Today's biologists today use a system to name and classify living organisms that was developed by the Swedish botanist Carolus Linnaeus in 1758. In this system organism names fall into seven nested categories or taxa. The seven taxa are kingdom, phyla, class, order, family, genus, and species.
Until the 1960's, biologists thought there were two kingdoms: plants and animals. Fungi such as mushrooms were included with plants. However, closer study of fungi revealed that they do not use the sun's energy as their immediate source of energy (i.e., they do not photosynthesize). Fungi get their food energy by using enzymes to break down other living or dead material; they also have a cell structure that is different from plants and animals. Thus, improved understanding of fungi prompted biologists to create a new kingdom.
Likewise, for many years the ecology of the oceans was primarily studied using fine mesh nets and optical microscopes to count and identify the plankton. Unfortunately, the organisms that went THROUGH the nets were discounted as being insignificant. In the 1970's and 1980's improved methods and tools were developed which allowed scientists to quantitatively study the oceans' smallest organisms. Since then, better detection of the abundance, diversity and activity of such microorganisms has led to a major restructuring of our concept of food webs and how organisms within it should be classified.
MICROSCOPIC DISCOVERIES

When microscopes allowed the discovery of tiny bacterial cells, scientists were not sure if they were plants or animals. As microscopy and scientific tests improved, bacteria emerged as totally different from plant or animal cells. They lack many of the subcellular structures -- such as nucleus -- that exist in plants, animals, and fungi. So now there is another kingdom, Monera, that includes bacteria, blue-green algae, and simple cells. Some members of the Monera kingdom make their own food by using pigments and chemical systems to photosynthesize or chemosynthesize; others use food molecules produced by other living things.

About forty years ago, questions arose about tiny photosynthetic organisms such as single-cell Euglena, a "green alga" (>>). When conditions permit, it photosynthesizes like a plant; in other instances, it uses its flagellum (i.e., whip-like hair) to swim and find food particles which it takes into its gullet and slowly digests like an animal. So, scientists pondered, what major group or kingdom should Euglena and similar cells be categorized? A new kingdom, Protists, was designated. By the 1970's, scientists decided that maybe all life could be categorized into five kingdoms: Monera, Protists, Fungi, Plants, and Animals.
THE NEW "DOMAINS"
In the 1990's scientists investigating the biochemistry of a group of organisms discovered a new organism that is very different from any known cells. As of 1999 biologists generally agreed that based on cell structure, function, and biochemistry there are three "domains": Archaea, Prokaryote (the Monerans), and Eukaryote (everything else) into which all living things can be classified. (NOTE: Viruses are not cells, and are not considered to be "alive." they consist of a protein coat around a clump of genetic material. In order to replicate or reproduce, virus particles must inject their genetic information into living cells. The viral genetic material uses the internal components of the living cell to reproduce the viral genetic material.)
As you read this, major breakthroughs are occurring in the biochemical genetics of plants. Such new discoveries may throw the current classification system into chaos. Scientists have agreed to not begin renaming plants... yet.

Adapted from Sea Soup Teacher's Guide: Discovering the Watery World of Phytoplankton and Zooplankton, Copyright 1999, by Betsy T. Stevens, Tilbury House, Publishers. Inquiry-based activities for use with Sea Soup: Phytoplankton and Sea Soup: Zooplankton, children's picture books by Mary R. Cerullo, photography by Bill Curtsinger, Tilbury House, Publishers

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