Phylogenetic and functional heterogeneity of sediment biofilms along environmental gradients in a glacial stream

We used in situ hybridization with fluorescently labeled rRNA-targeted olig onucleotide probes concurrently with measurements of bacterial carbon produ ction, biomass, and extracellular polymeric substances (EPS) to describe th e bacterial community in sediments along a glacial stream. The abundance of sediment-associated Archaea, as detected with the ARCH915 probe, decreased downstream of the glacier snout, and a major storm increased their relativ e abundance by a factor of 5.5 to 7.9, Bacteria of the Cytophaga-Flavobacte rium group were also sixfold to eightfold more abundant in the storm afterm ath. Furthermore, elevated numbers of Archaea and members of the Cytophaga- Flavobacterium group characterized the phylogenetic composition of the supr aglacial ice community. We postulate that glacial meltwaters constitute a p ossible source of allochthonous bacteria to the stream biofilms, Although s tream water temperature increased dramatically from the glacier snout along the stream (3.5 km), sediment chlorophyll a was the best predictor for bac terial carbon production and specific growth rates along the stream. Concom itant with an increase in sediment chlorophyll a, the EPS carbohydrate-to-b acterial cell ratio declined 11- to 15-fold along the stream prior to the s torm, which is indicative of a larger biofilm matrix in upstream reaches. W e assume that a larger biofilm matrix is required to assure prolonged trans ient storage and enzymatic processing of allochthonous macromolecules, whic h are likely the major substrate for microbial heterotrophs, Bacteria of th e Cytophaga-Flavobacterium cluster, which are well known to degrade complex macromolecules, were most abundant in these stream reaches. Downstream, hi gher algal biomass continuously supplies heterotrophs with easily available exudates, therefore making a larger matrix unnecessary, As a result, bacte rial carbon production and specific growth rates were higher in downstream reaches.