Web Bit 4-1: Caulerpa, The World's Largest Single-celled Organism?
By Mari N. Jensen
Many cells vie for the title "World's Largest Single Cell." Near the head of the line are the enormous nerve cells of some animals—those in giraffes' legs may be several meters long. Yet these cells are not self-perpetuating organisms, but the pampered parts of a body, cared for by a complex system of blood vessels and immune cells. The ostrich egg might be a contender, to say nothing of the still larger eggs of Cretaceous dinosaurs. But an egg cell is as much the creation of a complex multicellular animal as a nerve cell. It is an individual, but it is not a mature, functioning individual.
Figure 4.1: The plant-sized algal cell Caulerpa can grow up to 1 m in length.
Some researchers give a score of 10 to the green alga Caulerpa—without question, one of the world's largest single-celled organisms. Caulerpa flourishes in warm, shallow, tropical and subtropical seas throughout the world (Figure 4.1). Some species grow to be a meter long—10,000 times larger than most other cells. If most cells are limited in size to less than 100 microns across, how does Caulerpa manage so well?
First, Caulerpa divides into compartments. Far from being a huge, undifferentiated blob, Caulerpa consists of a slender filament that grows along the ocean floor and thin, leaflike projections that extend up into the water. The green fronds in your dentist's saltwater aquarium may, in fact, be Caulerpa.
This giant cell is bounded not only by its plasma membrane, but by a cell wall, a rigid structure enclosing the membrane. The cell wall provides support. In addition, rods, acting as struts, protrude from the cell wall into the cytoplasm, spanning the cell and further stiffening it. Running both perpendicular and parallel to the long axis of the cell, the rods form a dense lattice throughout the cell and act as an internal skeleton.
Why isn't acquiring nutrients and excreting wastes an insurmountable problem for this giant cell? One answer is shape: being long and thin, not short and fat, helps Caulerpa by increasing its surface-to-volume ratio. Even so, it would appear that, to transport nutrients and wastes in and out of the cell, Caulerpa has to move them further than does a typical cell.
The answer may be Caulerpa's vacuoles, fluid-filled sacs inside the cell. The vacuoles take up most of the space inside the cell, squeezing the cytoplasm into a thin layer between the vacuole and the cell wall. In Caulerpa, that layer of cytoplasm is only five microns thick. As a result, whether a Caulerpa cell needs to absorb nutrients or excrete wastes, the distance from the cytosol to the external environment—the warm, tropical ocean—is similar to that of other, smaller cells.