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Phytoplankton: Plants of the Sea

by Prentice K. Stout

To the casual observer, the oceans and bays are vast trackless bodies of water.  Beneath their surfaces are countless fish.  But more numerous by far are the tiny microscopic animals and plants collectively called plankton, a word derived from the Greek meaning wandering.

The plant portion of this complex oceanic soup is called phytoplankton.  The term phyto comes from the Latin phyton meaning tree or plant.  This large grouping is composed mostly of single-celled algae and bacteria.

It is known that green plants liberate oxygen and produce carbohydrates, a basic link in the food chain of plants to animals to people.  Collectively, this chemical process is referred to as photosynthesis (photo = light, synthesis = to make).  In these tiny food factories, there is a chemical compound called chlorophyll that, in combination with sunlight, converts carbon dioxide, water, and minerals into edible carbohydrates, proteins, and fats.  Thus, these phytoplankton are the basis for the oceanic food chain.  Animals cannot perform this biological food-making process.  Two-thirds of all the photosynthesis that takes place on this earth occurs in the oceans that yearly create 80 to 160 billion tons of carbohydrates.  So numerous are these tiny plant forms that they often turn the water green, brown, or reddish.

Among the most abundant phytoplankton are the diatoms.  Some 20,000 species make up this plant group.   They consist of a tiny blob of protoplasm enclosed in a transparent pill-box structure made of silica.  This silica, the main ingredient of glass, is extracted from the surrounding seawater.  Minute holes or pores in their shells permit nutrient absorption and an exchange of carbon dioxide and oxygen to take place with the surrounding seawater.  Under favorable conditions, a single diatom can reproduce 100 million offspring in a month.  Clearly, such reproductive capacity creates vast numbers - as many as a billion of them in a gallon of seawater.  Those that are not eaten die and their virtually indestructible shells settle to the bottom of the ocean.  In some areas of the sea, their skeletal remains form layers up to 700-feet thick.  This diatomaceous earth has been used as a fine abrasive in toothpaste and automobile polish.  A drop of oil within the protoplasm in the shell may have created the earth's petroleum supply.  This oil in the diatom is eaten by a number of small fish; one is called the capelin, which, in turn, are eaten by codfish.  From the codfish bodies come cod liver oil detested by some, but rich in vitamins A and D.

While diatoms are essentially cool water inhabitants, their counterparts in tropical waters are called dinoflagellates.   Equipped with whip-like projections, they propel themselves about in a jerky motion.

Dr. C.P. Idyll, noted oceanographer, states, "The various species of dinoflagellates resemble chinese hats, carnival masks, children's tops, urns, pots, and vessels of many kinds, the spiky knobs of medieval war clubs, balloons on strings, hand grenades or lances."

Flagellates, like diatoms, are proficient in their ability to reproduce.  By splitting in half, a dinoflagellate can reproduce thirty-three million offspring in only twenty-five divisions.  One species of dinoflagellate, Gonyaulax, whose excessive reproductive ability can create havoc, produces the "red tide."  In the seventh chapter of the Book of Exodus, the people of Egypt were plagued with what could have been an outbreak of the red tide.  Charles Darwin, on his around the world voyage on the Beagle, wrote one of the first scientific accounts of a dinoflagellate outbreak that discolored the waters off Chile.

Another interesting dinoflagellate, if disturbed, emits light in the form of bioluminescence.  Called Noctiluca, this small organism combines two chemicals, luciferin and luciferase (Latin: lucifer = "bearer of light").  This is the same combination of chemicals that gives our familiar firefly its ability to blink during warm summer evenings. 

A simple tool can be used to study these plant forms.  One needs a coat hanger that has been worked into a circle.  Then over this frame stretch a pair of discarded pantyhose.  Cut off the legs of these stockings just below the knee and place a small plastic pill jar in the openings.   Tightly wind some thread around the fabric and the mouth of the jars and then tow it through the water.  The fine mesh stocking of the pantyhose will capture may of these small plant forms as well as their animal counterparts.  (See fact sheet on zooplankton.)  A hand lens or microscope will assist in the viewing of these fascinating organisms.

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Buschbaum, Ralph and Milne, Lorus J.  1960.   The Lower Animals, Doubleday and Co. New York.

Silverberg, Robert.  1972.  The World Beneath the Ocean Wave.  Weybright and Talley, New York.


Fun with Phytoplankton


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