The most significant importance of the kingdom Protista is as primary producers, and thus food sources, for other organisms, as well as generators of oxygen. Single-celled oceanic algae, which are members of Protista, actually produce the majority of atmospheric oxygen via photosynthesis. Protista are also important as pathogens, consumers and decomposers. In a few cases, as with coral and termites, they are also crucial symbiotes.
The kingdom Protista is not a well-defined group of related organisms, as are the other taxonomic kingdoms. Most single-celled organisms with a true nucleus, along with a number of multi-cellular algae, are classified as protists. However, a few fungi, specifically yeasts, are also single-celled. Protista is a kingdom of organisms that do not fit in any other kingdom.
Kingdom Protista consists of three major categories, none of which is very closely related to the others. The first, Protozoa, is somewhat animal-like, in that the organisms are heterotrophic and mobile. This group includes organisms such as amoeba and paramecia. The second group is the algae, which includes both single-celled plankton and multi-cellular seaweeds. Algae are incredibly important as photosynthetic producers, arguably even more important than plants. Finally, slime molds are somewhat fungus-like since they are decomposers that absorb nutrients from their environments. Slime molds actually grow as giant single cells with multiple nuclei, moving from place to place much like amoebae do.