SOURCES OF RAW MATERIALS FOR BIOMINERALIZATION: External Environment and/or Extra cellular Body Liquids as Sources of Ions.
Ion transport through the layers of epithelium can occur by one of two pathways: paracellular by difusion/flux of sea water or transcellular by active or facilitated transport (or both).
The first step involves the sequester and concentration of ions to be nucleated. In marine organisms, water in their surrounding environment is probably the most easily accessed source of ions. The ionic composition of seawater and marine organism body fluids are quite similar, with many marine organisms being, as a rule, in ionic equilibrium with the surrounding environment. Food is an important source of ions for freshwater organisms and those of land environments. The sequester mechanism that involves the drinking of water from the environment appears more likely for marine life than for freshwater or land organisms. Pinocytosis of seawater acts to form intercellular vacuoles rich in calcium and magnesium acting as precursors for intracellular mineral nucleation.
When present in water, the ions Ca2+ and CO32- spontaneously aggregate into stable clusters of amorphous calcium carbonate (ACC). This process also occurs in subsaturated solutions (by way of alterations in pH).
The ion absorption process by cells is only partially understood in most mineralizing organisms (e.g. coral, coccolithophore, echinoderms and foraminiferas). The most accepted theories about ion adsorption for biomineralization involves ion transporters and channels located in the membranes that would permit the entrance of ions into the cell, that would follow a path, frequently by way of endoplasmatic reticulum, to the point of mineralization.
Figure 1.14: – Representation of the three principle mechanisms of cellular ion adsorption. a) Transporters: specialized structures located in the cellular membrane. Coupled ions or molecules (1) alters the configuration of the transporter (generally a protein) that pulls the coupled load into the intracellular space (2). b) Ionic channels: proteins inserted in the membrane that act as specific channels, facilitating the entrance (or exit) of ions in accordance with the concentration gradient. c) Vesicles: by way of altered membranes (1), the cell can englobe and transport various particles (ions, molecules, proteins or other complex structures – like viruses or other cells) or liquid (2).
In the case of Ca2+, the ions flow freely in the cellular cytoplasm through special channels in the direction of the concentration gradient, but are actively pumped out of the system to the mineralization location by pumps and switches. An alternate route was observed in foraminiferas. In foraminiferas, large vesicles containing sea water have a pH of approximately 9, which is high, in relation to the surrounding intracellular environment pH.
According to observations1 in vitro, clusters observed in calcium carbonate solutions with a pH of 9 grew spontaneously around 8nm; this increase in pH in the vesicles can also induce the formation of large metastable clusters of calcium carbonate.
The extracted fluid from the extrapallial space of mollusks presented a large number of hemócitos3 , normally present in celomic fluid. Some of these cells contained refractory granules, and when observed in MEV they showed exocitose of calcite crystals. It was then proposed that hemocytes, after transepithelial migration by way of the mantle, act as a mineral source, that are deposited in the form of small granules in the shell matrix, subsequently remodeled.
Figure 1.15: Larval shell of Limnoperna fortunei .
Figure 1.16: Iron Hydroxide Deposits – Fe(OH)3 – in bacterial mucilage. Between various species, the Gallionella ferruginea is one of the microorganisms responsible for the production of ferruginous mud.
