Planktonic bacteria have an important role in aquatic ecosystems, since bacteria make an important link between the dissolved organic matter (DOM) and the higher trophic levels. The main goals of this thesis are: (1) to evaluate the bacterial growth and the DOC consumption in water samples from coastal lagoons and clear water Amazonian ecosystems; (2) to identify the main limiting nutrients to bacterial growth in coastal lagoons and clear water Amazonian ecosystems; (3) to evaluate the influence of enrichment with nutrients on the DOM consumption by planktonic bacteria in coastal lagoons and clear water Amazonian ecosystems; (4) to evaluate the sources and the quality of the DOM available to bacterial growth in coastal lagoons and clear water Amazonian ecosystems; (5) to evaluate the quality of the DOM leachated from aquatic and terrestrial primary producers to the bacterial growth in coastal lagoons and clear water Amazonian ecosystems; (6) to evaluate the influence of UV radiation on the quality of DOM leachated from aquatic macrophytes and its consequences to bacterial growth. We used natural samples and batch cultures prepared with water from these ecosystems enriched or not with C, N and P. We evaluated the bacterial growth on the natural samples and the cultures by changes on bacterial numbers, biomass, respiration, and secondary production. Based on the main results, we concluded that: (1) the planktonic bacteria have an important role on carbon cycling in coastal lagoons and Lake Batata and could represent and important source of energy to higher trophic levels of planktonic food chain in these ecosystems; (2) the low concentration of dissolved phosphorus is the main limiting factor to bacterial growth in coastal lagoons and clear water Amazonian ecosystems and it interferes directly on the DOC consumption by planktonic bacteria in these ecosystems; (3) only a small fraction of the DOC present in coastal lagoons and clear water Amazonian ecosystems is considered labile to bacterial growth. The watershed of these ecosystems is the main source of DOC (allochthonous DOC). The low quality of the bulk DOC to bacterial growth in these ecosystems is also considered an important limiting factor, and, together with the availability of inorganic nutrients, determines the amount of DOC oxidized to CO2 and the amount of DOC allocated in bacterial biomass; (4) other sources of DOC could be important to bacterial metabolism in these ecosystems. For instance, the aquatic macrophytes and the phytoplankton are considered important sources of autochthonous DOC in coastal lagoons and in Lake Batata, respectively. A large fraction of this autochthonous DOC is considered labile to bacterial growth. This DOC could stimulate or even sustain the bacterial growth and it could stimulate the consumption of older, more refractory DOC accumulated in these ecosystems; (5) the concentration of carbohydrates is a good predictor of the quality of DOC of the aquatic macrophytes leachates and phytoplankton exudates to bacterial growth; (6) the UV radiation interferes on the quality of DOC leachated from aquatic macrophytes, resulting in a lower bacterial growth than in the no-irradiated leachates, and (7) on the contrary of temperature in temperate environments, the interchange with the surrounding terrestrial ecosystem determines the dynamics of DOM and, consequently, the bacterial communities, in the tropical ecosystems studied: coastal lagoons and Lake Batata.
For further information: farjalla@biologia.ufrj.br