In this article we will discuss about the position of prokaryotes in the living world.
Traditionally, the living organisms were placed in two kingdoms, the plants (Plantae) and the animals (Animalia).
Leeuwenhoek’s discovery of microorganisms and the gradual discovery of large number of diverse microorganisms made it impending to think about creating a third kingdom to accommodate those organisms, because they differed radically from both plants and animals.
Haeckel (1886) realized that simple microorganisms—like fungi, simple algae, protozoa and bacteria—were so much unlike the higher plants and animals that a third kingdom was necessary for accommodating them. He proposed that a kingdom, Protista, be created for these simple organisms.
The invention and refinement of electron microscope in the 20th century provided a powerful tool for biologists to study the fine structures of cells. It was soon realized that biological cells could be distinguished basically into two types. One kind of cells possessed well-organised double-membrane bound nucleus and several other membrane-bound cell organelles, while the other kind of cells, as those of bacteria, did not have these features.
The organisms having the first type of cells were designated as eukaryotic (eu = true, caryon = nucleus). All animal organisms including the lower ones, like protozoa, amoeba etc. as well as plants, both higher and lower ones, like bryophytes, algae and fungi, have eukaryotic type of cells. The second type of cell, called prokaryotic (cells with a primitive nucleus) forms the structural units of all bacteria and the blue-green algae (Cyanophyceae, now called Cyanobacteria).
Electron microscopy also revealed the structural details of viruses and it was realized that viruses do not have a biological cell as a unit of structure. Viruses are, therefore, acellular and they are neither prokaryotic nor eukaryotic. Strictly speaking, viruses are not organisms, though some biologists prefer calling them acellular organisms. On the basis of the knowledge available about living organisms, Whittaker (1969) proposed a classification comprising five kingdoms.
These kingdoms and their characteristics are briefly mentioned below:
Kingdom I. Animalia:
All multicellular animal organisms having eukaryotic cells without a rigid wall, and a heterotrophic, ingestive type of nutrition.
Kingdom II. Plantae:
All multicellular plants with eukaryotic cells invested within a rigid cell wall, and having mainly photoautotrophic, absorptive type of nutrition.
Kingdom III. Monera:
All prokaryotic organisms i.e. bacteria and cyanobacteria. The organisms could be unicellular or multicellular, cells bound generally by a rigid wall and having an absorptive type of nutrition. Their nutrition might be heterotrophic, photoautotrophic or chemoautotrophic.
Kingdom IV. Protista:
Eukaryotic organisms, like protozoa, algae, lower fungi, amoebae etc. The organisms are generally unicellular, sometimes colony- forming and might have a heterotrophic or photoautotrophic mode of nutrition. They may have an ingestive or absorptive type of food uptake.
Kingdom V. Fungi:
Multicellular higher fungi having eukaryotic cells, bound by a wall, often multinucleate mycelium and a heterotrophic absorptive nutrition.
The five-kingdom classification of Whittaker is schematically represented in Fig. 3.1:
In Whittaker’s classification, the prokaryotes were included in a single kingdom, Monera. But Carl Woese and his associates on the basis of their study of the ribosomal RNA sequence analysis claimed in 1970s that Monera did not represent a homogeneous group, rather they consisted of two basically different types of prokaryotic organisms, — the archaebacteria and eubacteria.
They proposed that living organisms should be divided into three major groups—called domains—which were above the kingdom level in the taxonomic hierarchy. The three domains were named as Archaea, Eucarya and Procarya.
Under these domains are seven kingdoms as shown below:
On the basis of r-RNA homology, Woese et. al (1993) built a universal phylogenetic tree to include all living organisms.
A simplified form of it is shown in Fig. 3.2:
Since the pioneering works of Woese, the archaebacteria and the eubacteria have been recognized as two major distinct groups of prokaryotic organisms. On the other hand, the eukaryotic microorganisms have presented a problem in classifying them into homogeneous groups.
In Whattaker’s five-kingdom classification, all of them were dumped in a highly heterogeneous assemblage—Protista.
Cavaler-Smith proposed an eight-kingdom classification which included two Empires — Bacteria and Eucaryota.