured is influenced by abiotic factors such as temperature, pH, nutrient availability, etc., since the different taxa of organisms respond differently to environmental factors. Cascade arrangement generates limnological properties beyond those often reported for lentic systems. Greater mixing of water, that generates greater water turbidity in upstream reservoirs, nutrient pulses, and introduction of organisms from the previous reservoirs and the adjacencies of each reservoir are characteristic of cascade systems. Dry and rainy seasons should modulate such attributes through the amount of water that connects the reservoirs, so that the rainy season promotes greater upstream transport of water from the downstream due to the shorter residence time, as well as greater introduction of allochthonous material. The opposite is expected in the dry season, where the lower flow of water between the reservoirs increases the residence time of the reservoirs allowing the organisms to remain for a longer period in the reservoirs. In this study, we explored the composition of the microbial community of four cascading reservoirs of the medium-low Tietê River as well as the factors that governed microorganism’s turnover along the cascade in the dry and rainy periods. Finally, we analyzed the effect of the trophic gradient on the ecological networks of the microbial communities of each reservoir. We found that the cascade formation of these reservoirs influenced the structure of the microbial communities so that each subcommunity (bacteria attached to particles or free-living, cyanobacteria and eukaryotic phytoplankton) studied responded to different environmental factors that varied along the cascade or seasons. In the same way, the variation in subcommunity structures along the waterfall was guided by both local factors (local environmental conditions of each reservoir, promoting selection) and regional (dispersion of organisms), but with regional factors predominating. Finally, we observed the influence of the trophic gradient on the ecological networks of microbial interactions: highly eutrophic environments, by constricting the diversity, especially of primary producers, produce networks with high modularity, that is, subgroups interacting less inserted the global network. With the eutrophication reduction we observed more connected networks, with less modularity, where subgroups are immersed in the global network of the community.