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

Abstract

We used in situ hybridization with fluorescently labeled rRNA-targeted oligonucleotide probes concurrently with measurements of bacterial carbon production, biomass, and extracellular polymeric substances (EPS) to describe the 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 relative abundance by a factor of 5.5 to 7.9. Bacteria of the Cytophaga-Flavobacterium group were also sixfold to eightfold more abundant in the storm aftermath. Furthermore, elevated numbers of Archaea and members of the Cytophaga-Flavobacterium group characterized the phylogenetic composition of the supraglacial ice community. We postulate that glacial meltwaters constitute a possible source of allochthonous bacteria to the stream biofilms. Although stream water temperature increased dramatically from the glacier snout along the stream (3.5 km), sediment chlorophyll a was the best predictor for bacterial carbon production and specific growth rates along the stream. Concomitant with an increase in sediment chlorophyll a, the EPS carbohydrate-to-bacterial-cell ratio declined 11- to 15-fold along the stream prior to the storm, which is indicative of a larger biofilm matrix in upstream reaches. We assume that a larger biofilm matrix is required to assure prolonged transient storage and enzymatic processing of allochthonous macromolecules, which are likely the major substrate for microbial heterotrophs. Bacteria of the Cytophaga-Flavobacterium cluster, which are well known to degrade complex macromolecules, were most abundant in these stream reaches. Downstream, higher algal biomass continuously supplies heterotrophs with easily available exudates, therefore making a larger matrix unnecessary. As a result, bacterial carbon production and specific growth rates were higher in downstream reaches.

FIG. 1

Longitudinal patterns of sediment organic matter and bacterial biomass along the RT stream. Panels: a, chlorophyll a; b, EPS as carbohydrates; c, amino acids (AA) associated with EPS; d, cell carbon content; e, bacterial abundance; f, bacterial biomass; g, carbohydrate EPS normalized to bacterial cell number; h, EPS amino acids normalized to bacterial cell number.

FIG. 2

Bacterial cell size distribution (thick curve) and biomass allocation (thin curve) to cell length intervals of 0.1 μm along the RT stream. The shaded bar designates the consistent cell length peak along the stream in October.

FIG. 3

Longitudinal gradients of BCP and SGR in sediment biofilms along the RT stream.

FIG. 4

Phylogenetic composition of Bacteria and Archaea in sediment biofilms, glacier ice, and stream water as revealed by fluorescence in situ hybridization.