13.14: There are animal-like protists, fungus-like protists, and plant-like protists.

Six major lineages of protists have been named, but even that classification does not capture the huge diversity of the group, and some of the best-known protists do not fit into any of the six lineages. Protists include forms that are very much like animals, others that seem a lot like fungi, and still others that look like plants (FIGURE 13-21).

Animal-like protists Some protists propel themselves quickly around their environment and appear to hunt for prey. These animal-like protists, which include Paramecium, are the ciliates, and get their name from the cilia (hair-like projections) that cover their body surfaces and propel the cells through water. Paramecium feeds by the process of phagocytosis: cilia in a funnel-shaped structure called the gullet create an inward flow of water that carries bacteria and other small particles of food with it. These particles accumulate at the inner end of the gullet, where a portion of the plasma membrane bulges inward and eventually breaks free, forming a food vacuole that drifts into the interior of the cell. Enzymes and hydrochloric acid enter the food vacuole from the cytoplasm, and in this acidic environment the engulfed food items are broken down into molecules that diffuse from the vacuole into the cytoplasm. When all of the digestible material has been consumed, the undigested contents of the vacuole are expelled.

Fungus-like protists Some protists resemble fungi: living as heterotrophs (organisms that are unable to fix carbon through photosynthesis, but require carbon for growth), establishing sheet-like colonies of cells on surfaces (such as the grout in shower stalls), using spores to reproduce, and sometimes producing fruiting bodies. These are the slime molds. They generally spread without any individual cells moving, but rather by adding new cells at the edges of the colony. Some slime molds, however, called plasmodial slime molds, are oozing masses of gooey material that flow along a surface, engulfing bacteria, fungi, and small bits of organic material as they go. The streaming of a slime mold in its feeding phase is easy to observe with a microscope, and such a slime mold can flow around, over, or through almost anything—it can even flow through a window screen and reassemble itself on the other side! You may have seen an irregularly shaped blob of yellow material in a moist, shaded garden—that was a plasmodial slime mold.

Remarkably, a plasmodial slime mold is a very large, single cell (but has multiple nuclei). Slime molds divide by binary fission, just like other cells, but in this case a single cell may cover an area of several square centimeters! All of the nuclei in the cell undergo mitosis simultaneously.

Plant-like protists Other members of the protist kingdom grow in water and resemble plants. These include the protists referred to colloquially as algae and seaweeds. The term “seaweed” generally refers to the macroscopic, multicellular marine algae, many of which are used as a source of food for humans. And although they are all protists, seaweeds encompass several groups that do not share a common multicellular ancestor. These include some red algae and some green algae (from which the land plants most likely evolved), as well as some of the brown algae—such as the giant kelp that grows in water 30 meters deep. (The seaweeds, of course, represent parts of the protist kingdom that obviously are not “microbial.”)

Brown algae cover large portions of the rocks in the intertidal zone. Giant kelp, growing in temperate regions of the North and South Pacific and off the Atlantic coast of South Africa, are among the fastest growing organisms on earth, with some growing as much as 60 meters in a single year. Kelp forests are an enormously diverse habitat; more than 1,000 species of fishes, crustaceans, snails, and mammals make their homes in these marine “forests.” Sea otters wrap a kelp frond around their waist when they sleep, and gray whales hide in kelp forests to escape from killer whales.

Although many protists are unicellular, there are many exceptions. As we’ve seen, many green and brown algae are multicellular, composed of many different cells and cell types, performing different functions. Additionally, some other protists, such as spirogyra, are colonial, living as collections of cells, each of which can carry out all of its life processes independent of the other cells.

Also among the plant-like protists are the diatoms (FIGURE 13-22). Diatoms are unicellular organisms that live in ponds, lakes, and rivers as well as in the oceans. They are so small that, for some species, 30 individuals could be lined up across the width of a human hair. A characteristic of the diatoms is that they are enclosed in a shell made of silica. Many species of diatoms float in the water, forming part of the phytoplankton—the collection of microscopic organisms that fix carbon dioxide and release oxygen. Phytoplankton can reach densities of hundreds of thousands of cells per liter, and it accounts for about one-quarter of the photosynthetic production of oxygen on earth, occupying a critical position in marine food chains. Small fishes and shrimp-like organisms called copepods feed on phytoplankton, and these small predators are eaten by larger predators, which are eaten by still larger predators. Thus the diversity of life in the marine habitat relies on diatoms and the other microbes that make up the phytoplankton.