The intracellular OH
À is exchanged for extracellular HCO 3
À across the cell
membrane. The now extracellular OH
À generates an alkaline pericellular region
where CO 3
2À is generated from HCO 3
À (reaction 5.6):
HCO 3
À
þ OH
À
! CO 3
2À
þ H 2 O
(5.6)
The resulting CO 3
2À immediately reacts with Ca
2+ at the cell surface to form
CaCO 3 . The calcite deposits of Synechoccus can be in the form of marl sediment
and massive bioherms. Other cyanobacteria lay down CaCO 3 as concretions around
pebbles in streams.
In heterotrophy, Castanier et al. (1999) distinguished two bacterial carbonate
precipitation-enhancing processes which may occur often concurrently: (1) ionic
exchanges through the cell membrane (active); (2) metabolic pathways able to
produce carbonate and bicarbonate ions and induce chemical modifications in the
medium, mainly a pH increase (passive). According to Dupraz et al. (2009), all the
processes derived from the activities of living cells are considered active,
overcoming the previous distinction between active vs. passive processes. Process
(1) is discussed in Sect. 5.3.2. Here we discuss process (2). Metabolic pathways
belonging to the nitrogen and sulfur cycle are involved. In the nitrogen cycle,
bacterial precipitation follows three different pathways: (1) ammonification of
amino acids, (2) dissimilatory reduction of nitrate, (3) degradation of urea or uric
acid. These pathways are more often associated with precipitation in soils and
geological sediments, as well as precipitates linked to the urinary tract. These three
pathways induce production of carbonate and bicarbonate ions and, as a metabolic
end-product, ammonia, which induces a pH increase. When the H
+ concentration
decreases, the carbonate–bicarbonate equilibria are shifted toward the production of
CO 3
2À ions (reaction 5.7). If calcium ions are present, CCP occurs (reaction 5.8).
HCO 3
À
$ CO 3
2À
þ H
þ
(5.7)
Ca
2þ
þ CO 3
2À
$ CaCO 3
(5.8)
Urea or uric acid hydrolysis by the urease enzyme is a simple model and it has
often been used in technological application of BCCP. The reaction takes place
according to the following (5.9) and (5.10) (Wright 1999):
CO(NH 2 Þ 2 þ H 2 O ! CO 2 þ 2NH 3
(5.9)
2NH 3 þ CO 2 þ H 2 O ! 2NH 4
þ
þ CO 3
2À
(5.10)
In the sulfur cycle, the dissimilatory reduction of sulfate carried out by sulfatereducing bacteria (SRB) is recognized to affect CCP. The environment must be
anoxic and rich in organic matter, calcium, and sulfate. Using this pathway, bacteria
produce carbonate, bicarbonate ions and hydrogen sulfide (H 2 S). In presence of Ca
2+
ions, CCP depends on the behavior of the hydrogen sulfide. Removal of the
124
B. Perito and G. Mastromei
À is exchanged for extracellular HCO 3
À across the cell
membrane. The now extracellular OH
À generates an alkaline pericellular region
where CO 3
2À is generated from HCO 3
À (reaction 5.6):
HCO 3
À
þ OH
À
! CO 3
2À
þ H 2 O
(5.6)
The resulting CO 3
2À immediately reacts with Ca
2+ at the cell surface to form
CaCO 3 . The calcite deposits of Synechoccus can be in the form of marl sediment
and massive bioherms. Other cyanobacteria lay down CaCO 3 as concretions around
pebbles in streams.
In heterotrophy, Castanier et al. (1999) distinguished two bacterial carbonate
precipitation-enhancing processes which may occur often concurrently: (1) ionic
exchanges through the cell membrane (active); (2) metabolic pathways able to
produce carbonate and bicarbonate ions and induce chemical modifications in the
medium, mainly a pH increase (passive). According to Dupraz et al. (2009), all the
processes derived from the activities of living cells are considered active,
overcoming the previous distinction between active vs. passive processes. Process
(1) is discussed in Sect. 5.3.2. Here we discuss process (2). Metabolic pathways
belonging to the nitrogen and sulfur cycle are involved. In the nitrogen cycle,
bacterial precipitation follows three different pathways: (1) ammonification of
amino acids, (2) dissimilatory reduction of nitrate, (3) degradation of urea or uric
acid. These pathways are more often associated with precipitation in soils and
geological sediments, as well as precipitates linked to the urinary tract. These three
pathways induce production of carbonate and bicarbonate ions and, as a metabolic
end-product, ammonia, which induces a pH increase. When the H
+ concentration
decreases, the carbonate–bicarbonate equilibria are shifted toward the production of
CO 3
2À ions (reaction 5.7). If calcium ions are present, CCP occurs (reaction 5.8).
HCO 3
À
$ CO 3
2À
þ H
þ
(5.7)
Ca
2þ
þ CO 3
2À
$ CaCO 3
(5.8)
Urea or uric acid hydrolysis by the urease enzyme is a simple model and it has
often been used in technological application of BCCP. The reaction takes place
according to the following (5.9) and (5.10) (Wright 1999):
CO(NH 2 Þ 2 þ H 2 O ! CO 2 þ 2NH 3
(5.9)
2NH 3 þ CO 2 þ H 2 O ! 2NH 4
þ
þ CO 3
2À
(5.10)
In the sulfur cycle, the dissimilatory reduction of sulfate carried out by sulfatereducing bacteria (SRB) is recognized to affect CCP. The environment must be
anoxic and rich in organic matter, calcium, and sulfate. Using this pathway, bacteria
produce carbonate, bicarbonate ions and hydrogen sulfide (H 2 S). In presence of Ca
2+
ions, CCP depends on the behavior of the hydrogen sulfide. Removal of the
124
B. Perito and G. Mastromei