Identification in situ and dynamics of bacteria on limnetic organic aggregates (lake snow).

Microbial assemblages on large organic aggregates (lake snow) of Lake Constance, Germany, were analyzed with rRNA-directed fluorescent oligonucleotide probes specific for the domain Bacteria and the alpha-, beta-, and gamma-subclasses of the class Proteobacteria. Lake snow aggregates were either collected in situ by SCUBA diving or in a sediment trap at 50 m or formed of natural lake water incubated in rolling cylinders under simulated in situ conditions. For the latter aggregates, the time course of the microbial colonization was also examined. The natural aggregates and those made in rolling cylinders were composed of the particulate organic material present in the lake and thus reflected the composition of the ambient plankton community. All types of lake snow aggregates examined were heavily colonized by microbial cells and harbored between 0.5 x 10(6) and > 2 x 10(6) cells aggregate -1. Between 55 and 100% of the microbial cells stained with 4', 6-diamidino-2-phenylindole (DAPI) could be visualized with the domain Bacteria-specific probe. In most samples, beta-subclass proteobacteria dominated the microbial community, constituting 27 to 42% of total cells as counted by DAPI staining, irrespective of the composition of the aggregates. During the time course experiments with the laboratory-made aggregates, the fraction of beta-subclass proteobacteria usually increased over time. Except for a few samples, alpha- and gamma-subclass proteobacteria were far less abundant than beta-subclass proteobacteria, constituting 11 to 25 and 9 to 33% of total cells, respectively. Therefore, we assume that a specific aggregate-adapted microbial community was established on the aggregates. Because the compositions of the microbial assemblages on natural and laboratory-made aggregates were similar, we conclude that aggregates made in rolling cylinders are good model system with which to examine the formation and microbial colonization of macroscopic organic aggregates.