Phytoplankton composition and organic matter organization of mucous aggregates by means of light and cryo-scanning electron microscopy

Two types of northern Adriatic mucous macroaggregates - surface and cloud-like - collected in June 2000 at the sea surface and at the depth of the pycnocline, respectively, were studied by light and cryo-scanning electron microscopy. The combination of both techniques permitted investigation of species/particle composition and particle-organic matrix associations of copious mucous aggregates. Both types of macroaggregates had, on average, heterogeneous composition with diatoms the dominant biotic component, differing by the prevailing species and stage of degradation. Some differences in the organization of the basic fibrillar organic network of macroaggregates were observed. Clouds were characterized by a tighter organization of the polysaccharidic fibrils within the walls of the organic network constituting the cloud. This organization was looser in the organic network within surface aggregates. Plankton remains and inorganic particles could lead to lower porosity and greater rigidity of these aggregates. The more complex organization of the organic matter and the intense growth of Cylindrotheca closterium within the clouds probably reflect a longer physical diagenesis of the polysaccharidic fibrils in comparison to surface aggregates.