CHAPTER 7
Silica in Biology
INTRODUCTION
Since the survey by ller in 1955 ( I ) of silica in living organisms a brief survey of the
relation of silicon to life has been written by Hunter and Aberg ( 2 ) . Also there have
appeared a short monograph by Mohn ( 3 ) and a book by Voronkov, Zelchan, and
Lukevitz (4a). Mohn summarized mainly the literature of the last quarter-century,
including a brief review of silica chemistry as related to biology and extensive experiments on the uptake of silica by rats when various forms of it were added to the diet.
Voronkov and associates presented an exhaustive survey of the occurrence of silica
in nature, its possible role in the origin of life, distribution in all types of living
organisms, toxicity. and therapeutic uses of new organic derivatives of silica and
silicon. accompanied by more than SO00 references.
A summary of the literature of silica biogeochemistry was presented by Leo and
Barghoorn (4b), who discussed the cyclical movements of silica, including passage
through the biosphere. The biochemistry of silicon was reviewed at the 40th Nobel
Symposium in 1977 and published in 1978. (see Ref. 127)
In this chapter some of these aspects a r e reviewed briefly. Attention i s
concentrated on the chemistry of soluble and colloidal silica in relation to interaction
with biochemicals and biocolloids.
In many biological studies, data are given in terms of "silicon'' rather than
"silica." Since there is little evidence that silicon occurs in any biosystem in any
form other than in coordination with oxygen, data are referred to here in terms of
"silica" or SiO,.
ORIGIN OF LIFE
Even though silicon is one of the most abundant elements, it has been considered to
be nonessential i n most living organisms. whereas carbon, which is far less plentiful,
is the primary element upon which all life depends.
However, it has been suggested that compounds of silicon originally may have
played an important. perhaps necessary, part in the origin of life. As pointed out by
For an excellent survey a r e G . Bend7 and I Lindq\I\t. E d \ . . Bnlchcnll+ir). cit'Silicon dnd Rclatcd
Problems. Nobel Foundation Syniposiuni 40. Plcnuiii Prc\\. N C HYorL. London. I97X
730
Earliest Life Forms
73 1
Gamow ( 5 ) , the transition between nonliving and living matter may have been very
gradual. Oparin (6) has postulated that life first began through the association of
simple, naturally occurring carbon compounds with inorganic colloids. Bernal (7)
has speculated on the possible role that colloidal silicates played in catalyzing the
formation of complex organic molecules from simple ones. He presumes that the
original atmosphere prior to the appearance of life must have consisted of hydrides
such as methane, ammonia, hydrogen sulfide, and water vapor. As shown by Miller
(8) amino acids can be formed from methane, nitrogen, and water vapor under the
influence of electrical discharges, so that a wide variety of organic compounds may
have been present in the ancient seas. Bernal suggests that a concentration of simple
organic molecules might have been brought about by adsorption on colloidal clays,
which have an enormous surface area and an affinity for organic matter. He points
out that small molecules attached to the surface of clay are not held at random, but
are in definite positions relative not only to the clay but to each other, and are thus
held in a position so that they can interact to form more complex compounds, especially if energy is supplied in the form of light. The formation of asymmetric
molecules which are characteristic of compounds occurring in living organisms
might have first occurred, according to Bernal, through preferential adsorption of a
pair of asymmetric molecules on the surface of quartz, which is the only common
mineral possessing an asymmetric structure.
It must be recognized that, even though complex organic molecules might have
been formed originally through the agency of such inorganic catalysts, this was only
the first step out of hundreds or thousands of reactions which must have occurred
subsequently in order to produce the first true ‘‘living’’ organism, that is, one having
the power to reproduce itself. To say that this first step was the “origin of life” is
like saying that the first removal of a piece of iron ore from the ground was the
origin of the automobile.
The role of silica in prebiotic evolution was reviewed by Janet (9), and its role in
the evolution of life has been further considered by Sedlak (10) and Vysotskii,
Danilov, and Strelko ( 1 1). If the colloidal silicates first furnished a molecular pattern in the origin of life, the pattern was no longer needed once a highly ordered
‘‘living’’ arrangement of organic matter was established. However, if the hypothesis
is correct, the crystal pattern of the silicate should have left some imprint upon the
structure of living matter. For example, there should be some relationship between
the molecular structure of biological materials such as proteins and the atomic spacings characteristic of the surfaces of colloidal silicates such as bentonite,
paligorskite, or kaolinite.
Voronkov, Zelchan, and Lukevits (4a) have summarized all the beliefs and
hypotheses of the last 2000 years on this subject, including the possibilities of other
life forms in the universe based on Si-Si, Si-N, or Si-C polymers.
EARLIEST LIFE FORMS
By far the oldest fossil remains of living organisms are the blue-green algae embedded in chert (microcrystalline silica), discovered by Barghoorn and Tyler (12) and
Silica in Biology
132
Figure 7.1. Colloidal silica adsorbed on an alga membrane.
further studied by many others ( I 3-17). These microscopic life forms were obviously
closely associated with soluble and colloidal silica since they must originally have
been embedded in silica gel. These organisms lived 3.5 billion years ago, probably
only I billion years after the earth was formed and long before the Metazoan life
forms proliferated 0.6 billion years ago. It was during the intervening 3 billion years
that the blue-green algae are thought to have converted the original reducing
atmosphere to an oxidizing one by producing oxygen by photosynthesis. I t is now
thought that photolysis of water in the upper atmosphere, with loss of hydrogen into
space, could not have provided much of the oxygen during this Precambrian period
( 18).
There is as yet no evidence that silica is necessary to the metabolism of blue-green
algae, but at least one type observed by Iler grows readily in concentrated 30% sols
of colloidal silica. Well washed cell membranes are found to have absorbed colloidal
silica as shown in Figure 7. I
I
Association with Primitive Organisms
733
Oehler and Schopf have experimented with fossilizing algae in silica gel (19).
Algae were suspended in concentrated colloidal silica, gelled, and autoclaved at sufficiently high temperature to convert the gel to microcrystalline quartz. The result
was a chert-like mass containing embedded algae similar to the ancient fossils.
BIOLOGICAL DISINTEGRATION OF ROCKS
Although most of the secondary minerals such as the clays can be formed from the
primary silicate rocks by means of purely inorganic reactions in the presence of
water, this weathering process may nevertheless be catalyzed by organic agents.
Jacks (20) has reviewed the work of a group of Russian scientists, who believe that
the weathering of rocks may in many cases involve biological attack. Polynov (21)
believed that many of the unstable minerals now found on the surface of the earth
would have long ago disappeared if they were not being continuously synthesized by
living organisms. Aidinyan (22) reported that on rocks on which lichens were growing there was a colloidal mineral weathering product having a S O , : R,O,(iron and
aluminum oxides) ratio identical t o that in the ash of the lichen, indicating that the
colloidal mineral was of biological origin. Glazovskaya (23) concluded that algae
and diatoms were powerful weathering agents and produced amorphous silica and
synthesized aluminosilicates such as beidellite and montmorillonite. Yarilova (24)
found that lichens excreted acids which ate into solid rock and could split plagioclase
crystals into smaller particles. Some of the clay minerals of the nontronite-beidellite
type appeared to be synthesized in the tissues of the vegetation. Bolyshev ( 2 5 )
believed that blue-green algae decomposed soil minerals and brought silica and alumina into solution and that the silica was thus made available for utilization by
certain diatoms which accompanied the algae. Aleksandrov and Zak (26) isolated a
bacillus ( B . siliceus) which decomposed insoluble, potash-containing aluminosilicates
and made potassium available to plants; inoculation of soil fertilized with nitrogen
and phosphorus but low in soluble potash increased grain yields (wheat, maize) by
50-100%. In laboratory studies, Oberlies a n d Pohlmann (27) found that polished
feldspar specimens were attacked by various bacteria as in corresponding studies on
glass (28). A variety of microbes and minerals were included in extensive studies by
Kutuzova (29). who found the p H of the media was reduced in some cases to pH 2,
decomposing aluminosilicates. Even quartz released SiO, to Sarcina. The various
types of bacteria that attack silicates have been reviewed by Voronkov, Zelchan. and
Lukevits (4a).
ASSOCIATION WITH PRIMITIVE ORGANISMS
Whether or not microorganisms contain silica or have silica adsorbed on the exterior
as in Figure 7.1 is difficult to determine by chemical analysis. Separation of the
cellular organisms from contaminants such as colloidal clays is also a problem which
734
Silica in Biology
casts doubt on the significance of the reported silica content of the "ash," especially
in many earlier studies before the electron microscope became available.
Viruses
Silica is reported to be an essential component even in a virus. Faust and Adams
(30) isolated a crystalline virus consisting of polyhedral particles from lepidopterous
larvae (Bombyx mori, etc.) and found that it contained silicon, corresponding to
0,2-0.6% S O , . as an integral part of the protein matrix.
Bacteria
With certain soil bacteria, the uptake of silicon as soluble silica in a culture medium
is followed by the excretion of phosphorus. Factors that accelerated and inhibited
this exchange were studied by Heinen (31). In the absence of glucose, silicon was lost
in the presence of excess phosphate. Particulate fractions isolated from the bacterial
membranes were involved in the metabolism of silicon (32). Many more details were
given by Heinen in 14 papers between 1960 and 1967.
The essential role of silica i n the metabolism of certain bacteria and the interaction of bacteria and silica gels and minerals have been extensively investigated, especially in Russia, and have been summarized by Voronkov, Zelchan. and Lukevits
(4a).
Fungi and Lichens
The mere fact that fungi absorb silica when soluble silicates are added to the culture
may prove only that the resulting colloidal silica is adsorbed on the surface of the
cells. However, the fact that in the absence of phosphorous oxygen accelerates the
uptake of silicon suggests that silicon may play a role in the metabolism (4a).
Lichens, a symbiotic combination of fungi and algae, have probably existed from
earliest times and, as previously mentioned, are probably responsible for much of
the conversion of rock to soil. The algae are photosynthetic and supply energy in the
form of carbohydrates while the fungi attack the rock supplying mineral nutrients.
I n view of the highly entangled structures and close association with the mineral silicates, chemical analyses for silica content must be viewed with suspicion, but ash
contents of I0-20% silica are reported. Reports of quartz being attacked by lichen
could not be confirmed (4a).
Algae and Diatoms
Out of the thousands of types of algae, one group, the diatoms, constitute the class
Diatomaceae or Bacillariophyceae, which absorb soluble silica from water even at
Association with Primitive Organisms
735
extremely low concentrations and metabolize and deposit it as an external skeleton.
According to Calvert (33) there are more than 10,000 varieties of diatoms, some living in fresh water, some in salt water. Nearly all varieties are alike, in that their
walls are impregnated with silica. These plants are single-celled organisms consisting
of two parts with the edge of one part fitting inside the other like the two halves of a
pillbox. In addition to secreting the siliceous skeleton, each cell accumulates a
droplet of oil, and it is suspected that this oil, together with other hydrocarbons
formed by the decomposition of the organic matter of the diatom, might have been
responsible for the formation of a great part of our petroleum resources. Deposits of
diatomaceous mud as large as 400 miles long and 100 miles wide have been found
off the coast of Africa. The organic shales from which petroleum is obtained are
believed to have been formed from the dead bodies of these organisms which were
deposited on the bottom of ancient oceans. Photographs of some of the beautiful
forms of the microscopic skeletons of diatoms are shown by Calvert.
Seawater contains only 2-14 ppm of SiO, and is far below saturation with respect
to amorphous silica (34). Part of this is suspended fragments of siliceous organisms
(35). Siever has pointed out that the major mechanism for the precipitation of silica
on the surface of the earth is biochemical (36). The various organisms responsible
for silica deposition were reviewed by Voronkov, Zelchan, and Lukevits (4a). The
microcrystalline quartz minerals jasper and chalcedony are probably transformation
products of very early diatomite deposits. About two-thirds of the deposited silica is
from diatoms and the remainder from radiolaria and sponges.
A certain minimum concentration of silica in solution is essential to the growth of
each kind of diatom. Increasing silica content from 3.5 to 8.3 ppm doubles the rate
of growth of one type of diatoms of which the dry weight of the cells is 4-22% SiO,
(37). However, some species which contain only 0.4% SiO, can obtain enough silica
for growth from ordinary glassware. Lewin found that colloidal silica will not support growth until it depolymerizes to soluble silica. Navicula pelliculosa requires 35
ppm for maximum growth rate (38). Growth rate in seawater falls off as the silica
content is reduced by overpopulation (diatom bloom) (39). Diatoms can reduce silica
concentration in water down to less than 0.08 ppm, and when cell concentration is
high, an inhibitor is given off which retards silica uptake (40). The half-saturation of
silica uptake varies with different species, ranging from 0.05 to 0.2 ppm SiOl (41a).
Apparently below a concentration of about 0.06 ppm, SO,is not available to these
organisms (41b). When silicon becomes depleted, cells become coated with a
gelatinous capsule of polyuronide of glucuronic residues (42).
A monograph by Lewin and Lewin (43a) summarized what was known of the subject in 1962, and Voronkov, Zelchan, and Lukevits (4a) listed references up to 1975.
A monograph on the biochemistry and physiology of diatoms (43b) appeared in
1977.
The siliceous skeleton of diatoms is a marvel of complex design extending to
molecular dimensions. Electron micrographs showing the fine structure of some
diatoms are shown in Figure 7.2. It will be noted that the geometrical regularity of
the structure is characteristic not only of the larger portion of the skeleton, which
can be observed by the optical microscope, but is continued down to the smallest
736
Silica in Biology
Figurz 7.2. Electron micrograph of the silica skeleton o f a diatom. (Iler ( I ) , by permission of
Cornell University Press.]
units visible a t a magnification of 100,000~.The silica is tightly enclosed in organic
material and is not exposed directly to the surrounding water. The silica is thus
obviously deposited from within the tissues in well defined patterns (44). A review of
the silicification process has been prepared by Darley (45). The skeleton is a
microporous silica gel that exhibits ion-selective properties. T h e properties of
the silica isolated from freshly killed cells by dissolution of organic matter in 70%
H N O , are very different from those of diatomaceous earth which has undergone
densification and even crystallization on a submicroscopic scale. The freshly isolated
silica was observed by Iler to have a specific surface area, by nitrogen adsorption, of
more than 100 m2 g - l and if not dried, has further microporosity accounting for the
selective ion-exchange properties. The area of diatomaceous earthAhousands of
years old is much lower since the silica has become microcrystalline (46a).
Electron micrographs a t very high magnification by Pankratz reveal two kinds of
structures in acid-cleaned silica from a radiolarian. According to Hurd (46b), who
prepared the specimens, Figure 7 . 3 shows silica as 500 8, thick lamina separated by
open channels. The darker silica appears to consist of aggregated ultimate particles
about 200 A in diameter. This would correspond to a specific surface area of 140 rn2
g-I. Figure 7.4 shows an apparently continuous matrix of silica (darker material)
full of round pores or holes 20-500 8, in diameter.
The question is whether in the livirlg cell the open spaces in the silica are occupied
by living tissues or only an aqueous phase. The mechanism by which silica is
deposited in predetermined form is unknown. Observation of the early stages of frustule formation in a diatom have been described by Dawson (46c).
Association with Primitive Organisms
737
A peculiar dark green alga, found by Her growing in concentrated colloidal silica
at p H 9-10, was described by Kingsbury (47) a s being unusually small; the cells were
around 1 micron wide and 2 microns long. Iler has observed algae of this type in
which the cells are enclosed in a tubular casing or skin, from which fibrils extended
out all around to a total width of 3 microns. A section of the sheath, bearing
adsorbed silica, was shown in Figure 7. I . A supply of nitrate and phosphate greatly
accelerated growth.
The extraordinary occurrence of quartz crystals 100 nm in size in the cell wall of
the microorganism Chlorochytridion tuberculutum was described by Brandenberger
and Frey-Wyssling (48). It is extremely unlikely that quartz crystals of such small
size could have beep present as a contaminant and it can only be concluded that the
crystals were formeh in situ. The question arises as t o why the. siliceous skeletons of
decomposed diatoms d o not redissolve in the seawater, which is greatly undersaturated. Jorgenson found that the skeletons of freshly killed diatoms dissolved in water
(49), but as shown by Lewin (50) and discussed in Chapter I , the rate of dissolution
is very slow when traces of aluminum and iron are present.
Metabolism involving silica has been studied in diatoms by measuring the effects
of silica-deficient growth conditions. Lewin found that silica is not taken up by
washed cells until supplied with a sulfur compound. Cadmium inhibited uptake,
Figure 7.3. Biogenic silica, acid cleaned, from a radiolarian showing silica lamina made u p of
ultimate particles about 200 A in diameter. [Courtesy of Hurd and Pankratz (46b).]
738
Silica in Biology
Figure 7.4. A different area of silica shown in Figure 7 . 3 , where the silica is a continuous
matrix full of rounded pores (lighter areas) 20-500 A in diameter. [Courtesy o f Hurd and Pankratz (46b) ]
possibly by sequestering the sulfur compound. The uptake is also an aerobic process
(SI). Relation between uptakes of slSiO, and L4C0,indicated that during uptake of
silica more carbon went to amino acids but when uptake of S O , ceased, the carbon
went to form sugars ( 5 2 ) . In a study by Azam,Hemmingsen, and Volcani (53) the
ingested 31Si0,first accumulated in the cytoplasm, almost certainly indicating that
silicic aid was is some chemically combined state. I t was not in equilibrium with the
external aqueous medium and was concentrated more than 30-fold in the cytoplasm.
Uptake was inhibited by inhibitors of metabolism such as 2.4-dinitrophenol. Energy
is required for the uptake and deposition of silica a s evidenced by the consumption
of nucleoside triphosphate (54).
The critical role of silica in the early stages o f development of diatoms. algae. and
plants is suggested by abnormalities in the development of Cyclotella crJpiica (55.
56). As also observed by Azam, Hemminsen, and Volcani, germanic acid is an
inhibitor for silicic acid in diatoms.
The compound 2,3-cis,tramJ-3.4-dihydroxyproline
was identified in the cell walls
of diatoms (57). The question whether this could be related to the mechanism of
silica metabolism and transport is a matter o f speculation (58).
The concentration of silica in algae of many types and a review of the literature on
Association with Primitive Organisms
739
diatoms was presented by Voronkov, Zelchan, and Lukevits (4a). Silica was found
to stimulate T M P kinase and DNA polymerase in C. fisijormis according to
Sullivan and Volcani (59). The details of silicic acid requirements for these enzymes
were investigated by Sullivan (60). Apparently silica plays a very fundamental role
in the metabolism of algae. In the absence of silica, the entire cell becomes disorganized and cannot keep on dividing according to Reimann (61). It is possible that
silica plays a role in the DNA of algae as it may do in higher organisms.
Sponges
The silica from sponges is also the source of some silica minerals. The silica content
of sponges varies widely from 1 to 90% (4a). The hard, rigid sponges are reported to
have skeletons consisting of crystalline spicules of “cubic opal” or silicic acid of
cubic symmetry. The glass sponges are very rich in silica; the needle sponges consist
of “cubic opal” cemented by protein material known as spongin, and the four-ray
sponges contain “tetrahedral opal.” The strong, tough mineral flint, from which
arrowheads were made, is believed to have been derived from the siliceous spicules
of fossil sponges (62). The SiO, may only be “crystalline” in appearance.
Sponges may absorb so much silica as to lower the silica content of the water of
inland seas. Votintsev (63) reports that the water of Lake Baikal contains less SiO,
(2-4 mg I - l ) than its tributary river (7-10 mg I - l ) because of the presence of siliceous sponges. The silica content of the sponges was about 30% of the dry organic
material, and the dead remains formed a typical siliceous sediment on the bottom of
the lake. The silica is generally amorphous in spite of the particle shapes suggesting
possible crystals. One sponge, Geodia gibberosa, contains 55 micron spherules with
a solid glassy core covered with small projections 3.5 microns long which were
proposed for use in chromatographic columns (64). In some sponges the amorphous
silica is embedded in a protein (65). The size of the spicules increased with decrease
in number as the silicate content of the growth medium was increased, while growing
freshwater sponges (66).
The flint boulders embedded a t certain levels in chalk strata in England were
apparently formed by the gradual syneresis of sponge skeletons. Each skeleton
gradually shrank and turned into a rounded boulder. This is a remarkable example
of the decrease in surface area even with only a very small decrease in interfacial
surface energy over a period of 80 million years. Within the boulders trapped
belemnites, oysters, and other debris have been noted by Iler.
Gastropods, Sea Cucumbers, Limpets
T h e teeth of the limpet P. vulgafu were shown to consist of 80 nm fibers rich in S O ,
probably bonded together by Fe,O, (67). In the sea cucumber (Molpadiu, intermedia, Holothurioidea) it is interesting that silica occurs in granules in the skin in
the form of spheres 100-190 nm in diameter mixed with spherical particles of “ferritin” of the same size. The latter is a calcium-magnesium-basic iron phosphate
(68). The biological role of the silica is unknown, but since the ferritin may serve as
740
Silica in Biology
a reservoir of iron for the organism perhaps the silica is also kept as a reserve supply. This suggests that silica may play an unusual metabolic role in this organism (69).
PLANTS
I t appears that although silica may not be necessary t o the healthy growth of most
plants, it nevertheless often seems to have secondary effects. For example, some
plants employ silica for building certain parts of the skeletal structure; others take
up silica from the soil even though the silica has no apparent useful function. The
question of the nutritional value of silica is confused by the fact that in some plants
the presence of silica seems t o enhance resistance to fungus disease, making the
plants appear healthier. Also, on some soils, the addition of soluble silicates
increases plant growth indirectly by liberating phosphate ions adsorbed on the soil,
thus increasing the total amount of available phosphate. The need for silicon in
plants has not been demonstrated except in a few isolated instances; it is not easy t o
remove all traces of silicon from artificial growth media (70).Sprecher (71) believed
that silica has an important biological function in stimulating plants to greater
growth and probably plays a role in maintaining a “physiological equilibrium” in
the nutritive solutions in the soil.
I t is usually difficult to prove that silicon is essential to plant growth, but at least
in the case of beets (Bela vulgaris), silicon appears t o be a n indispensable element
for growth, according to Raleigh (72). The importance of silica in the physiology of
rice and barley has been emphasized by Okawa (73). For example it was
demonstrated that the silica is useful to the young plants a s a nutrient. Silica is also
necessary in order that the rice plants may open and, in general, appears to be
necessary for normal growth, especially in forming the ears. For some unexplained
reason, young barley plants appear to be protected from injury by cold if colloidal
silicic acid is present in the culture solution. Lipman (74) added sunflowers t o the list
of plants which appear to require silica; the yield of seed was increased in the
presence of silica. The possibility that silicon might be taken up as a substitute for
boron is suggested by the observations of Das and Montiramani (75). G r a m (mung
bean) plants which showed yellowing were found t o be richer in silica and lower in
boron than healthy plants.
However, the difficulty of determining whether silica is directly affecting the plant
itself or is merely modifying the environment is typified i n the investigation by
Onodera and Kageshima (76a) of the effect of colloidal silica on rice. The addition
of colloidal silica to rice grown in nutrient solution appeared to make the plant more
tolerant of potassium. However, it is equally possible that the silica acted as an ion
adsorbent and therefore kept the potassium ions out of solution and away from the
plants.
The pollen of certain plants, such as Lychnis alba, has 0.8% silicon (about 2%
SO,) apparently concentrated in the outer structures t o improve resistance to decay
or weathering (76b).
In many plants soluble silica appears to be taken into the plant merely as an inert
component in the water and then deposited wherever it is concentrated as water
Plants
74 1
evaporates from the leaves. In other cases, the deposition of silica is restricted to
certain characteristic regions and excluded from other regions. For example, as it is
concentrated and converted to colloidal form it cannot pass cell membranes and so
remains where it is concentrated. Finally, in some plants the silica must enter into
the plant metabolism since it is transported and deposited in very precise forms, as
in the case of the hollow stinging needles or nettles.
Nature of Silica Deposits in Plants
It is generally observed that silica deposited within plant tissues is amorphous.
Nevertheless, several cases of crystalline silica have been reported although there is
no way of knowing whether these might have been due to mechanical inclusions of
dust. Umemoto (77) claims that a low temperature plasma method of obtaining
plant ash is essential to avoid thermal effects. (This avoids the possible hazards of
using powerful oxidants.) Although silica was primarily amorphous Umemoto
reported that it was mixed with small amounts of alpha quartz. Lanning (78) and
Sterling (79) definitely report quartz i n various plants.
Silica deposits in plants occur most commonly in the form of particles of characteristic shapes (phytoliths). The shapes are characteristic of a given plant and vary
enormously between different species (80). In grass the silica content may be 2% and
cause death of calves from silica calculi in the urinary tract if salt is not added to
their diet to make them drink more water (81). The phytoliths pass through the intestinal tract and accumulate in the soil (82). The phytoliths are opal-like (hydrated
amorphous silica) and occur in the tissues of grasses in a three-dimensional distribution (83) in such a way as to suggest silica is excluded from the cells and is deposited
essentially as silica gel in spaces between cells (84-86). The silica is transported as
Si(OH), and then concentrated and gelled as water evaporates from the leaves (87). It
is not surprising that the edges of leaves of sorghum wheat and corn are most highly
silicified, because silica is found most highly concentrated where water is lost most
rapidly (88). The structure of silica in several plants has been shown to consist of a
dense gel with pores 1-10 nm diameter full of water; the silica is completely amorphous (89). The pattern of distribution has been studied by examining the spodograms
or ashed images (90) of leaves. The pattern varied even with different species of wheat.
Lanning compared the index of refraction and X-ray pattern of the silica and concluded it was biogenetic opal since it was amorphous and not very porous (91). It is
peculiar that on the same soil under identical conditions different plants accumulate
very different amounts of different elements. Analyses of dry matter by Cooper,
Paden, and Mitchell (92) were as follows as percent by weight of element
Element
Si
Ca
AI
P
N
Cotton
Wheat
0.08
1 .oo
1.21
0.13
0.08
0.41
2.42
0. IO
0.11
0.53
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Silica in Biology
The difference in silica content can surely not be explained by the difference in the
amount of water transpired. Holzapfel and Engel showed that the uptake and
deposition of silica in wheat could be influenced by experimental conditions (93a).
Strengthening Plant Parts
Though the deposition of silica as phytoliths does not necessarily benefit the plant,
silica that is distributed precisely through the structure, especially in stems, plays a
definite strengthening and stiffening role. This is a general effect in many common
plant tissues, including the stems of grasses and grains, the hulls or shells of certain
nuts, bamboo, certain species of wood, and the spines and stinging hairs of some
plants such as nettles. Silica-hardened tips of hairs or spines on some plants provide
protection against herbivores (93b).
Eq u iset u m
The Equisetum genus (horsetail) contains so much silica it was used in the kitchen
as “scouring rush.” Pioneers used it to clean the teeth. According to Frison (94)
these plants were used for centuries as abrasives, one type being employed for
polishing wood and another for household utensils.
Silica in E. arvense is deposited as long fibers within the epidermal membrane and
is also exuded as wormlike projections until the surface is covered with opaline silica
(95). Silica probably occurs in the epidermis in organic combination with the
cellulosic material of the cell wall, according to Viehoever and Prusky (96). This
conclusion was reached on the basis of the observation that the epidermal tissue
remaining after dissolving away cellulose in cuprammonium hydroxide solution
consisted of a combination of silica with organic matter. When treated with HF, it
became soft and then gave a positive test for cellulose. Also it showed considerable
resistance to attack by cellulose-destroying bacteria. Details of the deposition and distribution of silica were described by Kaufmann et al. (97a-e), who also reviewed the
literature on silica in Equisetum (97b). It was found that the silica is deposited suddenly at a certain stage in cell differentiation. The distribution of silica in perennial
scouring rush (Equisetum hyemale var. a/fine) was examined by the scanning electron
microscope along with the electron microprobe, which reveals the silicon concentration at each point in the structure. Silica is deposited only after the cell wall has been
fully elongated but not in those sections of the stem that are still growing. This is true
also in oats and rice plants.
The above-mentioned microfibrous silica in Equiserum arvense described by
Laroche (95) may be formed by the same process by which silica gel is excreted
within cells from the inner surface of the membranes in Avina sativa. As shown in
electron micrographs by Kaufman et at. ( 9 7 ~ ) .the silica appears to be extruded as
fibers around 120 A in diameter and I20 A apart, from the inner surface of the walls
of special “silica cells.’’ As the mass of parallel fibers grows away from the
membrane surface it seems to undergo syneresis like most freshly formed silica gels.
Plants
743
However, because of the oriented structure there is shrinkage in only two directions
and the fibers are drawn together into parallel bundles or rods about 600 A in
diameter. In the micrograph the dried silica is disrupted but in the living cell the
silica forms a lining within the cell in which the rods project from the surface like a
pile carpet.
The silica, having been formed by polymerization at ordinary temperature,
probably consists of close-packed ultimate particles of SiO, with surfaces of SiOH
groups and with water held tightly in the micropores between these small particles as
in microporous silica gels formed in the laboratory. I f suitably dried, such gel should
have a specific surface area of more than 400-600 mz g-I.
The fibers may be formed by biochemical concentration and release of Si(OH), on
the outer side of the cell membrane. This may then diffuse through closely spaced,
sieve-like holes 120 A in diameter in the membrane and polymerize continuously at
each hole at the inner surface of the membrane.
Kaufman suggested (97d) that silica polymerization is inhibited in regions where
the hormone gibberellic acid causes a lowering of the pH from 6.5 to 5.0 or less, as
noted in elongating cells, for example (97e). It may be significant that such a drop in
pH would stabilize the tropolone-type chelates of silicon and thus inhibit release of
monomer (see Weiss, in Ref. 127).
Bamboo
The hardness and stiffness of bamboo can be partly ascribed to silica in the fiber
structure. However, such excesses of silica are taken up that masses of silica gel are
often found in the hollow stems. This gelatinous material containing some organic
matter, known as tabasheer (also tabashir and tabaschir), used to be employed in the
Orient as a medicine. According to Frison (94) this material has been known from
antiquity in China and India and was reported by Odorico Porto, a fourteenthcentury contemporary of Marco Polo. Interest in this curious substance seems to be
revived periodically. It was studied by European chemists in the latter part of the
nineteenth century, then essentially forgotten until Rakusin (98), in 1926, reviewed
what was known about it. A year later, Wolter (99), studied the peculiar physical
properties of this gel, but since then it has received little attention and has not been
examined by modern techniques. According to Rakusin, tabasheer was also known
as bamboo sugar because of its sweet taste. Evidently the silica gel also contains
sugars or other organic material from the plant juices. The inorganic part of
tabasheer is 99.9% SiO,. Presumably because of its purity, tabasheer was patented
for use in making cracking catalysts (100) in the East Indies.
As long ago as 1791, tabasheer was studied by Macie (IOI), who prepared sodium
silicate from it. A century later, the physical properties were studied by Cohn (102)
and van Bemmelen (103). According to Cohn, the pore volume of the gel is 0.75 ml
g-l. The transparent homogeneous pieces are amorphous, and the pores cannot be
seen under the optical microscope. However, the porous mass imbibes different
liquids giving a glass-clear solid; in the course of the absorption, the mass appears
fluorescent. By coloring the mass with various materials, Cohn prepared synthetic
744
Silica i n Biology
opals and onyx. Cohn stated, “Neither in the vegetable nor animal kingdom is there
an odder material than tabashir.” Wolter (99) investigated tabasheer which was in
the form of glasslike pieces weighing 3-15 grams. This material in some respects
resembles the opal obtained by drying silica gel very slowly. The refractive index of
tabasheer is about 1.18. By putting it in various liquids, the refractive index can be
changed. Calcined tabasheer takes up about 166% of its weight of water. The
specific gravity of calcined tabasheer is 0.54, and the silica skeleton occupies 25.7%
of the volume. It has about the same adsorptive capacity for various liquids as commercial silica gel. Tabasheer is present in an astonishingly porous form i n the plant.
It takes up iodine solution, methylene blue, or phenol very readily. More recently
the properties of tabasheer were examined by Jones, Milne, and Sanders (l04), who
found it to consist of an opal-like silica gel consisting of clusters of 10 nm silica
particles.
Grasses
Many of the grasses, reeds, and straws owe their weather resistance (e.g., thatching
of roofs) to heavy impregnation with silica (94). Rice hulls are very high in silica.
The shiny epidermis of rattan, used for furniture, is impregnated with silica.
Both the straw and grain of wheat contain silica. The silica content of the straw
ranges from 2 to 3% and makes up about half of the total ash. The upper half of the
stalk contains twice as much silica as the lower part. Also, the grain contains from
0.07 to 0.025’70, according to Coppenet and co-workers (105). The silica content of
wheat at various stages of growth was studied by Chene (106).
A consequence of the silica content of grain is that beer is essentially a saturated
solution of silica: according to Stone and Gray (107, 108) assays of 14 types of beer
showed 60-100 ppm SiO, which came almost entirely from the malt husk. The
grasses such as oats and wheat are strengthened by deposition of silica in specialized
epidermal cells (109).
Spiny Plants
Certain plants secrete almost pure silica, particularly in spines or spicules. For
example, according to Noguera ( 1 IO), two South American plants, Melinis minufiflora and Pappophorum silicosum, form readily detachable spicules containing
75434% SiO,. The dried flowers contain 7.5 and 10% S i 0 2 , respectively.
Nettles are reported to have silica in the barbs. Dried nettle plant (stems and
leaves) contained 3 . 3 % by weight of SiO, (1 1 I). Figure 7.5 shows two of the barbs at
low magnification. I n transmitted light, the tip is glass-clear and, when fresh, filled
with liquid containing a few bubbles. According to Strasburger et al. ( I 12a) the tip
of the tube is siliceous, and the base (bulb) contains calcium. The liquid contents of
the tip are released when the tip penetrates the skin and breaks off. The liquid is
highly poisonous and contains a proteinaceous toxin. Some tropical nettles are not
only painful but dangerous, inducing cramps.
Plants
745
Figure 7.5. Photomicrograph of the stinging hair of nettle. [Her ( I ) by permission of Cornell
University Press.]
When the leaves touch the skin lightly, the fragile barbs penetrate the skin, but
when the contact is sudden and hard, the barbs tend to break off before the skin is
penetrated. This idea is expressed in an old rhyme (contributed by Mr. F. C .
Carlson, Wilmington. Del.):
Nettles
G r a s p it with a touch that's gentle.
And it stings you for your pains,
Grasp it as a man of mettle,
And it soft as silk remains.
Anon.
Without silica, nettles do not develop the ability to sting. Barber and Shone (1 12b)
described experiments in which the nettle Urticaria dioica was grown in a culture
solution nominally free of silica. The leaves showed little stinging ability. Then a
solution of silica was added to the culture medium and in 2 weeks the stinging hairs
746
Silica in Biology
had become effective. presumably because they had become stiff by the deposition of
silica.
I n the stiff hairs covering the stems of some types of poppy plants the silica fills
the space between closely packed fibers of cellulose ( I 13). According to Tingey and
Pillerner (1 14) sharp plant hairs or tichomes protect plants by impaling insects.
Obviously such hairs or needles as in nettles and thistles protect plants against being
eaten by animals. This role of silica in barbs may be universal, but has been actually
proved only in a few cases.
Job’s Tears
The seeds of this plant (Coir lacrymu L . ) , hard, brilliant, and neatly spotted, are
used for beads. The epidermis is so heavily impregnated with silica that opal can be
scratched with it, according to Frison (94).
The leaves of the palmyra palm of India, used for centuries as writing paper, contain
beautiful siliceous concretions. T h e endocarp of the ivory nut contains a layer of
elongated cells assembled in palisade-like formation, each cell having a funnelshaped lumen filled with silica. Frison developed a method for preparing specimens
for optical examination so that the beautifully formed spines of silica, each covered
with still finer spines, could be readily observed by microscopists. Siliceous concretions also appear in the endocarp of the coconut and in coco fiber, bass fiber, and
Manila hemp.
Wood
According to Frison (94) siliceous concretions in the form of dense silica particles
within the cells often occur in tropical woods and contribute to blunting of saws and
other tools. U p to 3.18% by weight of SiO, has been found in some types; more than
50 varieties of tropical woods (e.g., teak) contain more than 0.5% silica. However,
no trace of silica has been found in wood from the temperate zones. Certain tropical
woods have a high resistance to marine borers (teredo). It has been conclusively
demonstrated that this resistance is due to the presence of silica particles: wood
containing more than 0.5% of silica is practically immune. Amos ( 1 15) has listed 400
siliceous timbers (more than 0.05% SO,) belonging to 32 families, in regard to SiO,
content. resistance to borers, and working properties.
Amos and Dadswell ( I 16) have investigated the occurrence of silica in the wood of
the Australian turpentine (S.wm-pia laurijblia Ten.), which has a worldwide reputation for resistance to the marine borer. I t was shown that the resistance to the borer
was related to the silica content ( 0 . 5 9 7 ~SO,), since timber from this same species
grown in Hawaii, having a content of only 0.09% SiO,, exhibited low resistance to
the borer. I t has been suggested that the silica particles in the wood damaged the
minute cutting teeth of the borers, but these authors believe that the silica may act
Plants
141
as a poison, since it is soluble in very weak alkali and would therefore pass into solution in the alimentary tract of the organism.
Mechanism Of Absorption, Movement, And Deposition Of Silica
In view of the solubility of silica in water and the relatively large volumes of water
drawn u p into plants and transpired, it is remarkable that all plants are not highly
silicified. Frey-Wyssling ( 1 17) believes that the secretion of silica in plants should be
considered a s merely a separation of nonassimilable material taken in with the
transpiration stream. This point of view explains the accretion of silica within hollow
stems, as in the case of bamboo, but does not explain the formation of specific,
highly silicified elements of plant structure such as the stinging hairs of nettles.
However, as Frey-Wyssling points out, in most plants silica is deposited in
peripheral tissues and along conducting vessels, and in this regard resembles the
separation of calcium salts which are taken in inadvertently and are deposited in
some plants in much the same way.
The mechanism by which silica is brought into solution by the roots of rye and
sunflower was examined by Whittenberger ( I 18), who found that, with 450 ppm of
silica in the culture solution, the plants accumulated silica primarily in the roots. In
view of the fact that silica is soluble to the extent of only about 100 ppm, it is now
evident that much of this accumulation must have been due to the filtering out of
colloidal silica on the root membranes. However, at less than 150 ppm (corresponding approximately to a true solution of amorphous silica), silica accumulated only in
the shoots and leaves, indicating that soluble silica moved along with the transpiration stream. When clay was used as the source of silica, it was shown that the roots
secreted a substance which brought silica into solution. This was demonstrated by
separating the clay from the roots by a collodion membrane; under these conditions,
no silica was absorbed. It was concluded that under natural conditions silica is
probably absorbed by plants principally as soluble silicic acid and that soluble silicates are formed by the decomposition of complex silicates. This study emphasizes
the role which plants serve in weathering of rocks and building of soil. Holzapfel
believed silica was solubilized by certain sugars ( I 19). However, there are certain
catechol-like compounds that solubilize silica which are also likely to be involved
(120) (see Chapters I and 3).
Amos and Dadswell ( I 16) postulate that in plants which absorb silica the protoplasmic surface of the root hair is basic in character and has a preponderance of
hydroxyl groups which are exchanged for silicate ions. Those plants with root hairs
more acidic i n character probably have an ion activity which produces the situation
favorable to the adsorption of cations. It is pointed out that other plants may have
plasma membranes containing more nearly equal numbers of acidic and basic
groups, which would therefore take up anions and cations in similar amounts.
The amount of silica in solution is reduced by the addition of metal salts. Thus
spraying a rice plant and soil with copper sulfate solution reduced the amount of
silica deposited in the leaves (121). This effect is undoubtedly due to the formation
of insoluble copper silicate, which thus rendered the silica unavailable.
748
Silica in Biology
Very little is known about the occurrence of silicon compounds, other than free
silica, in plants. Malfitano and Catoire (122) reported that the most highly purified
potato and corn starches yielded an ash containing S O 2 , suggesting that the silica
might be present in a form chemically combined with the starch.
Engel (123) has studied the nature of silica in rye straw and deomonstrated that
organic complexes of silica are present. With hot water or methanol, after pretreatment with a methanol-benzene mixture, labile organic compounds of silica can be
obtained from the straw; these compounds are easily transformed into the inorganic,
insoluble polymeric condition of SiO,. A small a m o u n t of ether-soluble
organic-silica complex was also obtained in which galactose was found to be present
in the ratio of 2 moles of SiO, per mole of sugar. Whether the silica complex in the
ether extract also consisted of fatty components and phosphoric acid, along with a
small amount of a pentose associated in a more tightly bound manner, could not be
determined. After further growth, rye straw contains another silica complex in which
the ratio of SiO, to galactose is I : I . I t appears that the silicic acid combines with
the sugar components as well as other components in the physiological structure.
About 187~of the silica in the rye straw structure must be combined with the framework cellulose, because this amount of silica is separated when the cellulose is
dissolved in cuprammonium solution.
Engel ( 1 2 3 ) points out that, since the deposition of silica a t specific sites in the
plant is apparently well controlled, it must enter into certain metabolic processes,
and therefore organic compounds of silica must be involved.
I n a low-silica medium, w'heat plants can actually lose silica from the aboveground tissues, showing that it can be carried downward to the roots by the circulation within the plant (124).
To reduce the silica content of quack grass and make it more palatable, leaves
were sprayed with glyphosphate (125). The silica content of forage reduces digestibility; this has been confirmed by experiments in which soluble silicate was added
( I 26).
The first pure silicon compound identified in a plant was isolated and identified by
Weiss and Herzog as a silicon chelate of thujaplicine, an isopropyl tropolone in the
conifer Thujaplicara (127) (see also silicon metabolism discussed below).
Relation Of Soluble Silica To Soil Fertility
Alihough silica is apparently not essential to the growth of most plants, it has been
shown repeatedly that the addition of soluble silicate to soil or culture solutions had
a beneficial effect when there was a deficiency of available phosphorus. I t now seems
clear that this is not because the plant utilizes silicate instead of phosphate ion, as
first believed, but rather because silicate ion is able to displace phosphate ion from
the surface of soil or colloidal material. thus increasing the availability of the small
amount of phosphorus which is still present.
For example, Sreenivasan reviewed the available information on the role of silicon
in plant nutrition and concluded that silicate in the soil facilitates the uptake of
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Plants
749
phosphorus. In other investigations by this author (128), it was shown that soluble
silica (or silicate ion) is adsorbed by certain components of the soil, particularly
clays. The relation between the concentration and retention of silicate is logarithmic,
indicating absorption. It was demonstrated that alumina and iron oxide gels
adsorbed silicate in somewhat the same manner as soils, forming an adsorption complex from which silicate is not readily removed by washing. It was further shown
that, when soil is treated with soluble silicate, phosphate ion is less strongly
adsorbed. Silica gel does not adsorb phosphate ion. It therefore seems clear that the
addition of silicate may have a nutritional effect because it displaces phosphate ion
from the adsorbed condition on the soil, thus making phosphate more available to
the plant. It has also been shown by Bastisse (129) that phosphate ion can be
liberated from the adsorbed state on certain soils by the addition of soluble silica.
This is especially true of lateritic soils which adsorb phosphate ion rapidly, so that it
becomes unavailable to the plants because of the formation of insoluble iron and
aluminum phosphates. In soils of this type, the addition of silicate displaced the
adsorbed phosphate ion, with the result that corn yields were doubled or tripled in
silicate-modified alkaline media and increased up to fivefold in neutral media. There
was also a marked increase in the plant content of S O z , P,O,,and iron. The displacement of phosphate ions from certain soils by silicate was also demonstrated by
adsorption isotherms by Laws (130). Treatment of the soil by sodium and potassium
silicates decreased the capacity of the soil to adsorb phosphate from solution. It
appears that silicate masked the active adsorption centers of the colloid and was held
more strongly than the phosphate ion, thus tending to prevent the adsorption of
phosphate.
In a study of the displacement of anions from soil by soluble silicate, Toth (131)
showed that phosphate ion was released from the absorbed state only in slightly
alkaline media, so that displacement is by hydroxyl or silicate ions rather than silicic
acid. At about pH 7, soluble silica is essentially nonionized and has little tendency to
displace phosphate ion. Definite increases in yield of barley and Sudan grass were
noted when calcium or magnesium silicate was added to soil, these materials being
apparently sufficiently alkaline to furnish some silicate ions. There was marked
absorption of silica by rape, barley, and Sudan grass when grown in silicated soils.
Other observations regarding the effect of silica on the nutrition of plants are as
follows. In the water culture of barley, soluble silicate caused a significant increase
in dry weight of the plants if insufficient phosphorus was present (132). Leaf
development was retarded by phosphate deficiency and hastened by the addition of
silicate. In the presence of sufficient phosphorus, silicate had little effect. Silica gives
an increase in yield of certain crops, particularly legumes and cruciferous plants,
only when there is a deficiency in phosphoric acid, according to Lemmermann and
Wiessman (133). However, the beneficial effect of silica was much less when there
was a deficiency of potash or of nitrogen. These authors (134) do not believe that
silica modified the plant functioning, but instead has a solubilizing action on phosphate compounds.
Duchon (135) concluded that the favorable action of colloidal silica on crop yields
i n sand cultures with insufficient phosphoric acid fertilization is due mainly to