On the basis of recently gathered data (Dick et al. 2008a, b), the mnxG gene coding
for the MCO has been identified by us in the newly discovered S. domunculaassociated bacterium (Fig. 4.10b). The sponge-associated bacterium shared
the highest sequence similarity only to those bacteria that comprise and express
the MCO. The expression of this gene has been found to depend on the presence of
Mn in the culture medium. During Mn deposition, the bacteria change their
morphology from barrel-like (Fig. 4.12a) to a form which is characterized by
distinct elongated projections that we labeled as spores/or spore-like bacteria
(Fig. 4.12b–d).
At present, we do not yet know if these Mn-depositing bacteria, associated with
S. domuncula, have any relationship to those bacteria, which were suspected to be
involved in polymetallic nodule formation. Therefore, we attribute those bacteria a
crucial role in symbiosis with sponges. The symbiotic coexistence of sponges with
bacteria is well established (Althoff et al. 1998). Most of the sponge species living
in the Mediterranean Sea contain, if they are kept in an aquarium for more than
2 weeks (LePennec et al. 2003), only a few different bacterial strains which are nonabundant and, with regard to S. domuncula, located in specific cells, the
bacteriocytes. This is in contrast to a few species, like Aplysina aerophoba, that
abundantly contain up to 50% of the sponge body mass as microorganisms (Weiss
et al. 1996). From specimens that had been kept in quarantine, the Mn-precipitating
bacteria described here were isolated. Already, this finding suggests that those
bacteria display a crucial role in the physiology/metabolism of the sponge, perhaps
supporting our assumption that the Mn-precipitating bacteria act as a reversible Mn
store in S. domuncula. According to this view, the presence of BAC-SubDo-03
bacteria is required as a protection against higher, toxic concentrations of Mn; after
oxidation of Mn(II) to Mn(IV), the ion becomes insoluble. However, the bacteria
appear to be also essential for maintaining the physiological concentration in the
sponge. Since only minute levels of Mn exist usually in the surrounding seawater, a
substantial accumulation of Mn is proposed here to rise onto the bacteria. The
release of bacterial-precipitated Mn(IV) is postulated to supply the physiologically
needed Mn if needed.
4.5 Mineralization/Biomineralization Processes During
Formation of (Co-rich) Polymetallic Crusts
4.5.1 Deposits
The Co-rich crusts, also termed ferromanganese crusts, are formed at depths
between 800 and 2,400 m (Bau et al. 1996; Hein et al. 2000). They contain high
amounts of Mn oxide (20–30%) and Fe oxide (14–20%) and are rich in Co (>1%),
Cu, Ni, and Pt (Hein et al. 1997; Mills et al. 2001). Crusts are found mainly in the
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X. Wang et al.
for the MCO has been identified by us in the newly discovered S. domunculaassociated bacterium (Fig. 4.10b). The sponge-associated bacterium shared
the highest sequence similarity only to those bacteria that comprise and express
the MCO. The expression of this gene has been found to depend on the presence of
Mn in the culture medium. During Mn deposition, the bacteria change their
morphology from barrel-like (Fig. 4.12a) to a form which is characterized by
distinct elongated projections that we labeled as spores/or spore-like bacteria
(Fig. 4.12b–d).
At present, we do not yet know if these Mn-depositing bacteria, associated with
S. domuncula, have any relationship to those bacteria, which were suspected to be
involved in polymetallic nodule formation. Therefore, we attribute those bacteria a
crucial role in symbiosis with sponges. The symbiotic coexistence of sponges with
bacteria is well established (Althoff et al. 1998). Most of the sponge species living
in the Mediterranean Sea contain, if they are kept in an aquarium for more than
2 weeks (LePennec et al. 2003), only a few different bacterial strains which are nonabundant and, with regard to S. domuncula, located in specific cells, the
bacteriocytes. This is in contrast to a few species, like Aplysina aerophoba, that
abundantly contain up to 50% of the sponge body mass as microorganisms (Weiss
et al. 1996). From specimens that had been kept in quarantine, the Mn-precipitating
bacteria described here were isolated. Already, this finding suggests that those
bacteria display a crucial role in the physiology/metabolism of the sponge, perhaps
supporting our assumption that the Mn-precipitating bacteria act as a reversible Mn
store in S. domuncula. According to this view, the presence of BAC-SubDo-03
bacteria is required as a protection against higher, toxic concentrations of Mn; after
oxidation of Mn(II) to Mn(IV), the ion becomes insoluble. However, the bacteria
appear to be also essential for maintaining the physiological concentration in the
sponge. Since only minute levels of Mn exist usually in the surrounding seawater, a
substantial accumulation of Mn is proposed here to rise onto the bacteria. The
release of bacterial-precipitated Mn(IV) is postulated to supply the physiologically
needed Mn if needed.
4.5 Mineralization/Biomineralization Processes During
Formation of (Co-rich) Polymetallic Crusts
4.5.1 Deposits
The Co-rich crusts, also termed ferromanganese crusts, are formed at depths
between 800 and 2,400 m (Bau et al. 1996; Hein et al. 2000). They contain high
amounts of Mn oxide (20–30%) and Fe oxide (14–20%) and are rich in Co (>1%),
Cu, Ni, and Pt (Hein et al. 1997; Mills et al. 2001). Crusts are found mainly in the
96
X. Wang et al.