10 Amazing Defenses of One-Celled Organisms

Copyright 2016 by Gary L. Pullman

We know what lions and tigers and bears use to defend themselves. We know, too, that there are many other forms of animal defense besides teeth and claws. There are beaks, talons, foul-smelling odors, quills, venom, hooves, stingers, camouflage, mimicry, and even playing dead.

There are a few unconventional and downright bizarre animal defenses, too, spitting, spurting blood, oozing cyanide, vomiting (yuck!), and squirting “ink,” among them.

But what about organisms that are too small to grow teeth and claws, or don't have blood and can't vomit on themselves? What do they do to fend off predators?

Can one-celled organisms defend themselves at all?

The microscopic world is every bit as brutal and pitiless as the larger world we inhabit. There are many threats and predators. Are unicellular organisms destined to be nothing more than some other, bigger organism's next meal?

Fortunately for them, the answer is no. Most are capable of defending themselves. In fact, several one-celled organisms use more than one form of defense.

These are 10 of the amazing defenses of one-celled organisms.

10 Trichocysts

Trichocysts (stained blue)

Basically, a trichocyst is a harpoon.

Certain one-celled animals, such as the paramecium, can launch this weapon when they are threatened. Some of these organisms are equipped with a whole arsenal of trichocysts, “widely distributed” over the outside of the entire cell. Others have only several that are “restricted to certain areas” of their outer cells.

There are three types of trichocysts: mucoid, filamentous, and toxic.

Mucoidtrichocysts are long, and they can be seen as they launch. Filamentous trichocysts look like threads and have rod-like tips at their ends. Toxicysts, which are restricted to the mouth area, resemble tipped threads, too, but, unlike ordinary filamentous trichocysts, they can be used offensively as well as defensively, to paralyze or kill predators or prey.

For some unicellular organisms, this weapon has another use, too. The paramecium uses its filamentous trichocysts to anchor itself to one spot while it feeds.

Dinoflaggelates are also armed with trichocysts—and with two similar weapons,

nematocysts and mucocysts. (In addition, as we have seen, dinoflaggelates can light up the night to discourage predators.)

9 Nematocysts

Nematocyst

If the trichocyst is like a harpoon, a nematocyst is a like a missile or a bomb.

It's a tiny, long or round “capsule” loaded with a poisonous thread. The thread is “coiled, hollow, [and] usually barbed.” When a one-celled organism, such as a dinoflaggelate, is attacked or otherwise properly “stimulated,” the thread “quickly turns outward” from the capsule, so that the launched thread can fend off predators or ensnaring prey.

The barbs “act like a drill, penetrating into” the predator. In the process, the thread is pulled into the attacker. The toxin passes through the hollow thread, paralyzing the victim. Then, the thread detaches from the nematocyst.

8 Mucocysts

Mucocyst

A mucocyst is like a flame-thrower, except it throws something other than fire.

“Small sac-like organelles,” mucocysts contain a mucus-like substance that it “discharges” when an organism, such as the Tetrahymena pyriformis, is attacked or otherwise provoked. The mucus-like stuff forms a “protective covering” around the organism.

7 Spines

Rotifer

Rotifers' weapons are spines. Some predators release chemicals called kairomones that stimulate the rotifers' spines to lengthen. This modification is an example of “predator-induced morphologicaldefense.” This ability comes in handy among rotifers, because larger specimens often eat smaller ones. The elongation of the rotifer's spines prevent them from being ingested by larger rotifers, because the longer spines “either cannot enter the pharynx through the mouth or cannot be” swallowed and are, therefore, “maneuvered from the pharynx into the esophagus andstomach and are rejected.” In the case of some rotifers, the changed length of the spines last from before the rotifer's birth and continue throughout its lifespan. In other rotifers, this induced change in spine length can occur after the rotifer is born and disappear before its death.

Although rotifer spines lengthen in response to predators' kairomones, they also elongate when temperatures are low or food is scarce. Although some scientists speculate that “predator-induced morphological defense” may have a trade-off, costing the organism something, such as a lower reproductive rate, the results of research have been inconsistent and the issue has not been resolved.

6 Armor

Not only are dinoflaggelates armed with trichocysts, nematocysts, and mucocysts, but they are also capable of bioluminescence. And that's not all.

Dinoflaggelate

Dinoflaggelates are also armored! Their hard coating, the theca, protects them against predators. 

The armor consists of “two major. . . regions composed of 100 individual plates” of cellulose, which form both an anterior, or frontal, horn and “usually two posterior,” or rear, horns.The plates' edges “overlap, sliding apart as the cell increases in size” to permit the cell's expansion.

The spines adapt to their environment. When they are in “cold, salty water,” the spines are “shorter and thicker.” In “less salty, warmer water,” the spines are “longer and thinner.”

Any microorganism that wants to take on a dinoflaggelate had better be determined. It will have to be to get past this guy's arsenal!

While it is not exactly armored, the paramecium does “ have a pellicle with alveoli that helps to provide stiffness. In addition, it defends itself with its trichocysts.

5 Flight and Evacuation

Some one-celled organisms have no shame when it comes to defending themselves. They'll do whatever it takes, no matter how revolting or embarrassing their tactics might seem to us. These are survivalists supreme, and they have a few nasty tricks to ensure they don't end up as a predator's next meal.

Paramecium

If a paramecium can evade a predator, it will do so: “K

nown for their avoidance behavior,” if it “encounters a negative stimulus,” it will rotate “up to 360 degrees to find an escape route.” It's a good thing they're ready to flee for their lives, because their nemesis, the Didinium “ejects poisoned darts (trichocysts).” Then, the Didinium “engulfs” its prey and eats it. In a few hours, the Didinium, which are “voracious eaters,” are ready to dine again, perhaps on another paramecium that's not lucky (or skilled) enough to escape its clutches.

Paramecia have plenty of other predators, too: amoebas, water fleas, Eastern lamp mussels, golden shiners, euglena, greater bladderworts, black crappies, rotifers, copecods, scuds, predatory nematodes, and flatworms.

Fortunately, although they'd rather flee than fight, paramecia are not defenseless. They are armed with trichocysts. However, this weapon has no effect on Didiniums. The predator continues, full steam ahead, as it were, “gulping down as many as two paramecia a minute.” Normally, if the “harpoon-like trichocysts” fail, paramecia have another defensive system: “undigested food, digested waste and waste water,” which it unloads “through its several water expelling vesicles and anal pore.” Only a truly determined (or starving) predator is likely to persist after an attack like that!

4 Bioluminescence

Ceratium fusus defends itself on the principle that any enemy of its enemies is one of its friends. In a sense, its weapon is a flare.

By “undergoing a chemical reaction at night,” it defends itself against predators, attracting “larger predators that prey on the organisms that eat” it.

Ceratium fusus is not the only one-celled organisms that uses bioluminescence as a defense. Dinoflaggelates also light up the night to discourage predators.

3 Toxins

Euglena

The euglena isn't dangerous in itself. However, when certain species, such as and

E. sangueinea, join forces with millions of others of their species, together they can mount an impressive defense  against predators such as baby fish, water fleas, mussels, salamanders, tadpoles, and frogs. En masse, such euglena “develop large toxic populations of green and red 'blooms' in ponds or lakes with large nitrogen content.”

Although the stimuli responsible for the release of the euglena's toxin (

euglenophycin toxin) are not well understood, scientists know that the toxin is harmful to fish, one of the euglena's predators. The toxin is also hazardous to the health of algae, with which the euglena competes for food. It is possible, therefore, that euglena “blooms” release these toxins to defend both themselves from predatory fish and their food sources from rival consumers.

2 Detoxification

Not all of unicellular organisms' enemies are microscopic. In fact, we're one of them. Beneficial bacteria live in our bodies, which is all well and good, both for them and for us. However, our systems are also sometimes invaded by bacteria that are harmful to our health. In some cases, they could even be fatal. From their perspective, though, it's we, not they, who are the enemies, and they've come up with a defense against the hostile environment that we pose to them. They protect themselves from the nitric oxide (NO) in our immune systems by detoxifying it with flavohemoglobin. “A protein in the bacteria,” flavohemoglobin protects them from the NO “secreted by the immune system.”

Despite their extreme “sensitivity” to NO, “over time,” bacteria have developed a means of stopping “NO from damaging” them, and “this detoxification of NO allows bacteria to survive.” As a result, they can infect us, thriving in an environment that once would have easily killed them. Scientists hope to turn this bacteria's defense against them. If they can find a way to “remove this protein from bacteria,” these invaders can be killed easily.

1 Antiviral Defense System

Archaea

Several explanations have been advanced to account for the origin of the single-cell microbes known as archaea and those known as eukarya (organisms that have membrane-enclosed nuclei and other organelles). One of the latest proposals traces the antiviral defense system these organisms developed over millions of years. This unconventional explanation is at odds with the traditional view, arguing that archaea and eukarya's “common ancestor . . . was not an archaeon,” as many believe.

During the course of their evolution, to survive against viruses, unicellular organisms, as well as bacteria, had to develop an antiviral defense system. One of the first steps in this process was their formation of a nucleus: “The cell nucleus itself might have originated as a protective device allowing the cell to hide their chromosomes from viruses.”

The formation of a nucleus freed “many organisms from viruses that infected LAECA [the last archaeal-eukaryal common ancestor], except those that, in a first time, could replicate in the cytoplasm.”

Thereafter, according to the unconventional view of the origin of archaea and eukarya, “the ancestors of archaea (and bacteria)” invaded hot habitats to escape the earliest eukaryra predators. The hot habitats initiated their “reductive evolution toward” the phenotype of the “last archaeal common ancestor.” Depending on how these unicellular organisms subsequently interfaced with viruses, each took a different evolutionary pathway. Thus, eukarya, and bacteria not only survived against viruses by developing an antiviral system, but, by doing so, they also shaped their own future evolution!