As discussed in Sect. 5.2, some authors proposed a specific active involvement
of bacteria in the CCP process. In particular, both Castanier et al. (1999) and
McConnaughey and Whelan (1997) suggested that “active” precipitation could be
linked to ion transport (specifically Ca
2+ ) across cellular membranes. Castanier
et al. (1999) did not better precise the nature of these ionic exchanges, stating
that they follow still poorly known mechanisms. McConnaughey and Whelan
(1997) stated that calcification creates two types of products, minerals, and protons,
according to (5.14):
Ca
2þ
þ HCO 3
À
! CaCO 3 þ H
þ
(5.14)
While production of calcareous skeletons by mineralization has an evident role
in supporting and protecting the soft parts of many organisms, proton secretion is
less obvious but plays major roles in carbon and nutrient assimilation by plants and
photosynthetic symbioses. They postulated that such uses of calcification account
for much of the massive carbonate accumulation in alkaline environments ranging
from desert soils to coral reefs. The authors hypothesized that also photosynthetic
microorganisms, such as algae and cyanobacteria, use active calcium metabolism
coupled with photosynthetic CCP for the purpose of generating protons to assist the
organisms in nutrient and bicarbonate uptake, according to (5.14) and (5.15).
H
þ
þ HCO 3
À
! CH 2 O þ O 2
(5.15)
In alkaline waters, bicarbonate is the most abundant carbon source, but it should
be inaccessible without a source of protons. By discharging the protons from
calcification into their boundary layers, photosynthetic organisms can maintain or
even elevate CO 2 concentrations despite CO 2 photosynthetic uptake, which gives
to calcification a competitive advantage in light- and nutrient-deficient alkaline
environments. In this view, the authors considered structural and defensive uses for
calcareous skeletons sometimes overrated.
Some experimental support to this hypothesis can be found in the work of Yates
and Robbins (1999), who found that extracellular precipitated calcium originated
from inside the unicellular green alga Nannochloris atomus. The authors discussed
that the incorporation of intracellular calcium into extracellular CC indicated that
calcium expulsion from cells played a significant role in mineralization and raised
the question of how Ca
2+ cycling in cells could affect environments of CCP.
Based on McConnaughey and Whelan (1997), Hammes and Verstraete (2002)
proposed an alternative view on the role of calcium metabolism in BCCP, based on
the hypothesis that alkaline pH stress and subsequent bacterial calcium metabolism
are key points in the precipitation process and on the relevance of precipitation
in regard to the precipitating organism and its microenvironment. This microenvironment is constituted by the thin watery layer surrounding bacteria which forms an
interface between the bacterial cell and the outside environment, where different
concentrations of protons (pH), DIC, and Ca
2+ can prevail. Coupled with the
5 Molecular Basis of Bacterial Calcium Carbonate Precipitation
129
of bacteria in the CCP process. In particular, both Castanier et al. (1999) and
McConnaughey and Whelan (1997) suggested that “active” precipitation could be
linked to ion transport (specifically Ca
2+ ) across cellular membranes. Castanier
et al. (1999) did not better precise the nature of these ionic exchanges, stating
that they follow still poorly known mechanisms. McConnaughey and Whelan
(1997) stated that calcification creates two types of products, minerals, and protons,
according to (5.14):
Ca
2þ
þ HCO 3
À
! CaCO 3 þ H
þ
(5.14)
While production of calcareous skeletons by mineralization has an evident role
in supporting and protecting the soft parts of many organisms, proton secretion is
less obvious but plays major roles in carbon and nutrient assimilation by plants and
photosynthetic symbioses. They postulated that such uses of calcification account
for much of the massive carbonate accumulation in alkaline environments ranging
from desert soils to coral reefs. The authors hypothesized that also photosynthetic
microorganisms, such as algae and cyanobacteria, use active calcium metabolism
coupled with photosynthetic CCP for the purpose of generating protons to assist the
organisms in nutrient and bicarbonate uptake, according to (5.14) and (5.15).
H
þ
þ HCO 3
À
! CH 2 O þ O 2
(5.15)
In alkaline waters, bicarbonate is the most abundant carbon source, but it should
be inaccessible without a source of protons. By discharging the protons from
calcification into their boundary layers, photosynthetic organisms can maintain or
even elevate CO 2 concentrations despite CO 2 photosynthetic uptake, which gives
to calcification a competitive advantage in light- and nutrient-deficient alkaline
environments. In this view, the authors considered structural and defensive uses for
calcareous skeletons sometimes overrated.
Some experimental support to this hypothesis can be found in the work of Yates
and Robbins (1999), who found that extracellular precipitated calcium originated
from inside the unicellular green alga Nannochloris atomus. The authors discussed
that the incorporation of intracellular calcium into extracellular CC indicated that
calcium expulsion from cells played a significant role in mineralization and raised
the question of how Ca
2+ cycling in cells could affect environments of CCP.
Based on McConnaughey and Whelan (1997), Hammes and Verstraete (2002)
proposed an alternative view on the role of calcium metabolism in BCCP, based on
the hypothesis that alkaline pH stress and subsequent bacterial calcium metabolism
are key points in the precipitation process and on the relevance of precipitation
in regard to the precipitating organism and its microenvironment. This microenvironment is constituted by the thin watery layer surrounding bacteria which forms an
interface between the bacterial cell and the outside environment, where different
concentrations of protons (pH), DIC, and Ca
2+ can prevail. Coupled with the
5 Molecular Basis of Bacterial Calcium Carbonate Precipitation
129