Seminar: Dr. Barbara MacGregor, UNC Dept. of Marine Sciences

UNC Marine Sciences’ is proud to host a seminar by one of our own faculty Dr. Barbara MacGregor.

Presenter Affiliation: University of North Carolina at Chapel Hill, Department of Marine Sciences

Title: Building big bacteria: Central vacuoles and nitrate respiration pathways in the large sulfur bacteria

Abstract: Most bacteria are microscopic, with cell diameters of at most a few microns, but this is biology – there are exceptions. For example, some sulfur-oxidizing Gammaproteobacteria are 100 microns or more across, and are visible to the naked eye. Most of their cell volume is taken up by a central vacuole, which may contain high concentrations of nitrate or phosphate. These large sulfur bacteria (LSB) gain energy by transferring electrons from reduced sulfur compounds (e.g. hydrogen sulfide) to oxidized nitrogen compounds (e.g. nitrate) or oxygen. Because these compounds are not typically found together at sufficient concentrations to support growth, sulfur-oxidizing, nitrate-reducing bacteria must shuttle between sulfidic and oxic environments; be long enough to span both; or live where sulfidic and oxic conditions alternate. Possession of a large nitrate storage vacuole could clearly be an advantage in any of these situations.

How did the vacuole evolve? How do DNA replication and cell division work with this extra cellular compartment in place? How are materials moved in and out of the vacuole? There has been excellent experimental work addressing some of these questions, but it has been hampered by the resistance of the LSB to cultivation. Over the past several years, genome sequences acquired for several species of different sizes and shapes have also yielded clues. One observation from the genomes is that, as predicted by experiments, the LSB seem to vary in their nitrate reduction potential: they may reduce it to nitrogen gas (complete denitrification); nitric or nitrous oxide (incomplete denitrification); and/or ammonium, via DNRA (dissimilatory nitrate reduction to ammonium). After introducing the cast of characters and their environments, I will discuss how this variety may have developed, and how transport in and out of the vacuole may be managed. As so often in bacteria, horizontal gene transfer has likely played a key role.