In order to identify patterns in bacterial community composition in freshwater habitats, we analyzed the available database of 16S rDNA sequences from freshwater plankton, including 24 new sequences from Parker River (Massachusetts, USA), 42 from Lake Soyang (South Korea) and 148 from Lake IJssel (The Netherlands). At this point, combined diversity studies using random cloning have deposited 689 bacterial and 75 plastid 16S rDNA sequences from the water column of rivers and lakes in North America, Europe and Asia. Systematic comparisons with the global database showed that the majority of the bacterial sequences were most closely related to other freshwater clones or isolates, while relatively few were closest to sequences recovered from soils or marine habitats. This habitat-specific clustering suggests that the clustered 16S rDNA sequences represent species or groups of species that are indigenous to freshwater. We have discerned 34 phylogenetic clusters of closely related sequences that are either restricted to freshwater or dominated by freshwater sequences. Of these clusters, 23 contained no cultivated organisms. These putative freshwater clusters were found among the alpha-, beta- and gamma-Proteobacteria, the Cytophaga-Flavobacterium-Bacteroides group, the Cyanobacteria, the Actinobacteria, the Verrucomicrobia, the green non-sulfur bacteria and candidate division OP10. This study shows that rivers and lakes have a specific planktonic bacterial community distinct from bacteria in neighboring environments such as soil and sediments. It also points out that these planktonic bacteria are distributed in diverse freshwater ecosystems around the world. [KEYWORDS: Microbial diversity · Ribosomal RNA gene · Freshwater · Habitat · Polymerase chain reaction · Phylogeny · Nucleotide sequence database

Illuminating the pathway to destruction: Arctic lakes are an important source of atmospheric CO ₂ and therefore play a role in climate change. It is thus vital to know how the rapid Arctic warming will affect them. Cory et al. now show that light is the biggest culprit in the breakdown of carbon from thawing permafrost soils (see the Perspective by Tranvik). This carbon then moves out into Arctic lakes and streams. Contrary to previous expectations, these photochemical processes cause much more destruction of the organic molecules in fresh water than bacterial respiration does. Science , this issue p. 925 ; see also p. 870

Shifts in bacterioplankton community composition along the salinity gradient of the Parker River estuary and Plum Island Sound, in northeastern Massachusetts, were related to residence time and bacterial community doubling time in spring, summer, and fall seasons. Bacterial community composition was characterized with denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S ribosomal DNA. Average community doubling time was calculated from bacterial production ([ ¹⁴ C]leucine incorporation) and bacterial abundance (direct counts). Freshwater and marine populations advected into the estuary represented a large fraction of the bacterioplankton community in all seasons. However, a unique estuarine community formed at intermediate salinities in summer and fall, when average doubling time was much shorter than water residence time, but not in spring, when doubling time was similar to residence time. Sequencing of DNA in DGGE bands demonstrated that most bands represented single phylotypes and that matching bands from different samples represented identical phylotypes. Most river and coastal ocean bacterioplankton were members of common freshwater and marine phylogenetic clusters within the phyla Proteobacteria , Bacteroidetes , and Actinobacteria . Estuarine bacterioplankton also belonged to these phyla but were related to clones and isolates from several different environments, including marine water columns, freshwater sediments, and soil.

Microbes are transported in hydrological networks through many environments, but the nature and dynamics of underlying microbial metacommunities and the impact of downslope inoculation on patterns of microbial diversity across landscapes are unknown. Pyrosequencing of small subunit ribosomal RNA gene hypervariable regions to characterize microbial communities along a hydrological continuum in arctic tundra showed a pattern of decreasing diversity downslope, with highest species richness in soil waters and headwater streams, and lowest richness in lake water. In a downstream lake, 58% and 43% of the bacterial and archaeal taxa, respectively, were also detected in diverse upslope communities, including most of the numerically dominant lake taxa. In contrast, only 18% of microbial eukaryotic taxa in the lake were detected upslope. We suggest that patterns of diversity in surface waters are structured by initial inoculation from microbial reservoirs in soils followed by a species-sorting process during downslope dispersal of both common and rare microbial taxa. Our results suggest that, unlike for metazoans, a substantial portion of bacterial and archaeal diversity in surface freshwaters may originate in complex soil environments.

Seasonal shifts in bacterioplankton community composition in Toolik Lake, a tundra lake on the North Slope of Alaska, were related to shifts in the source (terrestrial versus phytoplankton) and lability of dissolved organic matter (DOM). A shift in community composition, measured by denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes, occurred at 4°C in near-surface waters beneath seasonal ice and snow cover in spring. This shift was associated with an annual peak in bacterial productivity ([ ¹⁴ C]leucine incorporation) driven by the large influx of labile terrestrial DOM associated with snow meltwater. A second shift occurred after the flux of terrestrial DOM had ended in early summer as ice left the lake and as the phytoplankton community developed. Bacterioplankton communities were composed of persistent populations present throughout the year and transient populations that appeared and disappeared. Most of the transient populations could be divided into those that were advected into the lake with terrestrial DOM in spring and those that grew up from low concentrations during the development of the phytoplankton community in early summer. Sequencing of DNA in DGGE bands demonstrated that most bands represented single ribotypes and that matching bands from different samples represented identical ribotypes. Bacteria were identified as members of globally distributed freshwater phylogenetic clusters within the α- and β- Proteobacteria , the Cytophaga-Flavobacteria-Bacteroides group, and the Actinobacteria .

Dissolved organic matter (DOM) in the oceans is one of the largest pools of reduced carbon on Earth, comparable in size to the atmospheric CO ₂ reservoir. A vast number of compounds are present in DOM, and they play important roles in all major element cycles, contribute to the storage of atmospheric CO ₂ in the ocean, support marine ecosystems, and facilitate interactions between organisms. At the heart of the DOM cycle lie molecular-level relationships between the individual compounds in DOM and the members of the ocean microbiome that produce and consume them. In the past, these connections have eluded clear definition because of the sheer numerical complexity of both DOM molecules and microorganisms. Emerging tools in analytical chemistry, microbiology, and informatics are breaking down the barriers to a fuller appreciation of these connections. Here we highlight questions being addressed using recent methodological and technological developments in those fields and consider how these advances are transforming our understanding of some of the most important reactions of the marine carbon cycle.