ratios and the hyaline test has much lower ratios of Mg/Ca (Vogel and Uthicke
2012). Miliolids precipitate calcite in the form of 2–3 µm needles within cytoplasmic vesicles (Berthold 1976). These needles are accumulated within the cell
and then they form a new chamber after simultaneous transport outside the test and
assembly within the organic matrix (Angell 1980). The outer layer of the wall is
arranged in dense rows of needles, to what gives a porcelaneous structure of the test
surface (de Nooijer et al. 2009).
Foraminiferal calcification is preceded by extraction of calcium and bicarbonate
ions from sea water. Hyaline species tend to store calcium and carbonate in separate
intracellular organelles. During calcification Foraminifera build their new calcite
chamber over their previous shell (Bentov et al. 2009). Chamber formation starts
with the production of a primary organic sheet (POS) (de Nooijer et al. 2009).
During formation of the POS cytoplasm with a raised pH (≥9.0) is transported to
the site of calcification. Vesicles with high pH are formed mainly in the penultimate
chamber and transported through the ultimate chamber to its aperture where calcification occurs. Most probably protons are pumped out from the vesicles and
stored in a specialised cytosolic compartment with low pH (≤6.0). Throughout
chamber formation vesicles with elevated pH are continuously transported to the
calcification site until chamber formation is complete (de Nooijer et al. 2009).
Miliolids also need pH ≈ 9.0 for calcification, but their high pH vesicles are
conveyed around in the cytoplasm relatively fast and in a seemingly undirected
manner. However, the vesicles containing calcitic needles have considerably lower
pH (7.5–8.0). Elevating the pH is a widespread strategy to promote calcite precipitation in Foraminifera: it overcomes the inhibition by Mg
2+ of calcite precipitation that prevents spontaneous crystal nucleation and growth in modern seawater
Mg/Ca ratios (Zeebe and Sanyal 2002), and it also promotes the conversion of
bicarbonate into carbonates (Zeebe and Wolf-Gladrow 2001).
Most models of biomineralisation assume involvement of membrane ion transporters (channels and pumps) for delivery of Ca
2+ and other ions to the calcification
site. However, Bentov et al. (2009) observed another mechanism in the shallow
water, benthic foraminiferan Amphistegina lobifera (hyaline), in which transport of
vacuoles with sea water via fluid phase endocytosis may account for most of the
calcium and other ions. Initially, vacuoles are semi open to external sea water and
filling of the vacuoles may be mediated by narrow, tubular channels. During intracellular endocytosis sea water vacuoles undergo alkalisation and this further
enhances their calcifying potential. The alkalinisation of the vacuoles suggests that
the supply of CO 3
2− for calcification is also mediated by the sea water vacuoles
(Ferguson et al. 2008). The massive calcium transport through the cytosol would
require a large expenditure of energy. In addition, because of the lower cytoplasm pH
(7.2–7.5), direct transport through the cytoplasm may hinder the maintenance of the
high pH needed for calcification. Vacuolar transport obviously reduces these problems, and can bring Ca
2+ -enriched solution to the calcification site bypassing the
cytoplasm (Bentov et al. 2009). De Nooijer et al. (2008, 2009) recorded that the
phenomenon of alkaline vacuole (vesicles) is general, encompassing both hyaline and
miliolid species. Thus, sea water seems to be a calcifying solution, in agreement with
The Adaptations of the Foraminifera and Ostracoda …
105
2012). Miliolids precipitate calcite in the form of 2–3 µm needles within cytoplasmic vesicles (Berthold 1976). These needles are accumulated within the cell
and then they form a new chamber after simultaneous transport outside the test and
assembly within the organic matrix (Angell 1980). The outer layer of the wall is
arranged in dense rows of needles, to what gives a porcelaneous structure of the test
surface (de Nooijer et al. 2009).
Foraminiferal calcification is preceded by extraction of calcium and bicarbonate
ions from sea water. Hyaline species tend to store calcium and carbonate in separate
intracellular organelles. During calcification Foraminifera build their new calcite
chamber over their previous shell (Bentov et al. 2009). Chamber formation starts
with the production of a primary organic sheet (POS) (de Nooijer et al. 2009).
During formation of the POS cytoplasm with a raised pH (≥9.0) is transported to
the site of calcification. Vesicles with high pH are formed mainly in the penultimate
chamber and transported through the ultimate chamber to its aperture where calcification occurs. Most probably protons are pumped out from the vesicles and
stored in a specialised cytosolic compartment with low pH (≤6.0). Throughout
chamber formation vesicles with elevated pH are continuously transported to the
calcification site until chamber formation is complete (de Nooijer et al. 2009).
Miliolids also need pH ≈ 9.0 for calcification, but their high pH vesicles are
conveyed around in the cytoplasm relatively fast and in a seemingly undirected
manner. However, the vesicles containing calcitic needles have considerably lower
pH (7.5–8.0). Elevating the pH is a widespread strategy to promote calcite precipitation in Foraminifera: it overcomes the inhibition by Mg
2+ of calcite precipitation that prevents spontaneous crystal nucleation and growth in modern seawater
Mg/Ca ratios (Zeebe and Sanyal 2002), and it also promotes the conversion of
bicarbonate into carbonates (Zeebe and Wolf-Gladrow 2001).
Most models of biomineralisation assume involvement of membrane ion transporters (channels and pumps) for delivery of Ca
2+ and other ions to the calcification
site. However, Bentov et al. (2009) observed another mechanism in the shallow
water, benthic foraminiferan Amphistegina lobifera (hyaline), in which transport of
vacuoles with sea water via fluid phase endocytosis may account for most of the
calcium and other ions. Initially, vacuoles are semi open to external sea water and
filling of the vacuoles may be mediated by narrow, tubular channels. During intracellular endocytosis sea water vacuoles undergo alkalisation and this further
enhances their calcifying potential. The alkalinisation of the vacuoles suggests that
the supply of CO 3
2− for calcification is also mediated by the sea water vacuoles
(Ferguson et al. 2008). The massive calcium transport through the cytosol would
require a large expenditure of energy. In addition, because of the lower cytoplasm pH
(7.2–7.5), direct transport through the cytoplasm may hinder the maintenance of the
high pH needed for calcification. Vacuolar transport obviously reduces these problems, and can bring Ca
2+ -enriched solution to the calcification site bypassing the
cytoplasm (Bentov et al. 2009). De Nooijer et al. (2008, 2009) recorded that the
phenomenon of alkaline vacuole (vesicles) is general, encompassing both hyaline and
miliolid species. Thus, sea water seems to be a calcifying solution, in agreement with
The Adaptations of the Foraminifera and Ostracoda …
105