Chemistry Reference
In-Depth Information
magnetic minerals by the magnetotactic bacteria. Alternatively, lateral transfer of a group
or groups of genes responsible for magnetosome synthesis between diverse
microorganisms might also explain these findings.
Biogeochemical significance of the magnetotactic bacteria
This section focuses on aspects of the known physiology of the magnetotactic
bacteria and what they do in natural environments. Physiological experiments in general,
have shown they have great potential in playing significant roles in the biogeochemical
cycling of several key elements in natural aquatic habitats, more specifically at the oxic-
anoxic interface and below. These include nitrogen, sulfur, and carbon as well as iron.
Iron is not specifically mentioned here since it is discussed throughout this paper.
Several species of magnetotactic bacteria are known to facilitate important
transformations of nitrogen compounds. All species tested thus far show acetylene-
reducing activity, an indication they are capable of nitrogen-fixation (Bazylinski and
Blakemore 1983b; Bazylinski et al. 2000; Bazylinski and Frankel 2000b).
Magnetospirillum magnetotacticum
is capable of denitrification, an agriculturally-
important process, in which the organism respires with the fixed nitrogen oxides, nitrate
and/or nitrite, as terminal electron acceptors converting them to the nitrogenous gases,
nitrous oxide and/or dinitrogen (Bazylinski and Blakemore 1983a).
M. magneticum
strain
AMB-1 also respires with nitrate (Matsunaga and Tsujimura 1993) but the products of
nitrate respiration do not seem to have been reported. The marine vibrio, strain MV-1, is
capable of the last step of denitrification, the reduction of nitrous oxide to dinitrogen
(Bazylinski et al. 1988).
Many uncultured cells of magnetotactic bacteria collected from natural sulfidic
environments contain elemental sulfur globules (e.g., Cox et al. 2002) indicating they
oxidize reduced sulfur compounds. Several marine strains including MV-1 and MV-2,
the coccus MC-1, and the marine spirillum MV-4, have been shown to be capable of
lithotrophic growth on reduced sulfur compounds in the laboratory (Bazylinski and
Frankel 2000b).
Desulfovibrio magneticus
strain RS-1 is a sulfate-reducing bacterium
that grows anaerobically, respiring with sulfate and producing sulfide (Sakaguchi et al.
1993a). It is noteworthy that this organism produces iron-sulfide minerals via BIM when
grown on sulfate as the terminal electron acceptor (Sakaguchi et al. 1993a).
Magnetotactic bacteria are thus involved both in the oxidative and reductive parts of the
sulfur cycle.
When strains MV-1, MV-2, MC-1, and MV-1 are grown lithotrophically on reduced
sulfur compounds, they are also able to fix carbon dioxide as their major carbon source
and are therefore capable of autotrophic growth. Thus these organisms are
chemolithoautotrophs and can be considered as primary producers based on
chemosynthesis (not photosynthesis). Strains MV-1 and MV-2 use the Calvin-Benson-
Bassham pathway for autotrophy as do plants (Bazylinski et al. 2000b). Autotrophic
pathways have not been determined in the other strains.
THE MAGNETOSOME
Composition of the magnetosome mineral phase
Magnetotactic bacteria generally biomineralize either iron-oxide or iron-sulfide
magnetosomes. The iron-oxide type contains crystals of magnetite (Fe
3
O
4
) (e.g., Frankel et
al. 1979) whereas the iron-sulfide type contains crystals of greigite (Fe
3
S
4
) (Mann et al.
1990; Heywood et al. 1990). Several other iron-sulfide minerals have been identified in
iron-sulfide magnetosomes, including mackinawite (tetragonal FeS) and a cubic FeS, which
