alpha- and beta-Proteobacteria control the consumption and release of amino acids on lake snow aggregates

We analyzed the composition of aggregate (lake snow)-associated bacterial c ommunities in Lake Constance from 1994 until 1996 between a depth of 25 m a nd the sediment surface at 110 m by fluorescent in situ hybridization with rRNA-targeted oligonucleotide probes of various specificity. In addition, w e experimentally examined the turnover of dissolved amino acids and carbohy drates together with the microbial colonization of aggregates formed in rol ling tanks in the lab. Generally, between 40 and more than 80% of the micro bes enumerated by DAPI staining (4',6'-diamidino-2-phenylindole) were detec ted 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 f or alpha- and beta -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 sed iment surface at 110 m to 2.7% +/- 1.4% and 41.1% +/- 8.4%, indicating a cl ear dominance of beta -Proteobacteria at depths of 50 and 110 m, where aggr egates have an age of 3 to 5 and 8 to 11 days, respectively. From 50 m to t he sediment surface, cells detected by a Cytaphaga/Flavobacteria-specific p robe (CF319a) comprised increasing proportions up to 18% of the DAPI cell c ounts. gamma -Proteobacteria always comprised minor proportions of the aggr egate-associated bacterial community Using only two probes highly specific for clusters of bacteria closely related to Sphingomonas species and Brevun dimonas diminuta,,we identified between 16 and 60% of the alpha -Proteobact eria. In addition, with three probes highly specific for close relatives of the beta -Proteobacteria Duganella zoogloeoides (formerly Zoagloea ramiger a), Acidovorax facilis, and Hydrogenophaga palleroni, bacteria common in ac tivated sludge, 42 to 70% of the beta -Proteobacteria were identified. In t he early phase (<20 h) of 11 of the 15 experimental incubations of aggregat es, dissolved amino acids were consumed by the aggregate-associated bacteri a from the surrounding water. This stage,vas followed by a period of 1 to 3 days during which dissolved amino acids were released into the surrounding water, paralleled by an increasing dominance of <beta>-Proteobacteria. Hen ce, our results show that lake snow aggregates are inhabited by a community dominated by a limited number of alpha- and beta -Proteobacteria, which un dergo a distinct succession. They successively decompose the amino acids bo und in the aggregates and release substantial amounts into the surrounding water during aging and sinking.