|Branches on the Tree of Life: Protists|
The live photography was accomplished using a variety microscope techniques, including dark field (good for showing the natural color of subjects) and the most advanced forms of differential interference contrast (DIC gives contrast to transparent structures that would be invisible in normal bright field microscopy). While these forms of imaging living protists are particularly revealing for some aspects of micro-organism biology, all of the organisms seen here can be studied successfully with student microscopes.
IMPORTANT NOTE: This program packs a lot of information and a wealth of observational data into nine different modules. We recommend that the material be viewed module by module, stopping for discussion and replay as needed. “Discussion starters” for the various modules are provided in this guide.
What is a Protist
Cells with a nucleus, are known as “eukaryotic cells”. Single cell eukaryotes are called Protists. One of the problems in biology is how to classify protists. Analysis of their DNA and RNA shows that some lines of protists separated very far back in time, diverging from the earliest eukaryotes. Also, some lines of single cell life (green algae) are more closely related to multicellular organisms (plants) than to other lines of protists.
The lesson here is that single celled eukaryotes show such extreme evolutionary divergence, and that the old idea of lumping them into a single kingdom needs correcting. Biologists are now attempting to create a protist classification that looks more closely at their genetic material. The ultimate goal is to understand how the various lines of protists diverged from each other, and how these independently living cells and multicellular organisms are related.
The Euglenids (02:40)
Cells with a nucleus, are known as “eukaryotic cells”. Single cell eukaryotes are called Protists. One of the problems in biology is how to classify protists. Analysis of their DNA and RNA shows that some lines of protists separated very far back in time, diverging from the earliest eukaryotes. Also, some lines of single cell life (green algae) are more closely related to multicellular organisms (plants) than to other lines of protists.In green euglenids, the red eyespot and corresponding photosensitive region near the base of the flagellum create a guidance system that allows Euglena to home in on a light source and swim in that direction until it reaches a light level suitable for photosynthesis. The euglenid shown is E. rubra, which can live at the surface in full sunlight, a niche unavailable to other euglenids due to the high levels of ultra violet radiation. E. rubra survives the radiation using a shield of red pigment. The pigment can be withdrawn when the sun isn't shining, giving the chloroplasts more light for photosynthesis. A swimming E. rubra shows the red pigment as a concentrated core in the middle of the cell. Several other species are shown hinting at the diversity of green euglenids.
Non-green euglenids include Astasia, which evolved from a green euglenid that lost its chloroplasts. Without photosynthesis, Astasia obtains its nutrition by absorbing nutrient molecules directly from its surroundings. In another relative, Peranema, the exceptionally robust flagellum works like a propeller, pulling the cell through the water. Peranema is a predator, capturing and engulfing smaller euglenids. Two rods, located in the mouth area, are used to hold prey during engulfment.
In the euglenid line, as in most other flagellated protists, individuals divide longitudinally. Division begins with duplication of the basal body at the base of the flagellum creating a cell with two flagella that then splits right down the middle. Today’s euglenids are modern representative of an ancient line of life, so different from other protists that some biologists have suggested placing them in a kingdom of their own.
Diatoms and Their Unlikely Relatives (04:20)
Diatoms are important producers in aquatic food chains. In diatoms, the photosynthetic pigment is yellow, not green. Diatoms convert the products of photosynthesis into oil droplets--an energy reserve the cell can draw on when the sun isn’t shining. But the unique thing about a diatom is its house. These cells remove dissolved silica from the water and use it to construct finely sculptured glass cases.
Diatoms are not alone on their evolutionary branch. The lesson here is that even though organisms my look very different (diatoms and water molds), their genetic material may indicate that they occupy the same branch on the tree of life. Regrouping organisms on this branch is such a new game that biologists are yet to agree upon a name. One proposal for a branch name is “Chromista,” considered by some to be a phylum and by others, a kingdom.
Amoebas and Heliozoans (05:42)
This observational module shows amoeboid protists moving and feeding by the extension of pseudopodia. All of the amoeba observations are in real time. Shell-building amoebas such as Difflugia are common in pond water samples.
Heliozoans have long been classified with the amoebas, but genetic analysis places them on their own limb of the eukaryote tree. Their spines, made of microtubules, are often broken by a thrashing prey, but are quickly rebuilt.
Green Protists (07:36)
In unpolluted habitats it is common to find a diversity of unicellular green algae including crescent-shaped Closterium, a brilliant green cell with dancing crystals of gypsum in vacuoles at its tips.
Spirogyra is a filamentous alga composed of barrel-shaped cells with a band shaped chloroplast that spirals around the cell. Conjugation in Spirogyra usually occurs before seasonal disruption of the habitat. Conjugation results in zygotes that will assure a new population of Spirogyra when conditions improve.
Colonial Protists (08:42)
Colonial green protists let us imagine how simple plants evolved from single-celled green protists. In the same drop of pond water you can find: small colonies such as Gonium--usually with 16 cells; Eudorina--a 32 cells colony; and the queen of colonial protists--Volvox, with thousands of flagellated cells lining the sphere.
Volvox shows the beginnings of cellular differentiation, a key event in embryology of multicellular organisms. Within the Volvox colony germinal cells divide, producing daughter colonies that break out in an asexual cycle that repeats generation after generation. As winter comes on some daughters will produce packets of sperm, others eggs. The result of fertilization is a thick walled zygote that will carry Volvox through freezing and drying.
There are probably at least as many kinds if protists living inside animals as there are living in ponds, puddles, and oceans.
Plasmodium (three are seen in a stained blood smear) is a parasite of the red blood cells multiplying and breaking out in a daily cycle that produces the alternating chills and fever of malaria.
Another blood parasite, Trypanosoma (shown live in a drop of blood) produces an often fatal disease--sleeping sickness. Certain amoeboid protists colonize mammalian intestines, causing tissue damage and diarrhea. But not all insiders are harmful.
A termite’s gut is packed solid with flagellated protists (Trichonympha) that aid in breaking down wood. Without its helpers, a termite would continue to eat wood, but die of starvation. A variety of protists live in the termite gut, but not all of them digest wood. The relationships among the various species are still being worked out.
Ciliated Protists 11:38
In our view, ciliates are the dinosaurs of micro-space. Colpidium is representative of a large line of ciliated protists in which the cilia are uniformly distributed in rows. Lying just below the cilia rows are the mitochondria that supply the ATP that fuels the ciliary beat.
Animation shows cilia structure and explains how the power stroke is produced. The next section shows eight organisms (common in pond water samples) that demonstrate the diversity of ciliates.
Paramecium can be cultured by adding some boiled hay (or dry grass clippings) to pond, or aquarium water.
At the end of the narrated portion of the DVD is a 15 minute observation section featuring additional protists of particular interest. This section is provided for teachers who would like to involve students in observing living protists, an activity that leads to discussion and research. It can be used as an introduction to the narrated stories, or played as “ambiance” during laboratory work with protists. One thing everyone agrees upon is that observing this remarkable footage of living microorganisms has tremendous learning benefits.
Five common Flagellated Protists
Opalina: 100-200µ Although Opalina appears to be a ciliate, it is more closely related to certain flagellated organisms. Opalinids are intestinal parasite of amphibians. The species shown was taken from a tree frog tadpole. Note that Opalina is a multinucleated cell with hundreds of nuclei, a rare phenomenon in protists. Because most of the opalinid population occurs in the hosts rectum, Opalina is considered to be more commensal than a parasite.
To obtain opalinids for microscope study, without killing their host, use a glass eyedropper filled with 0.7% saline solution. The fire-polished end of the dropper can be gently inserted in the hosts cloaca. The idea is to provide a saline enema while sucking back a sample of the rectal content including any commensals it may contain. For a frog, the procedure may be a little unsettling, but certainly better than a postmortem extraction.
Questions: How might a multinucleated cell evolve? What kinds of nutrients are available to Opalina? How do tadpoles become infected?
Hexamita: around 20µ Hexamita is a diplomonad, related to Giardia, the notorious intestinal parasite known to travelers who indulge in untreated drinking water and spend the remainder of their trip in hospital bathrooms. The dark-field shot shows the typical rapid dancing behavior of unrestrained Hexamita. These came from a jar of putrid pond water in which a large freshwater bryozoan colony was decomposing (note the abundance of large spiral bacteria). Hexamita species are also common in the large intestine of frogs and salmonid fish.
Question: How might the transition from a free-living Hexamita to a parasite be accomplished? Might free living Hexamita have evolved the other way around--from parasite to free-living?
Dinobryon: Cells around 40µ Dinobryon colonies are found in the plankton of lakes. Colonial life may provide several advantages over living as a single isolated cell. Larger size may help prevent predation. Branching may help orient and stabilize the colony in the open water. Each new individual, resulting from division, builds on the rim of their sister and so derives all the benefits of colonial life.
Questions: Based on the video observation, in what group of protists would you place Dinobryon?
Questions: What steps might a colonial choanoflagellate have undergone in becoming a sponge-like organism?
Gymnodinium: around 40µ Gymnodinium is a common dinoflagellate found in lakes and ponds. The cells have a thin cellulose shell and two flagella. One flagellum is contained in a groove that circles the cell and produces rotation. The other flagellum projects posteriorly and provides forward locomotion. A surprising new finding, based on genetic analysis, is that dinoflagellates are more closely related to ciliates than to other flagellated protists.
Four “Unknowns” to identify:
Five Common Amoeboid Protists
Pelomyxa: 100-600µ These giant amoebas seldom put out pseudopods, creeping along by means of cytoplasmic waves. The one shown does indeed extend a broad pseudopod, but this may be in response to the coverglass pressing down on the organism. Pelomyxa is one of the few eukaryotic organisms lacking mitochondria, the ATP producing organelles that supply energy for cell use. Pelomyxa’s energy needs are met by symbiotic bacteria living in its cytoplasm. Some of these aerobic symbionts can be seen in the clear zone at Pelomyxa’s advancing surface.
Question: In what ways might Pelomyxa be a model for the evolution of mitochondria?
Hydramoeba: around 150µ We had been filming Hydra’s feeding and reproductive behavior. Our subjects, which numbered in the hundreds, suddenly went into an obvious decline. They stopped eating the copepods we sent their way and their tentacles withered to nubs. The day after we first noticed the decline all but a few had simply vanished. Examining the remaining sickly Hydras showed them to be stuffed with parasitic amoebas which oozed out when the Hydra was pressed by the coverglass. Other amoebas clung to the sickly hydras tentacles and body wall. Higher magnification showed that in addition to feeding on endothelial cells, the amoebas were ingesting the hydras nematocysts‹the stinging cells used to capture prey.
Questions: At what stage in the evolution of multicellular organisms did they become hosts for unicellular parasites? How might such relationships begin? How can a parasite benefit by killing its host?
Vampyrella: Named for its ability to enter and absorb the cytoplasm of algae cells, Vampyrella is an amazing find. Entering and emptying one cell in an algal filament, Vampyrella then moves to next and on down the line.
Question: Many filamentous algas (Spirogyra, Zygonema) produce a jelly-like coating. What evolutionary pressures might account for these jelly sheaths?
Raphidocystis: This small heliozoan is easily recognized by its two types of axiopodia, long axiopods with beads that move up and down the axiopod, and shorter processes with wineglass tips.
Choanocystis: This delicate heliozoan demonstrates the amazing ability of axiopods to repair themselves. The real-time observation shows how the microtubules that make up the axiopod quickly reassemble and repair the broken structure.
Two “Unknowns” to identify:
Twelve Additional Ciliated Protists
Nassula: 200µ Jellybean shaped Nassula is from an ancient line of ciliates, possibly going back to a time when cyanobacteria were a major food source for early eukaryotic cells. Nassula continues this ancient diet today by engulfing strands of Oscillatoria. Its food habits make it one of the most colorful ciliates, due to Oscillatoria fragments in various stages of digestion. When food runs low, or when the pond dries up, Nassula encysts, awaiting better times.
Questions: Does it appear that Nassula is restricted in its diet? Most ciliates engulf their food by phagocytosis, a process of surrounding the food with plasma membrane as it is taken into the cell. How then does Nassula, which engulfs its meal en toto break up the cyanobacteria strands and surround them with membrane?
Homalozoon: 400µ These large ciliates have a “nose” filled with toxicysts used to stun and kill small ciliate prey. In the dark-field observation, the “nose” touches a small ciliate which immediately disintegrates. Homalozoon then expands its anterior end and engulfs the prey. Homalozoon is adapted for living on, and moving slowly over, surfaces. In a petri dish it is often difficult to dislodge the cell, which adheres to the glass surface.
Question: How would you describe Homalozoons ecological niche? How does it “stick” to surfaces?
Loxodes: two species are shown, Loxodes magnus (500-700µ) and Loxodes vorax (150µ). After several hours of observation, we have yet to see Loxodes actually engulf food. The cytostome is located in the in-curved region and appears to have an extension that may “unzip” to engulf large items. Loxodes has an unusual feature‹a row of pinkish bodies known as Mullers vesicles. The pink bodies are barium salt crystals.
Question: What function might the Muller’s vesicles serve?
Trachelius: ovum 200µ. Trachelius ovum has a balloon-like cell body filled with large vacuoles. The curved proboscis contains a slit that moves suspended food items to a cytostome at its base. Look for Trachelius in plankton collections taken near weed beds. It hovers in the water using cilia to create a current from which it constantly filters out microorganisms, a feeding style practiced on a somewhat larger scale by the baleen whales.
Question: In what way might the shape of these large cells be adaptive for living in the open water?
Lembadion: 120µ Common in pond vegetation, Lembadion is a specialist at capturing small rapidly swimming ciliates such as Halteria. The cell is cupped, forming a large open cavity. On either side of the cavity are membrane flaps. When feeding, the flaps repeatedly open and close. Upon a chance encounter with a small ciliate, the flaps stay closed, preventing the prey from escaping.
Question: How might Lembadion’s unique feeding structure have evolved? Which organisms in this collection would you guess to be its closest relatives?
Bursaria trunkatella 500-1000µ, Bursaria swims through the water with its cavernous food trap sweeping up protists in its path. The actual cell mouth is at the rear of the cavity. Rows of cilia lining the narrowing food channel prevent prey from escaping. The macronucleus is long, curving around the cell interior. Contractile vacuoles are small and numerous lying near the surface. Bursaria is often found after seasonal rains fill temporary pools or catch basins, where it survives drying and freezing within the protection of thick walled cysts.
Spirostomum: The species shown here is a small one, extending to around 300µ. Some species of Spiostomum reach a length of 4mm. The species shown has a single macronucleus, but its longer relatives have multiple macronuclei strung together like a string of beads. A feeding groove, with associated cilia, extends part way down the elongated cell with a cytostome at its end. A collecting tube runs the length of the cell emptying into a contractile vacuole that fills the posterior of the cell. Myonemes (muscle-like contractile fibers visible in the observation) run the length of these long cells. Spirostomum’s worm-like shape seems well suited for living in the tangle of bottom debris where bacteria and small protists are usually plentiful. A thick population of Spirostomum resembles a plate of animated spaghetti. When disturbed Spirostomum suddenly contracts to about a quarter of its extended length. Find the place in the observation where the cells contract.
Question: How might the ability to contract benefit these extremely long cells?
Blepharisma: 130-200µ common in pond water. Two species are shown. Most ciliates are colorless, except for the colored food items they have ingested. Blepharisma is the rare exception. Cultured in bright sunlight it may appear colorless, but most individuals found in the shady aquatic jungles of ponds are pink. Blepharisma has a feeding channel, lined on one side by a membranous veil that traps small organisms and direct them to its mouth. On the other side of the channel, tufts of long cilia, beating in waves, help steer small food organisms into the cytostome. The second species shown in the observation cultures algae in its cytoplasm, a kind of symbiosis practice by a number of other large ciliates.
Strobilidium gyrans: 40µ Think of it as an escaped airplane motor running at full throttle. A ring of extremely powerful fused cilia create a feeding current that brings bacteria and small algae cells to Strobilidium's centrally located cytostome. Strobilidiumsecretes a tether, allowing it to remain in choice feeding zones. The tether is produced from a “tail” projection covered with “teeth” that appear to be used to snip the tether, allowing the cell go shooting away should danger threaten or local conditions deteriorate. In terms of body length traveled over time Strobilidium may well be world’s fastest protist. Under full power it can exceed 150 body lengths per second, not bad for a cell 40 micrometers in length moving through a viscous medium. Strobilidium is commonly found in aquatic plant infusions and seems more at home in well oxygenated water than in putrid cultures. This may be due to the high oxygen demands imposed by its powerful ciliature--the propellers that create its long-reaching feeding currents, and its remarkable speed.
Question: What is the advantage of living on a tether?
Halteria: 25-40µ, Common in stagnant pond water cultures. Halteria is shaped like a fat flower vase. The anterior is surrounded by cirri that produce a smooth gliding motion. Around the mid-section are groups of long stiff cirri that produce an entirely different form of locomotion--the sudden jumps for which this ciliate is noted. Bacteria and small algae cells are taken in through a curved slit bordered by a row of cirri that sweep in the food. These entertaining little cells go bouncing through their environment--but to what purpose? Filming several caught up in Stentor’s feeding currents we discovered once again how structure, behavior, and function are inevitably related.
Campanella: cells 200µ, colonies up to 4 cm, Campanella colonies are by far the largest we have seen in nature. In our lab’s outdoor fish pond, they often form blobs several centimeters in diameter. The cells are extremely robust, with rings of cilia-stiffened membrane that create a lot of local turbulence. The branching stalks (which began from a single individual) contain no myonemes and so can not contract. The individual cells, however, contain contractile proteins and jerk into tight balls if disturbed.
Epistylis: cells 100µ, Members of this genus are often found living on other organisms ranging from turtles to copepods. Like Campanella, the stalk does not contract and functions as a pedestal that holds the feeding individuals above their attachment site.
Six “Unknowns” to identify:
Lionotus 100-200µ. Lionotus can be distinguished from other elongated ciliates by having cilia restricted to the outer curve of its “neck”. The inside curve lacks cilia but is equipped with trichites distributed along the length of the cytostome--a long slit running along the slender neck. If you find a population of Lionotus, try to determine how it feeds and the kinds of food it goes after.
Dileptus 250-300µ One of the stealthy predators of the microworld, Dileptus hides in the bushes, extending its neck to snare passing rotifers and large protists. Once brought to the cytostome located at the base of the neck, the prey is engulfed with the aid of trichites.
Glacoma 50µ A number of small ciliates have cytostomes containing flapping membranes. See Colpidium in the narrated section of this program, and for an extreme case, Lembadion seen in the additional ciliates section
Cyclidium 25-30µ These tiny ciliates, almost always seen in masses, remind one of a cage of mice, all scampering about in total confusion. The illusion is enhanced by a particularly long cilium at the posterior end of the cell. Stop on clear frames to see Cyclidium’s structures, particularly the undulating membrane that channels food to its mouth and the long stiff cilia that produce the jumping locomotion characteristic of these small cells.
Metopus Two species are shown. Sizes vary from 100µ to 200µ . The anterior of Metopus is folded over, causing these cells to spiral when free swimming. The twist creates a trap for food organisms that are then transported to the mouth. Metopus thrives in low oxygen environments.
Carchesium cells 120µ, colonies up to 6mm Carchesium forms a branching colony in which the various branches contract independently. The observation shows a newly formed colony with just a few individuals. Over the next month, the colony grew to approximately a centimeter in diameter with thousands of individuals distributed on five main branches. Touching a few individuals on one branch caused a chain reaction with all members of that branch contracting causing the sub-branch to snap back into a ball. Occasionally the response would spread to the other branches and the grape sized colony would snap down to pea size before returning to feeding.