Submitted in partial fulfillment of the requirements of the degree of Doctor of Philosophy in Biological Oceanography
at the Massachusetts Institute of Technology
and the Woods Hole Oceanographic Institution
June 2006Magnetotactic 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 (up
to 105 cells ml−1) and high intracellular iron content suggest a potentially significant role in iron cycling, but very little is known about their population dynamics and regulation by environmental geochemistry.
The MTB community in Salt Pond (Falmouth, MA), a small stratified marine
basin, was used as a model system for quantitative community studies. Magnetiteproducing MTB predominate slightly above the oxic-anoxic interface and greigiteproducing MTB predominate in sulfidic waters. A quantitative PCR (QPCR) assay
was developed and applied to enumerate four major groups of MTB in Salt Pond:
magnetite-producing cocci, barbells, the greigite-producing many-celled magnetotactic
prokaryote (MMP), and a greigite-producing rod. The barbells were identified as δ-Proteobacteria while the rod was identified as the first MTB in the γ-Proteobacteria.
The MMP, previously thought to be a single species, consists of at least five clades
with greater than 5% divergence in their 16s rRNA. Fluorescent in situ hybridization
probes showed significant variation in clade abundances across a seasonal cycle in salt
marsh productivity. FISH also showed that aggregates consist of genetically identical
cells.
QPCR data indicated that populations are finely layered around the oxic-anoxic
interface: cocci immediately above the dissolved Fe(II) peak, barbells immediately
below, the MMP in microsulfidic waters, and the greigite-producing rod in low numbers
(100 cells ml−1) below the gradient region. The barbell reached 1-10% of total
eubacteria in the late season, and abundances of cocci and barbells appeared to vary
inversely. Calculations based on qPCR data suggest that MTB are significant unrecognized
contributors to iron flux in stratified environments.
Barbells can respond to high oxygen levels by swimming toward geomagnetic south, the opposite of all previously reported magnetotactic behavior. This behavior
is at least partially dependent on environmental oxidation-reduction potential. The co-existence of MTB with opposing polarities in the same redox environment conflicts with current models of the adaptive value of magnetotaxis.Funding for the research described
in this thesis was provided by the Rinehart Coastal Research Center at WHOI, the
WHOI Ocean Life Institute, and the WHOI Ocean Venture Fund