Science of the Seasons: Pros, cons of protozoa

By David Wartinbee, for the Redoubt Reporter

Photo courtesy of David Wartinbee. This species of protozoa, called Amoeba proteus, is a nonparasite and is fairly common in freshwater lakes and ponds. It is seen through a microscope at 300 magnification. The bumps and tubelike extensions around the single-celled creature are the pseudopodia used for locomotion and to capture dinner. The bubble-looking things inside are organelles and storage structures where they store proteins or engulfed materials.

In the late 1600s and early 1700s, a microcopist named Anton van Leeuwenhoek wrote letters to the Royal Society of London about tiny animalcules he was observing with his microscope.

His microscope was constructed around a relatively simple, but well-polished, bead of glass. By the time Jabez Hogg wrote his book “The Microscope,” in 1861, instrument construction, lenses, condensers and slide preparation were much more sophisticated. Today, we use a variety of even more powerful microscopes, like scanning electron microscopes and transmission electron microscopes, that can give several hundred thousand magnifications with three-dimensional images. While the equipment quality and magnifications have improved dramatically in 400 years, some things have not changed. Looking at the creatures found in pond water was a favorite for van Leeuwenhoek, and that same water continues to intrigue scientists today.

While the earliest microscopes probably could only approach 300 magnifications, that was more than adequate to see a variety of protozoa. Just about every biology student today has seen these little creatures buzz around in the water and come in and out of our magnified field of view.

Several hundred years ago scientists were not sure what kind of creature they were seeing move around with such ease. Today, with fancy equipment for the observations, we still wrestle with where these little organisms fit in the wide spectrum of living organisms. They are all made up of a single cell, although many will cooperatively hang out together in what we call a colony.

While so simple, many are able to perform feats we usually reserve for much more sophisticated creatures. Many protozoa use intricate, hairlike cillia on their body surface that enables them to glide through the water with ease. Their movements show amazing nimbleness and dexterity. They are attracted to areas with food and will detect and avoid areas with predators or harmful chemicals. While they have none of our complex sensory systems, nor anything like a brain, their behavioral repertoires are simply amazing.

There are others that have the ability to stretch out their body membrane in a variety of directions, all at the same time. These are called amoebas. They form and then break down ultrasmall protein fibers called microtubules within their body. These microtubules change the outside shape of the cell and enable them to surround and engulf food particles. These small body projections are called pseudopodia, and they use them for locomotion and feeding.

For microscopic protozoa, organic detritus and even smaller bacteria are common table fare. Because of the constant input of leaves, twigs, stems and other organic material into the water, protozoa populations in our streams and lake bottoms are astronomical. Surely that’s one of the reasons we like to look at pond water under a microscope, since we always know they’ll be present. The vast majority of these miniscule creatures have no medical impact on humans.

However, like any large group, there are some black sheep in the family. I am speaking of protozoa that have become human parasites. One that we sometimes hear about here in Alaska is Giardia lamblia. This creature is found in streams throughout Alaska and is taken in when we drink unfiltered water. The cyst form we drink then hatches out and reproduces rapidly in the gastrointestinal tract. The resulting adult forms cause stomachache and diarrhea, which is sometimes called beaver fever. Giardiasis can ruin an otherwise great camping trip.

An amoeba called Entamoeba histolytica can cause epidemic outbreaks when drinking water is contaminated. There were probably more American soldiers that died during the Spanish-American war due to ameobic dysentary than from bullets. Another member of the group that thrives in inadequately treated water supplies is Cryptosporidium. In 1993, more than 400,000 residents got sick from drinking city water in Milwaukee because of this protozoan. The addition of more chlorine in the city water solved the problem.

I have to describe one more group of bad actors among the protozoa. These are the Plasmodium group that cause malaria. These creatures have a fairly complex life cycle but, basically, they are transferred from infected mosquitoes to humans and then back again to the mosquito vector. While malaria is not a major concern here in Alaska, malaria kills over 5 million people in the tropical parts of the world every year.

Microscopic protozoa are found in virtually all of our lakes and streams. The vast majority of them are simply breaking down detritus and feasting on bacteria. Their activities enable minerals to cycle through living organisms, then back to a nonliving form, and then into a living form again. Most are completely harmless, but a few are worthy of precautions. Perhaps the most important take-home message today is to filter or sterilize all the water you drink when out in the field. Otherwise some of these curiously simple creatures may show their dark side.

David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River and Cook Inlet watershed.

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1 Comment

Filed under science, science of the seasons

One response to “Science of the Seasons: Pros, cons of protozoa

  1. dawson

    this is useless how can you teach kids this junk

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