What are diatoms?
Diatoms are single celled algae that have two hard coverings
of silica, SiO2, (almost identical to opal). [2,3] The two sides are called the
epitheca and hypotheca; they fit together like a box and lid or petri dish, the
epitheca overlapping the hypotheca. [2]
Like
other algae and photosynthetic primary producers, they have a nucleus,
cytoplasm, vacuoles, and chromatoplasts.[5] The silicon walls are very porous with intricate designs
consisting of punctae—(pores and depressions), striae—(lines of delicate
close-spaced puncta), costae—(rib-like thickenings), and septa—(inward growths
of the cell wall).The pores are passages for gas and nutrients. [2]
Diatoms can be generally classified in two groups—pinnate
and centric. Centric diatoms are round with radial symmetry and pinnate are
elongated. Some pinnate forms are
mobile employing a caterpillar tractor tread apparatus. Cytoplasm streams along the grooves in
the surface of the cell wall enabling some to reach a top speed of 25 micrometers
per second.[4]
Life History
There are at least three
common species of diatoms in Emigration Creek: Rhoicosphenia curvata
which is epiphytic—meaning it grows on another plant (Cladophora, a green filamentous algae)—and epilithic meaning attached to an inorganic
substrate like a rock-- and often in clusters, Coccones also epiphytic, and Navicula lanceolata,
found on rocks although free-swimming and not attached.
The attaching stalks of epiphytic and epilithic diatoms are composed
of mucilage secreted from the raphe—slits in the silica shell. [2]
Rhoicosphenia curvata (400X)
Close up Rhoicosphenia curvata (400X)
Epiphytic Coccones (400X)
Navicula lanceolata (100X)
Navicula lanceolata close up (1000X)
The leaves that fall into the creek in the autumn have a major
impact on diatom reproduction.Decomposing
leaves, acted upon by bacteria and fungi, release nutrients into the water. Also because the
leaves are in the creek, and not on the trees, there is additional light and
nutrients for the diatoms to carry on photosynthesis. As a result, the diatom populations explode. Diatoms have two
ways to reproduce—sexually and asexually. They can simply divide by mitosis or produce egg and sperm
(meiosis). When a diatom reproduces
asexually,(mitosis) the DNA replicates and the chromosomes are divided into
two identical halves (half goes with each side). It then grows a second shell for each
half of the existing shell. This
eventually pushes the now separate diatoms apart. Since the newly grown half fits inside the older half each
time a split occurs, the “daughter cells” become slightly smaller. Once a certain minimum size is reached,
(60 to 80% of the original parent), the diatoms enter into a sexual reproductive
cycle. The male produces and
releases sperm; the female bends, creating an opening in the cell wall for
the sperm to enter. Inside an
envelope-type structure, the
fertilized egg grows its own shell and nucleus. This new diatom will grow to full size
and start the cycle of division again. [3]
Diatom
reproduction (asexual and sexual) from National Geographic [3]
When
environmental conditions will not allow a diatom to grow—ice cover, not enough
light, etc.—it
can become a
spore or resting cell. The
cytoplasm condenses into a dark brown mass
containing large
droplets of lipids (fats) and polyphosphates. [2]
Place in the Food
Web
Diatoms
are primary producers—they convert light, water and carbon dioxide into
carbohydrates during
photosynthesis. They also require
nutrients from decomposers like
nitrogen, zinc,
and phosphorous and silica for their cell walls from ground water dissolved as
H2SiO4. A
by-product of photosynthesis is oxygen, which is essential to aquatic and
terrestrial life:
fish, aquatic invertebrates, snail, leeches, worms, etc. Although
diatoms do not have any specific
predators, they are grazed upon by snails, caddis fly larvae, protozoa, rotifers, and crustaceans.
They also get eaten by anything that grazes on Cladaphora, or filter feeds such as bryophytes and
clams.
BIBLIOGRAPHY
1.
Flowers, Seville. Algae of Utah. Duplicate copy University of Utah aprox.
1960
2. Graham, Linda, and Lee W. Wilcox. 2000. Algae. Prentice-Hall, Inc., Upper Saddle River, NJ.
3.
Hoover, Richard B. June 1979. Those Marvelous, Myriad Diatoms. National Geographic. Vol. 155,
No. 6:871-878.
4.
Patrick Ruth and Charles W. Reimer. 1966. Diatoms of the United States.
The Academy of
Natural Sciences of Philadelphia.
5.
Robbins, Wilfred, W., T. Elliot
Weier, and C. Ralph Stocking.
1966. Botany. John Wiley & Sons, Inc.
6.
Villee, Claude A., and Vincent G. Dethier. 1971.
Biological Principles and Processes. W. B. Saunders
Company.
7.
Vinyard, William C. Diatoms of
North America.