The kingdom Protist is an odd collection of organisms, including organisms resembling fungi, plants and animals. All protistans are eukaryotes, and many are unicellular. Otherwise, it is difficult to develop broad characteristics, as there is such great diversity within the group.

Some protists are similar to fungi in that they are heterotrophs (often saprobes, meaning they digest food outside of their bodies and then absorb the nutrients after external digestion), have cell walls (often containing chitin, as in true fungi) and often display hypae (root-like structures). Some are water molds that cause diseases such as "ich" in aquariums. Others live in moist conditions, such as the mold responsible for the potato blight in Ireland.

A number of protistans have animal-like characteristics. These groups include the amoeboids, ciliated protozoans, flagellates, and sporozoans. All of these organisms are unicellular, heterotrophic, lack a cell wall, and eukaryotes. Unlike many ‘animals’, they commonly reproduce through binary fission (asexual) but can also go through sexual reproduction.

The amoeboids are the ‘blob’ critters often viewed under microscopes. They are mobile, eukaryotes that move by extending a pseudopodia (a false foot consisting of cell membrane and flowing cytoplasm). They engulf food by a process known as phagocytosis. Members of this group make up some of the material in plankton communities. Amoebic dysentery is a disease caused by this group of organisms.

Cilia are extensions from the cell membrane. Paramecium (and other ciliated protozoans) move by beating many cilia simultaneously. Additionally, they have a contractile vacuole (used to move excess water against an osmotic gradient), allowing them to inhabit a wide range of environmental conditions. Like most of the animal-like protists, they usually undergo asexual reproduction during the process of binary fission. In the process of conjugation, organisms are able to go through limited sexual recombination. In this process, extrachromosomal DNA (micronuclei) can be exchanged, leading to new gene combinations in the next generation.

Protozoans that move with flagella and pseudopods. Many water sources are infected with the organism Giardia, a protistan that moves through the use of a single flagella. These organisms are often persistent because they can form cysts, which is the encapsulated nucleus with limited cytoplasm, that are able to withstand periods of unfavorable environmental conditions. For example, even if a water source doesn’t provide a useful habitat for a protist, they can form a cyst that will ‘hatch’ later, when conditions are more ideal.

This is a vitally important group for human health. Sporozoans must live part of their lives in the cells of another organism. The disease malaria is caused by a protistan that goes through different life stages within the human body. The organism can be transferred in the saliva of certain mosquitoes. These organisms can also form persistent cysts, capable of withstanding adverse conditions for great deals of time. The feces of cats contain on such cyst that can cause the disease toxoplasmosis, which can cause birth defects or a miscarriage for pregnant women. Thus, they are warned to avoid stray cats and not change litter boxes.

Algae are the plant-like protist. They contain photosynthetic pigments and chloroplasts, but they don’t have the tissue found in other plants (phloem, xylem, etc). Again, we don’t spend as much time on this group of organisms as we will on the ‘true’ plants, but this doesn’t mean they aren’t important. Phytoplankton are responsible for a great deal of photosynthesis on the planet, and they form the base of many food chains. Overall, 75% of our planet is water, so algae, not trees, are responsible for most of the photosynthesis and primary productivity on planet earth.

Euglena are odd organisms (like many protist) in that they have a flagella for movement, a contractile vacuole to regulate osmotic pressure and chloroplasts for photosynthesis. They can even move towards light to maximize photosynthesis due to the presence of a light-sensing cells.

When using a ‘buffing agent’ for cleaning surfaces, you are often using the remnants of ancient algae known as diatoms. These algae have calcareous shells that last for millions of years. Diamtoms are found in local ponds and remarkable for their amazing silica shells. Another interesting group of algae are the dinoflagellates. They have flagella, cellulose and chloroplasts. These organisms are responsible for the ‘red tides’ sometimes observed in coastal areas. The algae release toxins to slow fish and then go through multiple stages to feed off the sloughed cells and finally the dead fish. These organisms are an excellent example of the dual nature of ‘plant-like’ and ‘animal-like’ features of some protist.

Red algae are most common in marine environments. They exist as long strands in leaf-like structures, and attached to rocks on coral reefs. They are important producers in coral reefs. The most important reference in this section is figure 23.19. This alternation of generations is seen in mosses, ferns, fungi and other plants, so learn it now. In short, the organism alternates between a haploid generation, called the gametophyte generation (produces gametes), and a diploid sporophyte generation (produces spores through meiosis).

The brown and green algae are important groups, especially for aquatic ecosystems. Many of these are multicellular and look much like modern land plants (kelp, for example). However, the ‘leaves’ are actually blades and the ‘stems’ are stipes. A true leaf is not made of the same material as a blade, so this is a case of convergent evolution (two distinct evolutionary lines converging upon a common design to meet a similar need, such as collecting sunlight).