Regulação da estequiometria (C:N:P) da biomassa de bactérias heterotróficas em ecossistemas de água doce de baixa latitude

Abstract

The heterotrophic bacteria are important nutrient mineralizers (e.g. nitrogen [N] and phosphorus [P]) to the aquatic environment, subsidizing primary production and incorporating organic carbon from the organic matter into its biomass through secondary production. These processes are affected by environmental factors such as temperature and nutrient availability; are also determined by species identity and physiological status. The availability of nutrients can affect the chemical composition of bacteria, as these can affect the chemical composition of their predators and so on; This stoichiometric imbalance between the resource and the consumer influences the patterns of nutrient recycling in ecosystems, affecting their functioning and biogeochemical cycles. One way to deal with nutrient variation is the regulating ability that individuals have, and in the face of disturbances in the stoichiometry of their resource, bacteria can behave in a homeostatic or flexible way. For bacteria these characteristics seem to be dictated by the composition of the community, influenced by the trophic state of the ecosystems. Thus, homeostatic behavior is expected to be predominant in eutrophic environments, while flexible behavior is predominant in oligotrophic environments. In order to determine the degree of community homeostasis, controlled trials are necessary to evaluate the composition of bacterial C, N and P against the exposure of a community to substrates with different C: N: P ratios. This type of experiment is performed on chemostats, which represent a method of culturing microorganisms under controlled growth stationary conditions in a stable chemical environment. The objective of this work was to test the effect of the degree of productivity of the system on the variability of the stoichiometry of bacteria and their resources in tropical lakes of low latitude. Hypotheses: (i) The variability of the stoichiometric ratios of the bacteria and their resources are smaller in a hypereutrophic reservoir than in an oligotrophic pond; and (ii) The degree of homeostasis of bacterial communities increases with the degree of productivity of the system. We first investigated the variability of stoichiometric ratios in two lakes, one hypereutrophic (Gargalheiras) and one oligotrophic (Bonfim) under an environmental approach, where monthly measurements were made of the chemical composition of the bacteria and their resource. Under an experimental approach, it was tested by manipulating the C:P ratios of the culture medium in chemostats, the degree of homeostasis of bacterial communities from 11 lakes distributed along a productivity gradient from the coast to the semi-arid region of Rio Grande do Norte. The results show that Bonfim has a high variation in the stoichiometry of the bacteria and its resource; in Gargalheiras, the C:N:P ratio of the bacteria varies little in relation to a greater variation of their resource, this shows an indicative of homeostasis in these communities. For the chemostats, the communities are homeostatic up to a C:P ratio (atomic ratio) of approximately 1000:1, with the increase of this ratio, they seem to accompany their resource, showing part homeostatic, part flexible, independent of the trophic state of the environment from where they came.