Protista
Introduction
Protista, also known as Protoctista, is a diverse group of eukaryotic microorganisms, excluding the plants, animals, and fungi. Protists are usually single-celled organisms, but there are exceptions, and they are found in virtually any environment that contains liquid water. Many protists, such as the algae, are photosynthetic and are vital primary producers in ecosystems, particularly in the ocean as part of the plankton.
Classification
The classification of protists has always been problematic, and they were traditionally subdivided into several groups based on similarities to the "higher" kingdoms such as the Protozoa, the algae, the fungi, the plants, and the animals. However, modern molecular studies have shown that many of the traditional groups of protists are not monophyletic.
Characteristics
Protists are incredibly diverse and can exhibit many different types of metabolic strategies, cell structures, and life cycles. Some protists, such as the algae, are photosynthetic, others are heterotrophic and consume organic materials (like amoebae) to obtain nutrition, and others are mixotrophic. Many protists, such as the paramecia, are motile and move using cilia or flagella. Others, such as the amoebae, move using pseudopods.
Ecology
Protists play key roles in ecological communities. They are particularly important in aquatic ecosystems, where they often form the base of the food web. Many protists are also symbionts and live in association with other organisms. For example, many species of foraminifera and radiolaria have photosynthetic symbionts and obtain energy from these algae.
Evolution
The evolutionary history of protists is still not completely understood. However, it is clear that protists include several independent lineages, and they are not a natural group at all. Instead, they are a paraphyletic group, meaning that they include some, but not all, descendants of their most recent common ancestor.