V

~)G ]i y s

Figure 4.4 Schematic side view of three bioconvection plumes. The dark zones contain concentrated streams of microorganisms flowing downward in the culture medium.

Figure 4.4 Schematic side view of three bioconvection plumes. The dark zones contain concentrated streams of microorganisms flowing downward in the culture medium.

plicated form, drives the pattern of winds in thunderclouds.

We ought to pause, now, and consider the facts carefully. Convection cells in rooms and storm clouds are well-understood phenomena of fluid mechanics. Their similarity with the convection cells in our protozoan culture might tempt us to look for similarity in causes. That would be a mistake, for the following reason. To generate the convection cell in a room, you must do work on the room air, which you accomplish by heating it with the baseboard heater. In the generation of a storm cloud, the work is done by sunlight heating air close to the ground. All this is perfectly in line with the Second Law: the ordered patterns of air flow appear only when work is done on the system by some external agency. Stop doing the work (switch the heater off, let the sun go down), and the orderly convection cell disappears. Let me emphasize that what we observe in the petri dish is exactly opposite: the order appears only when we stop doing work on it. So there seems to be something to explain.

Because this phenomenon differs from the ordinary convection we see in, say, a room heated by a baseboard, it has a special name to distinguish it: bio-convection. We know a lot about bioconvection, in part because it is interesting in and of itself, and in part because it has some commercial applications, of which more momentarily. For bioconvection to occur, certain requirements must be met:

1. The liquid culture must contain swimming microorganisms—cells incapable of moving under their own power will not produce bioconvection in culture.

2. The organism must swim with some kind of taxis— that is, it must have a tendency to swim toward or away from something.

3. The organism must be slightly more dense than the culture it is in. This is easy: most cells are 5-10 percent denser than the water they live in.

4. The organism should have a center of mass that is offset from its center of buoyancy. This last requirement sounds rather obscure, but it is crucial to understanding how bioconvection works. We will explore it in detail momentarily.

Each of these requirements forms a piece of a puzzle. Before putting the pieces together, we must first understand them individually.

"Smart" versus "Dumb" Gravitaxis in Chlamydomonas Chlamydomonas move about their culture by doing a sort of breaststroke with their locomotory organs, a pair of flagella mounted at the "head" of the cell (Fig. 4.1). The speeds they can attain are, in their own way, impressive: a Chlamydomonas can swim at a top speed of about 200 micrometers (am) per second, or 1 millimeter every 5 seconds.2 The swimming is not random, however. The cells are able to sense conditions in the environment and to use this information to direct their swimming. As a general rule, the tendency is for the cell to stay where it is if conditions are "good" or to swim away from "bad" conditions, toward areas of the culture where conditions are "better."

2. Scaled up to the size of a human being (with an average body length of 1.5 m), this works out to a swimming speed of 135 kmh-1.

This tendency to swim preferentially toward or away from a particular type of environment is common enough to deserve a name, taxis (from the Greek tassein, "to arrange"). Taxes (plural of taxis, not what the government takes from you every year) come in a variety of forms, and they are named according to the way the arranging is done. For example, phototaxis is swimming toward or away from light (literally, "arranged by light"). Gravitaxis ("arranged by gravity," sometimes erroneously called geotaxis) is swimming up or down with respect to the Earth's gravitational field. By convention, taxes can be either positive or negative. For example, positive phototaxis is swimming toward light, while negative phototaxis obviously is swimming away from light. Similarly, positive gravitaxis is swimming down, or toward the Earth, while negative gravitaxis is swimming up, or away from the Earth.

Taxes may be managed in a number of ways. In some cases, the taxis is "smart": the cell has a sensory apparatus that takes in information about the environment and directs the swimming organs to drive the cell where conditions are good or away from conditions that are bad. For example, light falling on one side of an algal cell can influence the pattern of beating of its flagella, making it swim toward the light. In other cases, the taxis is "dumb," as in the magnetotactic bacteria Aquaspirillum magnetotacticum. These mud-dwelling spirochaetes contain within them a small crystal of magnetite. The Earth's magnetic field exerts a torque on these small crystals, just as a magnetic field imposes a torque on a compass needle. Thus, the bacterium's body is pointed in a particular direction by the magnetic field, and the bacterium simply swims in whatever direction it is pointed. In Chlamydomonas, the taxis is partially dumb and partially smart. The protozoan does make a sensory assessment of the environment— is there sufficient oxygen? or is there sufficient light?—and swimming is initiated if the answer is no But Chlamydomonas swimming is dumb in that the distribution of mass within the cell biases its direction of swimming. Let us now see how.

All bodies that have mass will experience a downward force due to the acceleration of gravity. In analyzing how this force acts, it is convenient to treat an organism's mass as being concentrated in an imaginary point known as the center of gravity, or CG (Fig. 4.5 a). If we suspend a Chlamydomonas cell by its flagella, the cell will come to rest so that its center of gravity sits directly below the point from which the cell is suspended. In Chlamydomonas, the center of gravity happens to be located at the end of the cell opposite to the flagella.

When an object of a certain density, say an air-filled inner tube, sits in a fluid that is more dense than air, say water, the downward gravitational force acting on the inner tube is offset by the upwardly directed force of buoyancy.3 Just as gravitational forces act through a single center of gravity, so too do the buoyant forces act through a single point, the center of buoyancy (CB; Fig. 4.5a). Generally, objects suspended in a fluid will orient so that the respective centers of buoyancy and gravity align vertically with respect to one another, with the center of buoyancy located directly above the center of gravity. If they do not align in this way, the object will experience a net rotational force, or torque, that will return the object to the orientation that aligns these two points properly.

Like all cells, Chlamydomonas have a density that is slightly higher than water's. Therefore, there will be a net gravitational force that will make the cell slowly sink. Because of the disparate centers of gravity and buoyancy, an undisturbed Chlamydomonas will also be oriented so that its flagella point upward (Fig. 4.5a). In the absence of any other force, a Chlamydomonas that is

3. Anything embedded in a fluid medium will experience buoyancy proportional to the mass of the fluid it displaces. Thus, a person standing in air will experience a buoyant force lifting him up, but air being much lighter than flesh, the force will be small. The weight of an object does not simply result from gravity, but from the difference between gravitational force and buoyant force. The difference constitutes the body's specific gravity. Thus, the specific gravity of any substance is less in water than it is in air.

v fr offset CB&CG

Figure 4.5 Orientation of a microorganism with noncon-gruent centers of buoyancy (CB) and gravity (CG). a: In still or uniformly moving fluid, the center of gravity will always come to rest directly below the center of buoyancy. b: When the microorganism is subjected to shear (indicated by the different lengths of the velocity vectors, v), it will be rotated by a torque proportional to the magnitude of the shear (darkly shaded arrow). The rotation displaces the centers of buoyancy and mass from their resting positions, which imparts an oppositely directed torque to the microorganism (lightly shaded arrow). The organism will come to rest at the angle where the two torques are balanced.

not swimming will always exhibit a positive gravitaxis. In other words, it will sink. When it swims, however, because it is pointed up, it will always swim up, in a negative, albeit a "dumb," gravitaxis.

Hydrodynamic Focusing in Cultures of Chlamydomonas We now turn to another piece of the puzzle, a phenomenon known as hydrodynamic focusing. Unlike bioconvection cells, which arise spontaneously in shallow cultures, hydrodynamic focusing requires work to be done on the culture, and it is work of a special kind.

As before, you start with a culture of swimming protozoans like Chlamydomonas. This time, however, the culture is placed in a device consisting of two vertical glass tubes that are connected so that the culture can be made to circulate between them, flowing downward in one tube and upward in the other (Fig. 4.6). We will be focusing our attention on the tube containing the downward flow. If a well-mixed culture is circulated between these tubes, it starts, as it did in the flat culture dish, as a uniformly turbid suspension. After a few moments, however, the cells in the downwardly flowing tube focus in the center into a densely packed column of cells. This is hydrodynamic focusing.

Hydrodynamic focusing is of great commercial interest because it offers a cheap and convenient method for harvesting microorganisms in culture. When microorganisms are grown commercially, they must at some point be harvested, usually at great expense. For example, many commercial cultures must be processed as a batch, in which a culture is started, allowed to grow for a time, and then stopped for harvesting. Hydrodynamic focusing offers a way around this problem: if you could place a small siphon at the center of one of the downward-flowing columns of culture (Fig. 4.6), you could drain off a highly concentrated suspension of cells without having to stop the whole batch. However, the practical applications of hydrodynamic focusing concern us less than how the plume originates.

The focusing arises from an interaction between the organism's swimming and the characteristic patterns

offset CB&CG

Figure 4.5 Orientation of a microorganism with noncon-gruent centers of buoyancy (CB) and gravity (CG). a: In still or uniformly moving fluid, the center of gravity will always come to rest directly below the center of buoyancy. b: When the microorganism is subjected to shear (indicated by the different lengths of the velocity vectors, v), it will be rotated by a torque proportional to the magnitude of the shear (darkly shaded arrow). The rotation displaces the centers of buoyancy and mass from their resting positions, which imparts an oppositely directed torque to the microorganism (lightly shaded arrow). The organism will come to rest at the angle where the two torques are balanced.

of fluid flow through tubes. When fluid flows through a tube, its velocity is not everywhere the same (Fig. 4.7). Fluid in the center moves faster than does fluid near the tube's walls. If velocity is plotted against the tube's radius, we see a characteristic velocity profile. In ideal circumstances, velocity is null at the inside walls and reaches a maximum at the tube's center. How does this distribution of flow affect the Chlamydomonas suspended in a culture broth?

If a Chlamydomonas sits anywhere in that tube but at the exact center, the flow on one side of the cell (that facing the center) will be slightly faster than the flow on the other side (that facing the wall). This disparity of velocities, called shear, will impose a torque on the cell that ordinarily would cause it to rotate (Fig. 4.5b).

You will recall, however, that disparate centers of mass and buoyancy make the protozoan resist this torque. Consequently, the cell will tilt, coming to rest at an angle where the torque imposed on the cell by the shear in the velocity profile is exactly offset by the torque arising from the misalignment of the centers of gravity and buoyancy. Because the shear is highest at the tube's walls, the tilt will be greatest near the walls. Because gravity always biases Chlamydomonas to point upward, the shear in the downward-flowing tube will point all the cells toward the center. Because Chlamydomonas swim wherever they are pointed, they will concentrate at the center of the tube, forming the plume (Fig. 4.6). The opposite happens in the upward-flowing tube. Here, the upward-pointing veloc-

Was this article helpful?

0 0
Oplan Termites

Oplan Termites

You Might Start Missing Your Termites After Kickin'em Out. After All, They Have Been Your Roommates For Quite A While. Enraged With How The Termites Have Eaten Up Your Antique Furniture? Can't Wait To Have Them Exterminated Completely From The Face Of The Earth? Fret Not. We Will Tell You How To Get Rid Of Them From Your House At Least. If Not From The Face The Earth.

Get My Free Ebook


Post a comment