Leslie Heiden, age 10, of Newport Beach, California, for her question:
What do protozoa eat?
The miniature protozoa live in miniature worlds of their own. A cupful of pond water is a teeming city; a shovel of rich, damp dirt is a thriving continent. Certain protozoa live secretly inside ordinary sized plants, animals and even people. But all of nature's children must have food, even the tiny protozoa that are too small for our eyes to see. And the right food for each type must be nearby in his miniature world.
It is not easy to imagine things we cannot see. Before the microscope was invented, nobody guessed that we share our world with zillions of protozoa. Now we know that there are thousands of different varieties in assorted shapes and sizes. Each type lives its own life in its own way„ Each type has its own favorite foods. Most protozoa prefer a mixed diet of meat and vegetables, though some types can digest simple chemicals from the soil and water Some of the midget types live inside larger plants and animals and thrive on food prepared by their hosts.
A snake may be able to gulp down a meal bigger than he is. But most of nature's children must take their food in small helpings. And of all things, the world of tiny protozoa is populated with even tinier living things. There are teeming bacteria, miniature molds and algae and even midget cousins of lobsters and other larger creatures. Water and soil also teem with scraps of decayed plants and animals. Most protozoa live on these morsels of plant or animals food, alive or dead.
Among the protozoa themselves, there are mousy midgets and whopping giants. The giants and the medium protozoa devour their smaller relatives. A giant protozoa may be 3/25th of an inch wide big enough for our eyes to see. The midgets measure 25,000 to an inch. The medium sizes range from 200 to 300 times larger than the midgets. To an average protozoa, one of his midget cousins equals about a human sized meal. So does an average sized bacterium, or a tiny green alga or a fungus type yeast cell..
Most protozoa live freely and make their own livings. Some flow along like moving blots of jelly, engulfing food as they go. Others wave hairy threads to capture scraps of floating food. One of these protozoa may engulf a bacterium, which goes into a tiny little stomach called a food vacuole. While the bacterium is being digested, the hungry little hunter may engulf a yeast or an alga cell, a smaller protozoa or a scrap of decayed meat or vegetable. Inside the average protozoa, there may be several food vacuoles stuffed with meals at various stages of digestion.
Other protozoa do not hunt for a living. Some types manufacture at least part of their food from minerals and other chemicals dissolved.in water or moist soil. Other types live as parasites, helping themselves to food they did not capture or prepare. A few of these parasite protozoa harm the cells of larger plants and animals. But most of them do no harm, and some even pay for their board and lodging by helping their. hosts to digest certain foods and perform other useful duties.