Bacterioplankton Ecology and Phylogenetics
Bacteria are critical
to the ecological function of aquatic systems carrying out a broad array
of essential chemical transformations. Bacteria are the most important
decomposers of organic matter, thus providing balance with phytoplankton
and other primary producers that create new organic matter. Bacteria
are the only organisms capable of fixing nitrogen from dinitrogen gas
(N2) and are also the only organisms capable of returning it to the inert
dinitrogen form (i.e., denitrification). Bacteria also transform sulfur,
iron, manganese, mercury and many other abundant and not so abundant
elements in aquatic systems. They are nature’s
ultimate cyclers and recyclers.
Before the 1990s, microbial ecology research
had a split personality. There were those who studied pure cultures of
bacteria in the laboratory, and there were those that measured the rates
of bacterial activities (e.g., respiration, denitrification) in nature,
bypassing the then impossible step of identifying the tiny subjects of
their research. In the early 1990s, DNA sequencing revealed that most
bacteria in nature are different than those studied in the laboratory.
Now an array of state-of-the-art molecular techniques are used to identify
bacterial communities, characterize their metabolic capabilities, and
study how the composition of these communities relates to their ecological
function in the natural environment.
Horn Point laboratory has a long history of leadership
in the field of microbial ecology. Scientists here are presently conducting research
on the bacteria in several marine, estuarine and freshwater ecosystems with the
general goals of quantifying the activity of bacterial communities and identifying
the links between ecological function and phylogenetic diversity.
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We are also working with the Arctic LTER program in the watershed
of Toolik Lake, a tundra lake on the north slope of Alaska, to identify
controls on heterotrophic bacterial activity and bacterioplankton diversity
through a chain of lakes and streams. We are presently expanding this
project to other major bacterial environments including lake sediments,
hyporheic stream sediments, and soil water. |
Biological-Physical Interactions.
Underlying
all our work is the concept that bacteria are constantly under the influence
of hydrodynamics. Aquatic systems are rarely static. Groundwater flows
into streams and streams flow into estuaries where they mix with seawater.
In all these systems hydrodynamics influence not just the chemical environment
of bacteria but also the ‘species’ composition of bacterial
communities through advection and mixing. In one study of Plum Island
Sound, we used two different molecular techniques to identify a native
estuarine bacterioplankton community, but found that it only developed
when water residence time in the estuary was long enough to allow that
community to develop. In spring, strong river flow flushes the estuary
rapidly, and the bacterioplankton community is a simple mixture of marine
and freshwater populations. Bacterioplankton are excellent subjects for
those interested in studying biological-physical interactions, particularly
in dynamic ecosystems like the Chesapeake Bay.
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