cation and, in terms of their anions, carbonates are among the most abundant forms.
Nevertheless, even if CC mineralization is a widespread process among organisms
from bacteria to Chordata, it is generally accepted that the capacities of prokaryotes
and eukaryotes in mineralization are different (Zavarzin 2002). Eukaryotes, prevalently tissue-forming multicellular eukaryotes, carry out biologically controlled
CC mineralization (Lowenstam and Weiner 1989; Mann 2001). In this case,
cellular activity controls the process to a high degree and directs the nucleation,
growth, morphology, and final location of the mineral (Decho 2010). The mineral
particles formed are synthesized or deposited on or within organic matrices or
vesicles in a specific location with regard to the cell and usually intracellularly.
CCP is often used for specific purposes and leads to the production of complex and
specialized CaCO 3 structures (e.g., formation of protozoan and mollusc shells or
coral reefs) clearly visible. Every organism synthesizes biogenic minerals in a form
that is unique to that species, independently from environmental conditions.
Because of these features, both the synthesis and the form of every specific biogenic
mineral are thought to be under specific metabolic and genetic control (B€ auerlein
2004). A known example of biologically controlled CaCO 3 mineralization by
eukaryotic microorganisms is that of the unicellular algae coccolithophores
(B€ auerlein 2004).
In contrast, biologically induced mineralization is usually carried out in an open
environment and no specialized cell structure or specific molecular mechanism is
thought to be involved. Microbially induced mineralization is a specific type of the
biologically induced mineralization, referring to precipitation that results from
interactions between microbial activities and the environment (Weiner and Dove
2003; Dupraz et al. 2009). With the singular exception of Achromatium oxaliferum,
CC deposition by bacteria has been generally regarded to be induced and the type of
mineral produced largely dependent on environmental conditions (Ben Omar et al.
1997; Brennan et al. 2004; Rivadeneyra et al. 1994). Different mechanisms of
bacterial involvement in calcification have been proposed (Ehrlich 1996). However, the role played by bacteria in calcium mineralization is still debated, ranging
from passive to active. Some authors emphasize that carbonate precipitation
by bacteria is an unwanted by-product of bacterial physiological activities under
special environmental conditions, a simple physiological accident. Bacteria would
not precipitate carbonate particles by a specific mechanism and the supply of a
structure by bacteria would not be necessary (von Knorre and Krumbein 2000).
Other authors emphasize that the role of bacteria in CCP can be specific with
ecological benefits for the precipitating organisms (McConnaughey and Whelan
1997; Castanier et al. 1999; Barabesi et al. 2007). Castanier et al. (1999) distinguish
between passive and active precipitation mechanisms which may occur, often
concurrently, in heterotrophic bacteria. Passive precipitation (or passive
carbonatogenesis) operates by producing carbonate and bicarbonate ions and
inducing chemical modifications in the medium through metabolic pathways
(e.g., linked to nitrogen and sulfur cycles, see Sect. 5.3). In active precipitation
(or active carbonatogenesis), the carbonate particles would be produced by ionic
exchanges through the cell membrane by activation of calcium and/or magnesium
116
B. Perito and G. Mastromei
Nevertheless, even if CC mineralization is a widespread process among organisms
from bacteria to Chordata, it is generally accepted that the capacities of prokaryotes
and eukaryotes in mineralization are different (Zavarzin 2002). Eukaryotes, prevalently tissue-forming multicellular eukaryotes, carry out biologically controlled
CC mineralization (Lowenstam and Weiner 1989; Mann 2001). In this case,
cellular activity controls the process to a high degree and directs the nucleation,
growth, morphology, and final location of the mineral (Decho 2010). The mineral
particles formed are synthesized or deposited on or within organic matrices or
vesicles in a specific location with regard to the cell and usually intracellularly.
CCP is often used for specific purposes and leads to the production of complex and
specialized CaCO 3 structures (e.g., formation of protozoan and mollusc shells or
coral reefs) clearly visible. Every organism synthesizes biogenic minerals in a form
that is unique to that species, independently from environmental conditions.
Because of these features, both the synthesis and the form of every specific biogenic
mineral are thought to be under specific metabolic and genetic control (B€ auerlein
2004). A known example of biologically controlled CaCO 3 mineralization by
eukaryotic microorganisms is that of the unicellular algae coccolithophores
(B€ auerlein 2004).
In contrast, biologically induced mineralization is usually carried out in an open
environment and no specialized cell structure or specific molecular mechanism is
thought to be involved. Microbially induced mineralization is a specific type of the
biologically induced mineralization, referring to precipitation that results from
interactions between microbial activities and the environment (Weiner and Dove
2003; Dupraz et al. 2009). With the singular exception of Achromatium oxaliferum,
CC deposition by bacteria has been generally regarded to be induced and the type of
mineral produced largely dependent on environmental conditions (Ben Omar et al.
1997; Brennan et al. 2004; Rivadeneyra et al. 1994). Different mechanisms of
bacterial involvement in calcification have been proposed (Ehrlich 1996). However, the role played by bacteria in calcium mineralization is still debated, ranging
from passive to active. Some authors emphasize that carbonate precipitation
by bacteria is an unwanted by-product of bacterial physiological activities under
special environmental conditions, a simple physiological accident. Bacteria would
not precipitate carbonate particles by a specific mechanism and the supply of a
structure by bacteria would not be necessary (von Knorre and Krumbein 2000).
Other authors emphasize that the role of bacteria in CCP can be specific with
ecological benefits for the precipitating organisms (McConnaughey and Whelan
1997; Castanier et al. 1999; Barabesi et al. 2007). Castanier et al. (1999) distinguish
between passive and active precipitation mechanisms which may occur, often
concurrently, in heterotrophic bacteria. Passive precipitation (or passive
carbonatogenesis) operates by producing carbonate and bicarbonate ions and
inducing chemical modifications in the medium through metabolic pathways
(e.g., linked to nitrogen and sulfur cycles, see Sect. 5.3). In active precipitation
(or active carbonatogenesis), the carbonate particles would be produced by ionic
exchanges through the cell membrane by activation of calcium and/or magnesium
116
B. Perito and G. Mastromei