et al. 1998); e.g., the Mg
2+ /Ca
2+ ratio and the Ca
2+ concentration in the seawater
seem to influence biocalcification in cyanobacteria (Arp et al. 2001).
Commonly, carbonate precipitates start to build up and develop on the external
surface of bacterial cells by successive stratification, suggesting that the bacterial
cell acts as heterogeneous crystallization center, and bacteria can be embedded and
calcified in growing carbonate crystals (Castanier et al. 1999; Pentecost and Bauld
1988; Rivadeneyra et al. 1998). Although there is agreement that bacteria become
the nucleus of growing crystals (going toward a kind of sacrifice), there is still
some uncertainty about bacterial survival inside and some authors suggest that a
mineralized coat would protect bacteria from environmental injuries and stress,
even if for a short period of time (Zamarren ˜o et al. 2009; Phoenix and Konhauser
2008).
Crystal formation has been studied in natural samples (Castanier et al. 1999) and
under laboratory conditions (Rivadeneyra et al. 1998). Castanier et al. (1999)
studied the development of mineral products by bacterial carbonatogenesis in
eutrophicated karstic originated waters from a natural pool. Solid products were
observed at SEM. The first ones were probably amorphous and hydrated at the
beginning. They appeared on the surface of the bacterial bodies as patches or stripes
that extended and coalesced until forming, in a few steps, a rigid coating as a kind of
cocoon (Fig. 5.1). Solid particles formed inside the cellular body and excreted from
the cell were also noted (excretates; Fig. 5.1). These tiny particles, including more
or less calcified bacterial cells, assembled into biomineral aggregates or buildups
which progressively displayed more crystalline structures. Biomineral assemblage
appeared to turn into true crystals, either well shaped or poorly organized, which
could enclose bacterial cells within the mineral structure (Castanier et al. 1999).
The authors propose that different types of crystallogenetic sequences could follow
the different metabolic pathways present in the natural sample.
Rivadeneyra et al. (1998) studied the sequence of crystal formation in a
liquid culture of the moderately halophilic species Halomonas eurihalina
under culture conditions in which the mineral phases were aragonite (96%)
and magnesium calcite (6%). The sequence of biolith formation basically
comprised three phases: (1) calcified filaments or chains, (2) discs, and (3)
spherulites (Fig. 5.2).
5.3 Bacterial Metabolism and Precipitation
5.3.1 Role of Bacterial Metabolism on Calcium Carbonate
Precipitation
Several authors recognized specific metabolic pathways to be involved in BCCP,
with the pH change of the medium as the main mechanism favoring precipitation.
In general, metabolic pathways able to shift the environmental pH toward alkalinity
120
B. Perito and G. Mastromei
2+ /Ca
2+ ratio and the Ca
2+ concentration in the seawater
seem to influence biocalcification in cyanobacteria (Arp et al. 2001).
Commonly, carbonate precipitates start to build up and develop on the external
surface of bacterial cells by successive stratification, suggesting that the bacterial
cell acts as heterogeneous crystallization center, and bacteria can be embedded and
calcified in growing carbonate crystals (Castanier et al. 1999; Pentecost and Bauld
1988; Rivadeneyra et al. 1998). Although there is agreement that bacteria become
the nucleus of growing crystals (going toward a kind of sacrifice), there is still
some uncertainty about bacterial survival inside and some authors suggest that a
mineralized coat would protect bacteria from environmental injuries and stress,
even if for a short period of time (Zamarren ˜o et al. 2009; Phoenix and Konhauser
2008).
Crystal formation has been studied in natural samples (Castanier et al. 1999) and
under laboratory conditions (Rivadeneyra et al. 1998). Castanier et al. (1999)
studied the development of mineral products by bacterial carbonatogenesis in
eutrophicated karstic originated waters from a natural pool. Solid products were
observed at SEM. The first ones were probably amorphous and hydrated at the
beginning. They appeared on the surface of the bacterial bodies as patches or stripes
that extended and coalesced until forming, in a few steps, a rigid coating as a kind of
cocoon (Fig. 5.1). Solid particles formed inside the cellular body and excreted from
the cell were also noted (excretates; Fig. 5.1). These tiny particles, including more
or less calcified bacterial cells, assembled into biomineral aggregates or buildups
which progressively displayed more crystalline structures. Biomineral assemblage
appeared to turn into true crystals, either well shaped or poorly organized, which
could enclose bacterial cells within the mineral structure (Castanier et al. 1999).
The authors propose that different types of crystallogenetic sequences could follow
the different metabolic pathways present in the natural sample.
Rivadeneyra et al. (1998) studied the sequence of crystal formation in a
liquid culture of the moderately halophilic species Halomonas eurihalina
under culture conditions in which the mineral phases were aragonite (96%)
and magnesium calcite (6%). The sequence of biolith formation basically
comprised three phases: (1) calcified filaments or chains, (2) discs, and (3)
spherulites (Fig. 5.2).
5.3 Bacterial Metabolism and Precipitation
5.3.1 Role of Bacterial Metabolism on Calcium Carbonate
Precipitation
Several authors recognized specific metabolic pathways to be involved in BCCP,
with the pH change of the medium as the main mechanism favoring precipitation.
In general, metabolic pathways able to shift the environmental pH toward alkalinity
120
B. Perito and G. Mastromei