α- and β- Proteobacteria Control the Consumption and Release of Amino Acids on Lake Snow Aggregates

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Abstract
We analyzed the composition of aggregate (lake snow)-associated bacterial communities in Lake Constance from 1994 until 1996 between a depth of 25 m and the sediment surface at 110 m by fluorescent in situ hybridization with rRNA-targeted oligonucleotide probes of various specificity. In addition, we experimentally examined the turnover of dissolved amino acids and carbohydrates together with the microbial colonization of aggregates formed in rolling tanks in the lab. Generally, between 40 and more than 80% of the microbes enumerated by DAPI staining (4′,6′-diamidino-2-phenylindole) were detected as Bacteria by the probe EUB338. At a depth of 25 m, 10.5% ± 7.9% and 14.2% ± 10.2% of the DAPI cell counts were detected by probes specific for α- and β- Proteobacteria . These proportions increased to 12.0% ± 3.3% and 54.0% ± 5.9% at a depth of 50 m but decreased again at the sediment surface at 110 m to 2.7% ± 1.4% and 41.1% ± 8.4%, indicating a clear dominance of β- Proteobacteria at depths of 50 and 110 m, where aggregates have an age of 3 to 5 and 8 to 11 days, respectively. From 50 m to the sediment surface, cells detected by a Cytophaga/Flavobacteria -specific probe (CF319a) comprised increasing proportions up to 18% of the DAPI cell counts. γ- Proteobacteria always comprised minor proportions of the aggregate-associated bacterial community. Using only two probes highly specific for clusters of bacteria closely related to Sphingomonas species and Brevundimonas diminuta , we identified between 16 and 60% of the α- Proteobacteria . In addition, with three probes highly specific for close relatives of the β- Proteobacteria Duganella zoogloeoides (formerly Zoogloea ramigera ), Acidovorax facilis , and Hydrogenophaga palleroni , bacteria common in activated sludge, 42 to 70% of the β- Proteobacteria were identified. In the early phase (Proteobacteria . Hence, our results show that lake snow aggregates are inhabited by a community dominated by a limited number of α- and β- Proteobacteria , which undergo a distinct succession. They successively decompose the amino acids bound in the aggregates and release substantial amounts into the surrounding water during aging and sinking.
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