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We studied the composition, microbial colonization and organic matter fluxes of lake snow in Lake Constance, Germany. in 1993. Aggregates were collected between 6 and 25 m by SCUBA or made in the lab in rolling cylinders from samples from the epilimnion. Aggregates were composed of living and senescent phytoplankton, zooplankton molts and carcasses, and other unidentifiable debris. Dry weight and particulate organic carbon (POC) were 3 to 20 1-19 agg.-' and 1.5 to 33 pg C agg.-', respectively. Carbon in particulate combined amino acids (PCAA) was 8 to 51 % of POC on aggregates but only <lO':.a, in bulk POC Aggregates wcre densely colonized, with 5 to 80 X 106 bacteria agg:', and numbers usually increased with depth. Production and growth rates of bacteria on aggregates (aggregate bacteria) were low compared to free-living bacteria. However, the organic matter tul-nover within the aggregates and release into the surrounding water were high due to intense activities of the ectohydrolases (aminopeptidase. alkaline phosphatase, aand (3-glucosidase, and chitinase) of aggregate bacteria. Activities of the aminopeptidase were consistently higher than those of the other enzymes. In laboratory experiments dissolved amino acids, mainly DCAA, were released from aggregates into the surrounding water; turnover times of PCAA on aggregates due to hydrolysate release usually ranged from <5 to 25 h. Lake snow thus was a potential source of amino acids for free-living bacteria in the epiand hypolirnnion of Lake Constance. The colonization of lab-made aggregates showed a transition from small to large rods within the first 2 to 5 d after aggregation to filamentous, grazing-resistant bacteria. This community was largely composed of microbes of the domain Bacteria, determined by in situ hybridization with rRNA-targeted fluorescent oligonucleotide probes. P-Proteobactena were usually dominant, particularly in aged aggregates. At earlier stages a-Proteobacteria also comprised substantial fractions of the community. This community structure is similar to that of activated sludge flocs, suggesting that lake snow has a function in lacustrine ecosystems comparable to that of activated sludge flocs in sewage treatment plants.

Bacterial colonization and microbial decomposition of limnetic organic aggregates (lake snow)