Photochemical and microbial processes act in concert on carbon recycle and transformation in aquatic ecosystems. Solar radiation induces several DOM chemical reactions resulting in lost of color (photobleaching) and their oxidation. The loss of colour leads to an increase of water column UV penetration with consequent harmful effects to the biological component. Several reactive species are produced and involved in these reactions, which amplify the degradation of organic molecules and inhibition of bacterial activity. However, these processes also lead to the release of nutrients and to the formation of biological labile compounds, which increase microbial activity and intensify carbon and energy transformation to trophic webs and ecosystems productivity. The major aim of this project is to evaluate the impact of solar radiation on carbon cycle in the estuarine environment. Therefore, the interactions between solar radiation, dissolved organic matter (DOM) and bacterioplankton will be investigated in the natural environment and simulated experimentally. In order to identify the factors that influence solar radiation penetration in water and the seasonal and spatial profiles of variation of bacterioplankton abundance, diversity and activity, field studies will be conducted in two different zones with distinct characteristics in the estuarine system Ria de Aveiro during one year. Additionally, laboratory experimental assays will be performed to elucidate the mechanisms involved in DOM photochemical reactions, the responses of bacterioplankton to DOM photochemical transformations and direct UV radiation exposure, and the role of different reactive species on DOM photoreactions and their impact on bacterial activity. The contributions of the different wavelength ranges (UV-B, UV-A and PAR) to DOM photochemical transformations will be also evaluate during the experimental procedures. Changes in the composition of the bulk DOM induced by photoreactions will be determined by UV/visible absorbance and excitation emission matrix fluorescence spectroscopy (EEMS).
Variations in DOM molecular weight will be evaluated by ultrafiltration and HPLC-SEC. The response of bacterioplankton to both DOM photochemical transformation and direct exposure to UV radiation will be assed by the determination of abundance, activity and diversity parameters. Bacterial abundance will be estimated by epifluorescence microscopy. Bacterial activity will be characterized by the rates of biomass production (incorporation of 3H-leucine), extracellular degradation of polymers (degradation of model-substrates methyl-umbelliferyl- and methyl- coumarinyl-substrates) and uptake of monomers (incorporation of 14C-labelled substrates).
Bacterial diversity will be characterized by culture-independent methods such as Denaturing Gel Gradient Electrophoresis (DGGE) after DNA extraction and amplification of 16S rRNA gene V3 fragments. The contribution of the different reactive species to DOM photochemical reactions will be evaluated by the addition of several scavengers and their impact on bacterial activity will be assed by the determination of several cellular stress indicators (production of intracellular reactive oxygen species, lipid peroxidation, protein peroxidation, activity of antioxidant enzymes and DNA lesions). Previous investigations conducted by the team members allowed explaining the factors that influence microbial density, activity and biodiversity and photochemical transformations occurring in the estuarine environment as well as the direct effect of UV radiation on bacteria distribution. However, the global importance of DOM photochemical transformations, the mechanisms involved on photoreaction and the interactions with bacterial communities, as well as, the balance between positive and negative effects resulting from irradiated DOM on bacteria, remain to be investigated. A global approach, which evaluates simultaneously stimulation and inhibition effects of photochemical reactions on microbial processes, carried
out by two teams with solid expertise in complementary scientific areas, which will apply recent and innovative analytic methodologies, will contribute to introduce novelties in scientific knowledge on the mechanisms involved on DOM photoreactions and bacterial responses.

CESAM members on this project

Adelaide Almeida

Angela Cunha

Grant holder

Maria Eduarda B. H. Santos

Valdemar Esteves