Protists
Introduction
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy. Protists live in almost any environment that contains liquid water. Many protists, such as algae, are photosynthetic and are vital primary producers in ecosystems, particularly in the ocean as part of the plankton.
Characteristics
Protists are a diverse group of organisms, comprising those eukaryotes that cannot be classified within the other kingdoms. They are usually microscopic and unicellular, but some protists are multicellular, others are macroscopic. Protists can be either heterotrophic or autotrophic. Some protists, like the diatoms and brown algae, are photosynthetic. Others, such as the amoebae, ingest other organisms, while still others, such as the sporozoans, absorb nutrients directly from their hosts.
Classification
The classification of protists is still debated. They were traditionally subdivided into several groups based on similarities to the "higher" kingdoms such as the Protozoa, the lower algae, the slime molds, and the water molds. However, modern molecular systematics research has shown that many groups of protists are related to plants, animals or fungi, and that some protists do not fit comfortably into any one of these groups.
Evolution
Protists are thought to be some of the most ancient organisms on Earth. The oldest known protists were the acritarchs, microfossils found in 1.7 billion-year-old rock. The acritarchs continued to diversify and dominate the marine ecosystem until the rise of the algae.
Ecology
Protists have a wide range of ecological roles. Photosynthetic protists, such as the green algae, euglenids and diatoms, form the base of the food chain in many ecosystems. Other protists, such as the ciliates and amoebae, are important grazers of bacteria and other microorganisms. Parasitic protists, such as the kinetoplastids and apicomplexans, can cause serious diseases in humans and other animals.
Impact on Humans
Some protists are beneficial to humans and the environment, such as those that contribute to nutrient cycling in ecosystems. Others are harmful, such as the parasitic protists that cause diseases like malaria and sleeping sickness. Protists also have commercial applications; for example, they are used in aquaculture and are a source of nutraceuticals and pharmaceuticals.