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.

The 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.

Seasonal Oxygen Gradient in Chesapeake Bay

In 2004, an analysis of the seasonal oxygen gradient and the deep hypoxic-anoxic zone that forms every summer in the Chesapeake Bay revealed high rates of bacterial production in the anoxic zone associated with a gradual shift in the composition of the bacterioplankton community.

Watershed-scale Bacterioplankton Research

HPL scientists are working with the Plum Island Sound Long Term Ecological Research (LTER) program and the Plum Island Microbial Observatory (PIMO) to describe bacterioplankton activity and diversity across watersheds from first order streams to rivers, through the estuarine salinity gradient and out to the coastal ocean.      

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.

Ciliate Phylogenetics and Microbial Eukaryotic Diversity

A fluorescent micrograph of a Myrionecta rubra cell hybridized with a SSU rRNA oligonucleotide probe labeled with cy-5; on left is a cell labeled with the probe and on right is a layered image of the probe, a DIC cell image, and the red fluorescence of chloroplasts.

Ciliates are important members of the microzooplankton, an important and diverse assemblage of microorganisms responsible for consuming most of the primary production in aquatic systems. While we now appreciate the pivotal ecological roles that ciliates play in marine microbial food webs, relatively little is known concerning the phylogenetic construct of marine and estuarine ciliates. Currently we are involved in a projects to better understand diversity and function of common pelagic ciliates. One surprising result so far is that the common marine ciliates Myrionecta rubra and Mesodinium pulex have SSU rRNA gene sequences that indicate they are not closely related to other ciliates in the Litostomatea, the class in which they were placed based on morphology.  Another interesting result is that the Myrionecta/Mesodinium SSU rRNA sequences match "mystery" eukaryotic sequences from deep sea and polar waters, providing valuable insight into microbial eukaryotic diversity in the world’s oceans.

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