Geobiology of Marine Magnetotactic Bacteria

Sheri Simmons, Ph.D., 2006
Katrina Edwards, Advisor

Magnetotactic bacteria (MTB) biomineralize intracellular membrane-bound crystals of magnetite (Fe3O4) or greigite (Fe3S4), and are abundant in the suboxic to anoxic zones of stratified marine environments worldwide. Their population densities and high intracellular iron content suggest a potentially significant role in iron cycling. The MTB community in seasonally stratified Salt Pond (Falmouth, MA) was used as a model system for quantitative studies of MTB population dynamics and geochemistry. Two novel types of MTB were phylogenetically identified, and a quantitative PCR assay targeting four major MTB groups was developed and applied to a time-series of samples from Salt Pond. MTB populations were finely layered around the chemocline, with magnetite-producing populations in the upper part of the chemocline and greigite-producing populations in microsulfidic to sulfidic waters. The “barbell” bacterium, the first MTB observed to swim south in response to high oxygen levels, reached 1-10% of total bacteria. Calculations based on qPCR data suggest that MTB are significant unrecognized contributors to iron flux in stratified environments. Additionally, the many-celled magnetotactic prokaryote consists of several clades with greater than 5% divergence in their 16s rRNA. Observations of MTB with south polarity and non-gradient MTB conflict with current models of the adaptive value of magnetotaxis.