Stream Bacterial Communities: When is Function Linked to Structure?

We have carried out a diversity of small and mesocosm-scale experiments, in conjunction with regionally distributed field sampling, to assess when the composition of stream benthic bacterial communities corresponds with differences in stream metabolic activities. We apply multiple measures of functional similarity including assays of extra cellular enzymes to determine what classes of dissolved organic matter are being degraded. We find large differences in carbon processing among streams with weaker differences in community composition.

wappinger creek

East Branch of the Wappinger Creek, NY

Ecologists are interested in the connection between how ecosystems function and the particular group of species present. Can ecosystem processes change without a shift in taxonomic composition, which would potentially facilitate a more rapid response to shifting environmental conditions? Conversely, if species change in relative abundance will there be a change in function or will the processes be resilient?

Sampling streams across broad regions together with mesocosm experiments have been used to examine whether there are parallel changes in stream microbial function and structure.

Experimental mesocosm for examining response
of stream bacteria to additions of carbon and
nutrients

Arrays of extra cellular enzymes are measured to determine what classes of dissolved organic matter are being degraded and, not surprisingly, this varies depending on land cover, geographic region and in response to specific experimental manipulations. The taxonomic composition, as measured by variations in DNA banding patterns (RAPDs), also varies broadly and in some cases we have found a strong concordance between compositional and functional shifts. There are also broad differences among regions and the similarity among communities is at least weakly related to land cover type.

These findings imply that as differences in carbon availability cause changes in the metabolic activity of bacteria, some taxa increase in relative abundance presumably due to higher growth and survival under those specific conditions. In other cases, we see no parallel between structural and functional variability, implying either that all taxa are equally adept at growth as organic carbon changes or, non-trophic selective forces overwhelm differences in carbon supply.

bacteria

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