Chemistry Reference
In-Depth Information
the OATZ is found, or is seasonally located, in the water column. The Pettaquamscutt
Estuary (Narragansett Bay, RI, USA) (Bazylinski et al. 1995) and Salt Pond (Woods
Hole, MA, USA) (Wakeham et al. 1984, 1987) are typical examples of this situation.
Hydrogen sulfide, produced by sulfate-reducing bacteria in the anaerobic zone and
sediment, diffuses upward while oxygen diffuses downward from the surface resulting in
a double, vertical, chemical concentration gradient with a co-existing redox gradient.
Strong pycnoclines and other physical factors, probably including the microorganisms
themselves, stabilize the vertical chemical gradients and the resulting OATZ.
Several morphotypes of iron-oxide and iron-sulfide magnetotactic bacteria are found
at sites like the Pettaquamscutt Estuary and Salt Pond. The magnetite-producing
magnetotactic bacteria are generally present at the OATZ proper and appear to behave as
microaerophiles. Experiments with pure cultures support this observation. Two strains of
magnetotactic bacteria have been isolated from the Pettaquamscutt and are now in pure
culture. One is a vibrio designated strain MV-2 (Meldrum et al. 1993b; Dean and
Bazylinski 1999) and the other is a coccus designated strain MC-1 (DeLong et al. 1993;
Meldrum et al. 1993a; Frankel et al. 1997; Dean and Bazylinski 1999). Both strains grow
as microaerophiles although strain MV-2 can also grow anaerobically with nitrous oxide
(N
2
O) as a terminal electron acceptor. Other cultured magnetotactic bacterial strains,
including spirilla (e.g., Blakemore et al. 1979; Schleifer et al. 1991; Schüler et al. 1999)
and rods (Sakaguchi et al. 1993a), are microaerophiles or anaerobes or both. The greigite-
producers appear to be strict anaerobes positioned in the more sulfidic waters just below
the OATZ where oxygen is barely or not detectable (Bazylinski et al. 1990, 1992;
Bazylinski 1995). To date, no greigite-producing magnetotactic bacterium has been
grown in pure culture.
Phylogeny of the magnetotactic bacteria
Because magnetite particles resembling those of magnetotactic bacteria in ancient
and recent sediments have been interpreted as magnetofossils (discussed later in the
chapter), it is important to understand the phylogeny of the magnetotactic bacteria.
Phylogenetic analysis, based on the sequences of 16S ribosomal RNA genes of many
cultured and uncultured magnetotactic bacteria, initially showed that the magnetite-
producing strains are associated with the
-subdivision of the Proteobacteria (Burgess et
al. 1993; DeLong et al. 1993; Eden et al. 1991; Schleifer et al. 1991; Spring and Schleifer
1995; Spring et al. 1992, 1994, 1998), a vast assemblage of Gram-negative prokaryotes in
the Domain Bacteria (Woese 1987), while an uncultured multicellular, greigite-producing
bacterium was found to be associated with the sulfate-reducing bacteria in the
α
-
subdivision of the Proteobacteria (DeLong et al. 1993). Since the different subdivisions
of the Proteobacteria are considered to be coherent, distinct evolutionary lines of descent
(Woese 1987; Zavarzin et al. 1991), DeLong et al. (1993) proposed that the evolutionary
origin of magnetotaxis was polyphyletic and that magnetotaxis based on iron oxide
production evolved separately from that based on iron sulfide production. However
,
recent studies have shown that not all magnetotactic bacteria with magnetite-containing
magnetosomes are associated with the
δ
-subgroup of the Proteobacteria. The cultured
,
sulfate-reducing, magnetotactic bacterium,
Desulfovibrio magneticus
strain RS-1
(Sakaguchi et al. 1993a)
,
has magnetite containing magnetosomes, yet is a member of the
δ
α
-subgroup of the Proteobacteria while an uncultured magnetotactic bacterium also with
magnetite containing magnetosomes,
Magnetobacterium bavaricum
(Spring et al. 1993),
is phylogenetically placed in the Domain Bacteria in the newly formed Nitrospira group
and not with the Proteobacteria (Spring and Bazylinski 2000). These results suggest that
magnetotaxis as a trait may have evolved several times and
,
moreover
,
may indicate that
there is more than one biochemical-chemical pathway for the biomineralization of
