General Account of Protoctista

The word protists, from the Greek meaning “the very first”, i.e. protists were the first eukaryote to evolve. Kingdom Protoctista (protista) consists of a vast assortment of primary organisms whose diverse body forms, types of reproduction, modes of nutrition and modes of life styles makes them difficult to characterize. Protists are unicellular or simple multi-cellular organisms that possess a eukaryotic cellular organization. It includes organisms which resemble early plants i.e. algae and early animals i.e. oomycota. It also includes a group of organisms known as slime molds which produce spore like fungi but can creep slowly over surfaces and therefore motile like animals.

 

Historical Prospective

Kingdom Protoctista was proposed for microscopic organisms by John Hogg in 1861. In 1866 Ernst Haeckel suggested creating another kingdom Protista to include micro-organisms that do not fit into plant or animal kingdom e.g. bacteria, Euglena etc. He, however, separated the prokaryotes from nucleated protists and placed them in a group called Monera within the kingdom protista.

 

In 1938 Herbert Copeland formed the kingdom Prokaryotes. Only unicellular eukaryotes were placed in kingdom protista by Robert Whitaker in 1969, who introduced Five Kingdom System. Margulis and Shawartz in 1982 modified the five kingdom system. Protista or Proctista is one of the five kingdoms having 27 phyla.

 

Size

Protists vary considerably in form from microscopic protozoa to giant kelp (brown algae), which can reach sixty meters in length. Although most protists are unicellular, some have a colonial organization, some are coenocytes (multinucleate but not multicellular) and still others are multicellular. Multicellular protists have simple body forms without specialized tissues.

 

The polyphyletic group

Kingdom protista is a polyphyletic group of organisms, that is, protists do not share a single ancestor. Eukaryotic organisms that are not considered as fungus, plants or animal are placed in the protist kingdom.

 

Nutrition

Methods of obtaining nutrients differ widely in kingdom protista. The autotrophic protists, e.g. the algae have chlorophyll and photosynthesize as plants do. Some of the heterotrophic protists, the water molds, obtain their food by absorption as fungi do. Other heterotrophs i.e. the protozoa and slime molds resemble animals i.e. they ingest food derived from the bodies of other organisms.

 

Mode of life

Many protists are free living while others form symbiotic association with different organisms. These associations range from mutualism, a more or less equal partnership in which both organisms benefit, to parasitism in which one organism lives on or in another and is metabolically dependent on it. Most protists are aquatic and live in oceans or fresh water. They make up a part of the plankton.

 

Reproduction

It is quite varied in the kingdom protists. All protists reproduce asexually and many also reproduce sexually with both meiosis and syngamy (the union of gametes). However most protists do not develop multicellular sex organs, nor do they form embryos.

 

Locomotion

Most protists are motile at some stage of their life cycle and have various means of locomotion. Movement may be accomplished by amoeboid motion i.e. extending cell protrusions by waving cilia or by lashing flagella. Many protists use a combination of two or more means of locomotion e.g. both flagella and amoeboid motion.

 

As a source of food

Because of their huge number, kingdom Protista is an important source of food for other organisms. Photosynthetic protists also supply oxygen to aquatic and terrestrial ecosystems. Certain protists are economically important while others cause diseases. The three major groups of kingdom protista are plant like protists (algae), fungi like protista and animal like protists.