Protists are eukaryotic organisms that do not fit into the kingdoms of plants, animals, or fungi. Instead, they are grouped based on how they obtain energy and their similarities to other organisms. The main categories include animal-like protists (protozoans), plant-like protists (algae), and fungus-like protists. Protists are very diverse and vary in how they move, obtain energy, and reproduce.

Protozoans are animal-like protists that are highly diverse and found in many environments, especially in water. They are heterotrophs, meaning they must consume other organisms for energy, and they lack cell walls. Many protozoans feed on bacteria and other protists. They are often classified based on how they move.  Some protozoans move by changing their cell shape. For example, amoebas use pseudopods, or “false feet,” which are temporary extensions of the cell used for movement and feeding. Zooflagellates move using one or more flagella, while ciliates, such as the paramecium, move using tiny hair-like structures called cilia. These cilia beat in a coordinated, wave-like motion, helping the organism move and sweep food into the cell. Some protozoans, like apicomplexans, do not move on their own and are parasites that live inside hosts, feeding on their cells and body fluids.

Algae are plant-like protists that contain chloroplasts and use photosynthesis to make their own food. They are commonly found in aquatic or moist environments and vary greatly in size and complexity. Some algae are unicellular, while others form colonies or are multicellular, such as seaweeds. Most have cell walls and specialized pigments that help them capture sunlight, allowing them to thrive in a wide range of environments.

For example, euglenoids are single-celled protists that move using one or two flagella. Unlike most algae, they lack a cell wall and can switch between making their own food through photosynthesis and consuming food when light is unavailable.

In contrast, dinoflagellates are also unicellular and photosynthetic, but they do have cell walls made of cellulose.  They use two flagella for movement and are commonly found drifting in aquatic environments, sometimes forming large populations known as blooms. Another important group is diatoms, which are unicellular autotrophs with distinctive glass-like cell walls made of silica. Their intricate shells fit together like the top and bottom of a box and are often beautifully patterned.

While many algae are microscopic, some forms are much larger. Seaweeds, for instance, are multicellular marine algae that can be brown, red, or green. Even though they can grow quite large, they still lack true roots, stems, and leaves. In fact, most algae exist as phytoplankton, tiny organisms that drift in water and serve as the foundation of many aquatic food webs.

Fungus-like protists obtain nutrients by absorbing them from their surroundings rather than producing their own food. This group includes slime molds, water molds, and downy mildew, all of which are heterotrophic and reproduce using spores.

Among these, slime molds are often brightly colored and commonly found on decaying logs, where they help break down organic matter. Similarly, water molds live in moist soil or on the surface of water, feeding on dead material. Downy mildew, on the other hand, is a plant pathogen that can damage crops.

Together, these fungus-like protists play an important role in ecosystems by decomposing organic matter and recycling nutrients.

Review:

1. What are protists?
2. How do different protists move in their environment?
3. Identify characteristics of fungi.