Effects of solar radiation on marine bacterial and phytoplankton heterotrophic activities

Abstract

Since the discovery, 20 years ago, that solar ultraviolet radiation (UVR, 280-400 nm) penetrates much deeper into the ocean’s water column than previously thought, knowledge on its effects on diverse surface processes and organisms has significantly increased, yet further research is needed for a good understanding of sun-organism interactions. Marine bacteria are considered some of the most susceptible to sunlight damage due to their small size and lack of pigments. Since they are primary consumers of dissolved organic matter (DOM) in seawater, one might expect that any effect on them will ultimately have implications for carbon and nutrient cycling. In this work, we contributed to increase the knowledge on the effects of sunlight on the heterotrophic uptake of DOM by bacteria, although we also devoted some effort to assess its impact on the osmoheterotrophic activity of phytoplankton. For that purpose, we combined bulk activity measurements with a single-cell approach involving microautoradiography for identification of both the responses at the community levels and particular responses and sensitivities to sunlight of different microorganisms. Our work highlights the role of solar radiation as a significant modulator of DOM fluxes through differentially affecting the diverse components of the planktonic food webs. The observed responses were shown to vary not only at different scales depending on changes in the light conditions, but also owing to the previous light-exposure history, the identity of the organisms involved, the substrates considered and the accuracy of our light incubation conditions. Whereas exposure to sunlight often led to a reduction in the amount of leucine incorporated, incubation under photosynthetically active radiation (PAR, 400-700 nm) conditions sometimes stimulated its uptake, and potential photoheterotrophy is discussed among other possibilities. In contrast, bulk assimilation of the organic compound dimethylsulfoniopropionate (DMSP) was often increased upon light exposure, particularly within the non-bacterial fraction. We also found that algal osmoheterotrophy seems to be widespread and more common than previously thought, particularly within the phytoplankton assemblages inhabiting the DMSP-rich summer polar waters, and that this behaviour was highly dependant on the light levels. The present thesis sheds light on the complexity of sun-microbe interactions and provides evidence of the need to afford the study of solar radiation effects on the different compartments of the microbial food webs