Effect of different carbon sources on marine bacterioplankton diversity and function

Resumo

Bacteria are the most abundant organism on Earth and mediate many critical ecosystem processes. This makes of great importance the knowledge of the identity and population dynamics of the indigenous microorganisms. First studies of microbiology have been carried out by traditional cultivation techniques, until cell counts by epifluorescence techniques and molecular techniques have been found to be more representatives approaches of the environment. This is mainly because the culture media have nutrient content several times higher than the natural media, thus the growth of dominant organisms from oligotrophic environment is not easy, and there is a selection of bacterial organisms with fewer representativity. The selection of bacteria also occurs in the natural environment in greater scale, when different inputs take place. These can be due to natural processes as rivers run-offs, aerosol depositions or from antropogenic origins as oil spills. In this thesis we want to evaluate the effect on bacterial diversity and function of these additions. There is an increasing interest for bacterial isolation due to the advantages of having better information about metabolic capabilities of the organisms there growing. Thus, we decided to evaluate the representativity of isolates in the Mediterrranean sea by a comparison of these sequences with other sequences retrieved by molecular techniques like DGGE finger printing and cloning in previous studies. The comparison was carried out by ARB sequences analyses package. Thanks to isolation we increased our knowledge of the diversity in our study area. To address the question of how nutrient addition selects bacterial diversity we performed different mesocosm experiments, where we add different nutrients concentrations to water collected at he Blanes Bay (NW Mediterranean Sea). Another mesocosm experiment was carried out to evaluate the effect of the oil spill that took place in the accident of the Prestige tanker in front of the Galician coast. The mesocosm experiments have been widely used to evaluate addition effects due to the difficulties of mimic natural conditions. During all these experiments different environmental parameters were measured and also bacterial production and abundance were monitored. The bacterial community structure was determined by DGGE fingerprinting technique, which allows the comparison of different samples and further analysis of the richness by Shannon-Weaver index based on the presence and absence of bands. Pearson-Pairwise correlation has been done to relate the bacterial production and bacterial composition before and after predator's growth. This analysis shows an influence of the grazing on the bacterial selection after additions. In the oil spill mesocosm experiment just effect on bacterial structure, production and abundance was detected with additions four times higher than the concentration found after the Prestige tanker accident. This thesis also includes a study carried out in the North Atlantic during two different cruises. In order to determine the bottom-up or top-down effects on the bacterial composition and activities at two different depths we performed a transplant experiment. This experiment was carried out using dialysis bags where the interchange of nutrients was allowed but not organisms flow. The results showed a clear bottom-up effect on bacterial communities.