Effect of the interaction between phytoplankton and bacteria on the response of microbial communities to nutrient inputs

Resumo

Biogeochemical cycles have undergone numerous anthropogenic alterations, having an important impact on marine systems (coastal). Nitrogen cycle have undergone important changes caused by human activities like fertilizers or the cultivation of nitrogen-fixing plants. These alterations affect the coastal organisms. The study of the microbial plankton response is very interesting because we can observe the response in a small time-scale. In this work we will investigate the role of the interaction between phytoplankton and bacteria in the response of microbial communities to nutrient additions. Microbial plankton communities do not always respond to inorganic nutrient additions in coastal waters off NW Spain. However, enhanced growth of phytoplankton and heterotrophic bacteria has been observed when exposed to both inorganic (nitrate, ammonium and phosphate) and organic (amino acids and glucose) nutrient amendments, suggesting the existence of an interaction between both microbial compartments, where bacteria produces secondary metabolites that phytoplankton needs to growth. The aim of this workr is to assess the role of the phytoplankton–bacterioplankton interaction in the response of primary producers to nutrient additions. As well this work evaluate the temporal variability of the nature of the interaction and the effect of one bacterial metabolite (vitamin B12) as modulator of the interaction. Finally this thesis studies the effect of ultraviolet radiation over the nature of this interaction.