|Intimate Relationships Among the Very Small|
Written and Photographed by Bruce J. Russell
A characteristic of life is that the genetic program of one species is inevitably tied to genetic program of another.
No where is this principle better illustrated than in microorganisms that have evolved ways of living with each other. These living together relationships are called symbiosis. In symbiotic relationships there are degrees of dependence ranging from relationships where one species lives at the expense of another (parasitism) to cooperative forms of symbiosis where both species benefit from living together (mutualism). Somewhere in the middle are relationships in which one partner benefits and the other is not harmed (commensalism). A ramora fish hitching a ride on a shark is often used to illustrate this kind of living together. But is the shark really not harmed by its lazy riders? Imagine two sharks of equal size and ability chasing a food fish. One has several remoras clinging to its body and fins. In this contest the slight drag of the remoras could leave their host hungry. We might say the relationship is entering a gray zone where, under certain conditions, the guests are tending toward parasitism. These gray areas give us insight into how the different kinds of symbioses seen today have evolve, or are evolving. Some partners may have starting as commensal but gradually shifting in the direction of parasitism. Others may have gone the other way, progressing from commensalism to mutualism. So, lets take a look at some common cases of symbiosis in the small organisms easily found.
Mutualism, Living Together For Shared Benefit
Algae farming is a growing industry along seashores, where everything from giant kelps to small red algae full of karageenan (ice cream stabilizer!) is grown on ‘algae farms’. However, organisms other than Homo sapiens farm algae as well and some have been at it for a very long period of time. Take the case of a famous type of ciliated protist studied in biology classes--Paramecium.
Most Paramecium species are bacteria feeders--but one is an algae farmer, culturing hundreds of bright green Chlorella cells in its cytoplasm. Unlike its bacteria-feeding cousins, Paramecium bursaria responds to light, swimming toward it–a behavioral adaptation that nicely accommodates its photosynthetic guests. Paramecium bursaria derives the same kinds of benefits of would any farmer–nutrients. From the point of view of the symbiont, Chlorella, a Paramecium cell supplies a protected environment, free from pesky browsers, along with a ready supply of the raw materials needed for photosynthesis–waste products of Paramecium’s metabolism. Each supplies the other’s needs–a type of symbiosis called "mutualism".
However, if the experimenter places the culture jar in the dark, the host cells get hungry and begin digesting their guests–a snack that can sustain them for several weeks. Should the experimenter return the culture jar to the light, P.bursaria again becomes the nurturing agriculturist, nursing any remaining green cells into a new bumper crop of symbiotic algae.
Algae farming is surprisingly popular in the microworld: Small invertebrates such as hydras and flatworms farm algae. Lichens are the classic example of algae farmers. A lichen is a two part entity: a fungus host harboring a garden of single cell algae in its tissue.
Of course the most famous mutualistic relationship is between termites and their internal guests. Without its helpers, the bacteria and protozoans that digest cellulose, the main component of wood, a termite would fill up and die of logjam, unable to digest a splinter. You can find out more about these internal guests in a previous notebook entry, Glorius Guts.
Commensalism – Eating the Scraps From Your Host’s Table
Commensalism is defined as a type of symbiotic association in which one member benefits and the other is not harmed. The common brown Hydra has a single ended gut that opens into a mouth surrounded by tentacles. Having captured a waterflea or some other small water animal, Hydra swallows and digest what it can, then regurgitates the remains. The partially digested regurgitated mass creates an environment highly favorable to bacteria.
Running around on Hydra’s body and tentacles are ciliated protozoans the feed on the bacteria. Looking closely at one of these "epi-commensals" shows an amazing adaptation for hanging on–a ring of hooks surrounded by a suction cup–one of the most impressive structure one can see in a single living cell.
Stalked ciliates often colonize small water animals such as Cyclops. One might argue that if enough of these hitch hikers establish themselves, the Cyclop’s will be slowed and adversely affected–in which case we might begin thinking of the ciliates as parasites–such is the way that the quality of relationships can intergrade.
Parasitism, Living At Your Host’s Expense
Hydra’s commensals probably do it no harm, although at times, they appear to make their host writhe as though under attack by a swarm of ants. Other Hydra guests are less benign, for Hydras are prone to attack by a certain kind of amoeba that literally eats the Hydra alive, even engulfing its stinging cells.
Volvox is as pretty a microscopic pond dweller as can be imagined. It shows an early form of multicellularity--the kind of thing that bridged the gap between single cells and the multicellular kingdoms of plants and animals. The interesting thing is that these simple multicellular beings are host to several kinds of parasites, all easily seen moving about inside. They include bacteria, amoeboid parasites that eat the colony a cell at a time, and rotifers that munch away on the colony from the inside, depositing eggs so that the young can finish off the food source.
In the three kinds of relationships mentioned so far, keeping the host alive, at least until the symbiont can reproduce, is important. Another kind of relationship requires one of the participants to die. A good case study for school labs is the relationship between Paramecium and Didinium. Both show amazing adaptations that help them survive and coexist. The details are described in another notebook entry, Didinium: A Wolf Comes Calling.
Less often seen, is a class of relationships essential to the operation of ecological systems–the removal of dead organisms through the action of scavangers. Daphnia (the waterflea) spends its entire life filtering and feeding on microalgae. It converts much of this booty into eggs that hatch and continue on. But, age and birthing take their toll. A Daphnia that survives to old age (several weeks in some), will die of parasites and age-related causes and settle to the bottom. Here a number of fates await. It may be attacked by water molds that spread through the corpse, converting Daphnia tissue into free-swimming mold spores. It may become dinner for scavenging flatworms that probe the carcass with their mouth tubes, sucking out every available morsel. But the body crevices and hollow appendages still hold protein snacks–sustenance for bacteria and a special class of protozoans that feast on dead flesh–the histophages. They can be seen homing in on a Daphnia carcass within seconds after adding it to a petri dish of pond water. Gaining entrance through any crack in its exoskeleton, they consume every accessible bit of tissue.
What one learns from observing these the interactions and relationships among microorganisms is that the micro-world mimics the larger world in its ecological structure. No surprise, as microorganisms established life’s rule book long before larger organisms evolved on planet earth.