This thesis contributes to our understanding of the nature and quantity of organic matter in the sea, and of the role of marine organisms in its production and transformations. Synthesis of organic matter through fotosynthesis by algae and breakdown of this organic matter to CO2 and nutrients by bacteria are central processes in the marine carbon-cycle. Due to its effect on the atmospheric CO2 concentration, and on the functioning of the marine food-web, there is considerable scientific interest in the marine carbon-cycle.

After formation, the major part of the primary production is rapidly degraded and thus forms the carbon and energy source for the marine food web. Particulate organic matter is grazed by zooplankton to provide the base of the ‘grazing food chain’. Dissolved organic matter is taken up by bacteria and can be channeled into the grazing food chain via the ‘microbial loop’. Organic matter that is not taken up by marine organisms can be transported to the deep sea where inhibited degradation may exclude carbon from the global carbon cycle for a long time-period. The amount of organic matter that is processed in each of the pathways (grazing food chain, microbial loop, vertical transport) is mainly determined by the capacity of different organisms in the marine food web to utilize it. A considerable portion of the marine primary production consists of algal exudates, and an interesting and substantial source of such algal polymers is produced by Phaeocystis. During spring blooms, this marine microalga produces massive amounts of mucopolysaccharides, forming colonies in which thousands of cells are embedded. After blooms, the mucus may end up in the sediment or as foam on beaches, which indicates that the mucus is not easily degraded.

The research in this thesis focussed on the nature and degradability of the mucopolysaccharides, to provide explanations for observations that have been done repeatedly during blooms such as the variation between colony-associated bacteria during subsequent bloom stages, and the accumulation of organic matter after blooms. Phaeocystis mucopolysacharides appeared to be complex polysaccharides with a characteristic monosaccharide composition. Throughout a bloom and between different blooms this composition remained stable. It was shown that the variation in the contribution of glucose to the carbohydrate composition was due to the presence of an intracellular glucan functioning as energy storage. Phaeocystis mucopolysaccharides appeared to be degradable in enrichment cultures of marine bacteria under oxic as well as anoxic conditions. The initial degradation rate is sufficiently high to facilitate degradation in the time-span of a typical spring bloom. Slowing down of the degradation rate of mucopolysaccharides could not be explained by an accumulation of refractory fragments. Production of inhibitory compounds during degradation provided an explanation however. Therefore, although Phaeocystis mucus is not easily degraded, it is not the chemical composition of the material that causes it to accumulate. Other factors such as nutrient limitation may play a significant role. The bacteria responsible for mucus degradation could not be isolated in pure culture. However, using a correlation between mucus degradation in enrichment cultures and community profiles generated by molecular methods, the bacteria involved in mucopolysaccharide degradation could be identified.

The characterization of the breakdown of an important marine microalgal polysaccharide and the identification of the responsible bacteria provide insights into the functioning of the marine carbon cycle at the base of the food web.