DIATOMS

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]

 

More than one way to look at a diatom. This picture shows the side view (top left) and top view (bottom right) of a Navicula lanceolata

 

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.

Back to top of page

Back to Home Page