Fotodegradação e degradação microbiana da matéria orgânica dissolvida no baixo Rio Negro, Amazônia Central

Abstract

The dissolved organic matter (DOM) can, among other processes, be mineralized in the water column by respiration or by photo-degradation, or be consumed by microorganisms and transferred through the food web. The interaction between sunlight and DOM promotes its break, producing inorganic carbon and smaller organic molecules. This interaction is stronger in humic ecosystems, where DOM is predominantly refractory, as the characteristic of the environments investigated here. The photo-degradation process may enhance bacterial growth, since it can generate more labile substrate to be consumed for these organisms. The interactions between radiation-DOM-bacteria are directly influenced by the substrate quality (DOM). Despite the fact, that Amazon basin is the largest watershed on the planet, few studies about the degradation and / or photo-degradation of DOM were made in this basin, furthermore, studies exploring the temporal variation in a hydrological cycle are even more scarce. This thesis had the following objectives: (1) estimate the potential production of dissolved inorganic carbon (DIC) by photodegradation, from the incidence of solar radiation in Amazonian black water ecosystems, (2) evaluate the role of the flood pulse dynamics in the input of DOM to Amazonian black water ecosystems, and its influences on DOM quality, and their changes to the process of abiotic (photo-oxidation) and biotic degradation, (3) evaluate the effects of photo-degradation on DOM mineralization and consumption by planktonic bacteria in Amazonian black-water ecosystems. The study was conducted on samples collected from Lake Tupé and Rio Negro in its lower portion, near the city of Manaus, Amazonas - Brazil. To achieve the objectives 1 and part of the 2, samples were exposed without bacteria, under natural solar radiation for 6h, were photo-production of DIC was measured, as well as changes in absorbance characteristics (430 nm). To achieve the objectives 3 and part of 2, four experiments were made in three distinct periods of the hydrological pulse (September and December 2008 and April-September 2009). These samples, without bacteria, were exposed under sunlight, and then, inoculated with bacteria from the respective study sites to assess the effects of photo-chemical degradation of DOM on planktonic bacteria degradation. The intensity of sunlight and the color of DOM showed a wide variation in a hydrological cycle, causing significant effects in the intensity of the process studied here. The photodegradation of DOM stimulated bacterial growth in the four sampling periods for in two environments. The DOM was more photo-reactive during the flooding and in the falling-waters, which showed the highest production rates of DIC and the highest rates of respiration, indicating complementary action of the processes of photo-degradation and microbial degradation in DOM removal in black-water Amazonian ecosystems. During our study period we report two weather phenomena, La Niña and El Niño. In 2008 La Niña increased precipitation in Amazon basin, resulting in a abnormal increase of the river water level, which reached, in 2009, the highest water level value ever recorded. In 2009 El Niño caused a severe drought increasing solar radiation. As a consequence of the two phenomenons we recorded the highest rates of bacterial production due to increased supply of high quality substrates to bacterial metabolism, resulting from the inundation of the low land forest. The highest rates of photodegradation occurred at falling- water of 2009 due to higher incidence of solar radiation and the highest amount of DOM, resulted from the flooded period of 2009. We can conclude that the process of photo-degradation in these ecosystems results in acceleration of DOM mineralization since it is able to generate inorganic molecules and stimulating bacterial respiration, reducing bacterial growth efficiency (BGE). We can conclude that, there are seasonal differences in DOM photo-reactivity and DOM quality also varies in a hydrological cycle.