Bacterial colonization of early stages of limnetic diatom microaggregates

Macroscopic organic aggregates in pelagic environments are colonized by bacterial populations that differ from those in the surrounding water. To understand better how this well-adapted bacterial community is established, it is important to examine the initial colonization early in the aggregation process. We studied, therefore, the early formation and bacterial colonization of diatom microaggregates (MA) (< 150 pm) during the phytoplankton spring bloom in Lake Constance, Germany. Water samples were incubated in plexiglass cylinders rolled horizontally for 44 to 48 h and subsampled for MA and their bacterial colonization, which was examined by fluorescent in situ hybridization with oligonucleotides of various specificity. During the initial 24 h bacterial numbers remained <similar to>70 cells MA(-1) and finally increased to similar to 250 MA-1. Detection rates of Bacteria by probe EUB338 ranged from 40% to >80% of the DAPI-stainable cells. Initially, alpha -Proteobacteria detected by the probe ALF1b dominated the bacterial community on MA, whereas toward the end of the incubation beta -Proteobacteria increasingly dominated. Proportions of Cytophaga/Flavobacteria detected by the probe CF319a also increased systematically on MA toward the end but constituted lower proportions than beta -Proteobacteria. In the surrounding water beta -Proteobacteria dominated during the initial 24 h whereas Cytophaga/Flavobacteria consistently dominated in the late phase of the experiments. Applying highly specific probes for narrow clusters of close relatives of the genus Sphingomonas (alpha -Proteobacterium), Duganella zoogloeoides (formerly Zoogloea ramigera) and Acidovorax facilis (both beta -Proteobacteria), we found that these bacteria were present on MA already at the initial sampling or at the latest after 10 h and comprised substantial and sometimes dominant proportions of total alpha- and beta -Proteobacteria. These bacteria, also dominant on natural lake snow aggregates in Lake Constance, were never detected among the free-living bacterial community in the surrounding water. Hence, our results indicate that the bacterial community on lake snow aggregates develops largely from seeds on their precursor MA.