The Kingdom Protista includes several phylogenetically distinct groups, associated together because they are all simple eukaryotes. Like more complex eukaryotes, protists have membrane-bound organelles and a defined nucleus. However, they are not organized into complex tissues, as are higher plants and animals. For ease, we will divided them into the larger categories of algae, protists and slime molds. Algae are plant-like photoautotrophs and have cell walls. Algae are difficult to define, and several different classification systems exist. In this SparkNote, we will treat all eukaryotic photoautotrophic microorganisms as algae. This includes five groups: green algae, red algae, brown and yellow-brown algae, dinoflagellates, and euglenoids. Algae are extremely important as producers in the world's oceans and lakes. Protozoa are animal-like. Protozoa lack cell walls and are eukaryotic heterotrophic microorganisms. They include ciliates, kinetoplastids, and sarcodina. Many protozoa live as parasites within other animals. Slime molds are unusual microorganisms that resemble protozoa at some phases of their life cycles and fungi at other phases. The two groups of slime molds, cellular and acellular, are not closely related. Slime molds live in damp soils and decaying plant material.
The animal-like protists are single-celled or colonial organisms called protozoa. They live in fresh and salt water, in the soil, and in the bodies of other organisms. All protozoa are heterotrophic. Some absorb nutrients through their cell membranes, whereas others engulf larger particles of food. Most protozoa are motile. Amebas are unicellular organisms that continually change shape and engulf food particles. Amebas reproduce asexually by binary fission (cell division). They are commonly found in freshwater ponds, lakes, and streams. Another group of the animal-like protista is called zooflagellates. Trypanosoma gambiense is the protista responsible for African sleeping sickness in humans. These microorganisms are spread by the tsetse fly. Plasmodium is a protozoa that is parasitic and causes malaria. The spores from this parasite invade the red blood cells of the human host, multiply there, then break out and invade new cells. The destruction of the red blood cells releases toxic cell wastes into the bloodstream. These waste products cause fever, chills,and other symptoms of malaria. Malaria is a serious, sometimes fatal, disease. Although it can be treated with drugs, one method of prevention is to eliminate the Anopheles mosquito. In spite of the widespread use of pesticides in many countries, millions of people are still infected with malaria, especially in tropical areas.
The plant-like protists, commonly called algae, resemble plants because they are all photosynthetic. Like the protozoa, algae are very diverse. Some are tiny, single-celled organisms with flagella. Others are large, multicellular organisms like seaweed. Like plants,algae have chloroplasts, which contain the photosynthetic pigment chlorophyll. Another group in this category is called the euglenoids. These single-celled protists have both plant-like and animal-like characteristics. Like plants, they contain chloroplasts and photosynthetic pigment. However, they do not have cell walls. Like some of the protozoa, euglenoids move by means of flagella. One typical euglenoid is euglena, an organism common in pond water. The euglena is a single-celled organism having two flagella. The cell has a large, central nucleus and numerous chloroplasts, which contain chlorophyll. Chlorophyll gives euglenas their grass-green color. Euglenas are primarily photosynthetic. However, in the absence of light, they live as heterotrophs, absorbing dissolved nutrients from the environment.