14.3 Crystals formed through excretion processes 273

The observations that the sizes of magnetite crystals are uniform, corresponding to the size of a single magnetic domain when they grow as a result of biological activity, and that crystals connect to form a single rosary-like chain, indicate that the crystal growth of magnetite proceeds through cooperation with protein, and also that the presence of magnetite is indispensable for biological activity. The commonly observed Habitus and Tracht of magnetite, which deviate from the structural form deduced from the crystal structure only, also suggest that the growth of inorganic crystals proceeds under the control of protein. The size and form of the container (cell) appears to play a role in determining the size and form of magnetite crystals. In particular, the form of the cell is highly likely to determine conical-weight-like form.

14.3Crystals formed through excretion processes

There is no doubt that calculus formed in organs, such as gallstones, kidney stones, and urethra stones, are not formed as indispensable factors in biological activities, unlike teeth, bones, and shells. Instead, these are products of illness, formed by precipitation in the respective organs during the process of excretion of components formed in unnecessary amounts. Clearly, cooperation with protein is not expected in their formation. A wide variety of crystals are known to exist in living bodies due to these processes.

Gout is caused by precipitation, at the knuckle of the big toe, of needle crystals of sodium uric acid, which are formed in blood corpuscles and transported via blood flow. When the crystals precipitate, a polycrystalline aggregate results from the intermingling of the needle crystals, around which radiating growth of the needles occurs, resulting in precipitation of polycrystalline aggregates with a close-cropped chestnut form. The size and form of needle crystals of sodium uric acid, and their spherical precipitates, are not uniformly controlled, and they change as the illness progresses, suggesting that their growth proceeds outside the control of protein.

In earlier times, when dietary habits were poor, gallstones consisted mainly of pigment precipitation, but, in recent years, they mostly consist of cholesterol crystals. Thin platy crystals of cholesterol are formed in the gallbladder, and coagulate to form spherical aggregates. The growth of thin platy crystals of cholesterol was confirmed to be by the spiral growth mechanism [7], [12]. Thin platy crystals formed in the gallbladder move and agglutinate by movement of the environmental field around something that acts as a heterogeneous nucleation site, so forming gallstones. Although hydroxyapatite was suggested as a possible material that could act as the nucleus, this has not yet been confirmed. The sizes and forms of single crystals of cholesterol are uneven. When a small gallstone is discharged

274Crystals formed through biological activity

through the gallbladder tube, the patient experiences a sharp pain, whereas when gallstones grow larger than a certain size in the gallbladder, the stone does not cause pain.

Both kidney stones and urethra stones are spherical polycrystalline aggregates of calcium oxalate. They are not uniform in size and form, although they are referred to as spherical aggregates. It is known that hydroxyapatite, which is the main component of teeth and bone, is sometimes formed in muscle due to disease. In this case, hydroxyapatite crystals form a spherulitic polycrystalline aggregate. It is interesting to note that the same hydroxyapatite crystals in kidney and urethra stones are different in form and size from those that exist in teeth and bones. It seems that crystals formed in living bodies through the excretion of unnecessary components do not show characteristics indicating cooperation with protein in their formation.

14.4 Crystals acting as possible reservoirs for necessary components

Many examples have been reported of organic or inorganic crystals in plant cells. Crystals are formed within cells, on cell walls, or at cell boundaries. Calcium oxalate, weddellite, and whewellite crystals are observed in dahlia and spinach, and needle crystals of inuline are seen in begonias. Crystals are mostly sheaf-like aggregates (Fig. 14.6). Amorphous silica (opal, SiO2 nH2O ) is found in the cells of grasses, and their sizes and forms are variable depending on the species. In addition, stalactic aggregates of calcite, attaining sizes of a few 100 m (vaterite, amorphous CaCO3, tartaric acid, and citric acid) are known to grow in plants.

Judging from the sizes and forms of the crystals and aggregates, crystals formed in plants are mostly poor in uniformity, and thus it is not clear what sort of role these crystals play in the biological life cycle. If these crystals act as a reservoir for essential components of the plant, such as Ca and Si, we may expect to see characteristics showing growth and/or dissolution in crystal morphology. There have been no reports of investigations that might substantiate this claim; this would be a suitable subject for further discussion. Investigations into the decalcification process have been made with respect to hydroxyapatite forming teeth and bones, or polymorphs of CaCO3 constituting shells.

14.5Crystals whose functions are still unknown

Various bacteria, such as spherical bacteria, bacillus, and fibrous bacteria, selectively precipitate inorganic crystals around the surface of the bacterium, though the reasons for this are unclear [13]. Although many studies have been performed on the precipitation of inorganic crystals around bacteria in relation to

14.5 Crystals whose functions are still unknown 275

(a)

(b)

Figure 14.6. Crystals formed in plant cells. (a) Calcium oxalate in begonias. (b) Inuline in

dahlias.

276Crystals formed through biological activity

environmental or ore genesis problems, it is yet not well understood how they affect the bacteria and their existence. In the future, it will be necessary to establish whether the structure of protein on the surface of a bacteria cell acts as a heterogeneous nucleation site for inorganic crystals, or if inorganic crystals play an essential role for bacteria. The bacteria in question are anaerobic, and so they discharge oxide minerals formed by oxidation from the cell.

Many mineral species are known to be selectively crystallized by the presence of bacteria. Carbonate minerals, such as calcite, aragonite, hydroxycalcite, and siderite; oxide minerals, such as magnetite and todorokite; oxalate minerals, such as whewellite and weddellite; sulfide minerals, such as pyrite, sphalerite, wurtzite, greigite, and mackinawite; and other minerals, such as jarosite, iron-jarosite, and gypsum, are known to precipitate in the presence of bacteria. Therefore, investigations have been developed to analyze the formation of banded iron ore by the action of bacteria, and to analyze the ancient environmental conditions of the Earth through the study of fossilized bacteria.

Minerals formed by bacterial activity are formed outside the cell. Since they are so small, very little information has been obtained relating to the characteristics of their morphology, but it seems that they exhibit idiomorphic polyhedral form, although their alignment is not regular.

In this chapter, we have summarized the characteristics of crystals formed by biological activities, based on the morphology of single crystals and polycrystalline aggregates. Various morphologies exhibited by biominerals are well documented, and we have tried to illustrate these as best we can; however, the mechanism by which they are controlled has not been well understood. Based on the understanding of simple systems, using morphology as our key, we may properly understand the problems involved in complicated systems, such as biomineralization. The data are based on those reported in refs. [1]–[8], [12], and [13]. We shall await the judgment of our readers as to whether this approach was appropriate or not.

References

1T. Watabe, Biomineralization, Wonders of Formation of Minerals in Living Things, Tokyo, Tokai University Press, 1997 (in Japanese)

2S. Mann, Biomineralization, Principles and Concepts in Bioinorganic Materials, Oxford, Oxford University Press, 2001

3K. Wada and I. Kobayashi (eds.), Biomineralization and Hard Tissue of Marine Organisms, Tokyo, Tokai University Press, 1996 (in Japanese)

4S. Hilton, Teeth, Cambridge Manuals in Archaeology, Cambridge, Cambridge University Press, 1986

References 277

5M. Iizima and Y. Moriwaki, In vitro study of the formation mechanism of tooth enamel apatite crystals – Effects of organic matrices on crystal growth of octacalcium phosphate (OCP), J. Japan. Assoc. Crystal Growth, 26, 1999, 175–83 (in Japanese with English abstract)

6K. Wada, Science of Pearl – Mechanism of Formation and Method to Distinguish, Tokyo, Shinjyu Shuppan Co. (in Japanese)

7H. Komatsu, Crystallization of gallstones, J. Japan. Assoc. Crystal Growth, 12, 1985, 23–41 (in Japanese with English abstract)

8Y. Shiraiwa, Calcification by photosynthetic organisms and global CO2 environment,

J. Japan Assoc. Crystal Growth, 28, 2001, 53–60 (in Japanese with English abstract)

9R. Westbroek, F. W. de Jong, P. van der Wal, A. H. Borman, J. P. M. de Vrind, and D. Kok, Mechanism of calcification in the marine alga Emiliania huxleyi, Phil. Trans. Roy. Soc. London, B304, 1984, 435–44

10J. M. Didymus, J. R. Young, and S. Mann, Construction and morphogenesis of the chiral ultrastructure of coccoliths from the marine alga Emiliania huxleyi, Proc. Roy. Soc. London,

B258, 1994, 237–45

11J. R. Young, J. M. Didymus, P. R. Bown, B. Prins, and S. Mann, Crystal assembly and phylogenetic evolution in heterococcoliths, Nature, 356, 1992, 516–18

12H. Komatsu, Coincidence site conjugation of cholesterol monohydrate crystals, in Morphology and Growth Unit of Crystals, ed. I. Sunagawa, Tokyo, Terra Scientific Publications, 1989, pp. 761–75

13K. Tazaki, T. Mori, and T. Nonaka, Microbial jarosite and gypsum from corrosion of Portland cement concrete, Can. Min., 30, 1992, 431–44