Microbial communities in northern Adriatic mucilaginous aggregates: insight into the early phase of aggregate formation

Insight into the initial phase of aggregate formation was provided by comparison between bacterial communities from freshly formed aggregates dominated by the epipelic diatom Cylindrotheca closterium and associated water masses. This study was performed from 2000 to 2006 in the northern Adriatic. The chemotaxonomic structures and physiological conditions were inferred from the fatty acid profiles of the cultured bacterial communities of all implicated components, fresh aggregates, their adjacent waters, oligotrophic high-salinity waters and halocline waters. The results showed similarity between bacterial communities of fresh aggregates and oligotrophic high-salinity water, suggesting their common origin and involvement in the formation of aggregates. In contrast, the origin of the water adjacent to aggregates was different from that of the other components but was similar to the halocline layer and was likely derived from northern Adriatic waters. The presence and activity of heterotrophic bacteria belonging to Alteromonadaceae, which are regularly observed on fresh aggregates, suggest that the early phase of aggregate formation corresponds to an abrupt change of environmental conditions due to the mixing of central and northern Adriatic waters. The initial colonisation of fresh mucilage ascribes to Alteromonas an important ecological function in aggregate community development related to the succession of phytoplankton and heterotrophic bacteria.