| Summary: | Heterotrophic bacteria of the genus Vibrio are indigenous in the marine environment although environmental cues regulate their growth and distribution. The attention brought to this genus is due to its many species/strains that are pathogenic to humans and other organisms. Vibrio abundances are strongly coupled to water temperature and salinity but abundance dynamics occur even where these hydrographical parameters are stable. In this thesis, I have studied Vibrio dynamics in relation to other organisms such as phytoplankton (papers I, II and III) and a bivalve host-organism (paper IV) in a changing environment where increasing temperature (paper III) and ocean acidification (paper IV) may influence survival and proliferation of these bacteria. In particular paper I showed that in a tropical coastal area, where the water temperature and salinity were stable across seasons, abundances of Vibrio were tightly coupled to phytoplankton biomass and community composition. A diatom bloom during December seemed to support high numbers of vibrios in waters with otherwise low levels of dissolved organic carbon. Paper II further supports that some phytoplankton can favor Vibrio growth while others seem to have a negative influence on Vibrio abundances. For instance, Skeletonema tropicum, a common diatom in Indian coastal waters, easily eradicated Vibrio parahaemolyticus from sea water in our experiments. In temperate marine areas culturable Vibrio predominantly occurs in the water column during the warmer months. Sediments are suggested to be potential reservoirs when conditions in the water-column are harsh. Accordingly, in paper III we showed that cold-water sediments from geographically separate areas in a boreal region of Scandinavia all contained relative high abundances of total Vibrio spp. and that all sediments also included culturable Vibrio. In agreement with paper I, the fresh input of organic material from phytoplankton blooms, for which chlorophyll a was used as a proxy, seemed to positively influence Vibrio ...
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