Five Kingdom Monera Protista

Introduction — Why Classify Life?

If you set out to catalogue every living being on Earth, where would you begin? With millions of species already named and many more still undiscovered, the living world would be chaos without some way of sorting its members into orderly groups. The science of classification does exactly this — and over two thousand years it has undergone a remarkable evolution.

Aristotle — The First Scientific Attempt

The Greek philosopher Aristotle (~350 BCE) was the first to look at living things with a scientist's eye. He divided all organisms using simple, visible features (morphology):

• Plants were grouped by habit into herbs, shrubs and trees.
• Animals were separated into those with red blood (what we now call vertebrates) and those without red blood (invertebrates).

Linnaeus — The Two Kingdom System

Carolus Linnaeus, in the 18th century, formalised a Two Kingdom scheme: Plantae and Animalia. This system served biology for nearly two centuries but could not cope with the microscopic world it had barely begun to see. Its main shortcomings were:

• It lumped prokaryotes (bacteria) with eukaryotes.
• It placed photosynthetic organisms (green algae) with non-photosynthetic ones (fungi) in "Plantae".
• It ignored the deep gulf between unicellular and multicellular life.
• Organisms as different as a mushroom, a whale-shark and an oak were forced into just two kingdoms.

R.H. Whittaker — Five Kingdom Classification (1969)

The American ecologist R.H. Whittaker proposed a much richer scheme in 1969. He split life into five kingdoms — Monera, Protista, Fungi, Plantae and Animalia — on the basis of several criteria, not just shape:

• Cell type — prokaryotic or eukaryotic
• Cell wall — present/absent and its chemistry
• Body organisation — unicellular, colonial, multicellular (tissue-level or organ-level)
• Mode of nutrition — autotrophic (photosynthetic/chemosynthetic) or heterotrophic (saprophytic/parasitic/holozoic)
• Reproduction
• Phylogenetic (evolutionary) relationships

Comparison of the Five Kingdoms

2.1 Kingdom Monera — The Bacteria

Pick up a pinch of garden soil and you hold more bacteria than there are humans on Earth. Bacteria are the most abundant microorganisms known and the sole members of Kingdom Monera. Every cell is prokaryotic — no true nucleus, no membrane-bound organelles — and the cell wall is built from a unique peptide-sugar mesh called peptidoglycan.

2.1.1 Shapes of Bacteria

Under the microscope bacteria show four classic silhouettes:

2.1.2 Classification Based on Nutrition

(a) Archaebacteria — Life on the Edge

The archaebacteria are ancient lineages that thrive where almost nothing else survives. Their cell walls have an unusual chemistry that allows them to tolerate extreme environments:

• Halophiles — in hypersaline habitats such as salt pans.
• Thermoacidophiles — in hot, acidic springs and geysers.
• Methanogens — in the guts of ruminants like cows, where they produce methane (biogas).

(b) Eubacteria — The "True" Bacteria

These are the rigid-walled bacteria often with a flagellum for movement. They come in three nutritional styles:

• Cyanobacteria (blue-green algae) — photosynthetic, carrying chlorophyll-a; some fix atmospheric nitrogen (Nostoc, Anabaena). They multiply explosively in nutrient-rich polluted water to form "algal blooms".
• Chemosynthetic autotrophs — oxidise inorganic substances (nitrogen, iron, sulphur compounds) for energy; crucial recyclers of these elements.
• Heterotrophic bacteria — the most abundant group: decomposers, fermenters (curd from milk), nitrogen-fixers in legume nodules, as well as pathogens of cholera, typhoid, tetanus and citrus canker.

(c) Mycoplasma — The Smallest Living Cells

Mycoplasmas are the tiniest organisms known to be capable of independent life. Uniquely among bacteria, they lack a cell wall, can survive without oxygen, and several of them cause diseases in animals and plants.

2.1.3 Reproduction in Monera

Bacteria multiply mostly by binary fission. Under stress, many produce resistant spores. A primitive form of DNA transfer called conjugation allows two cells to swap genetic material through a protein bridge — a distant ancestor of sexual reproduction.

2.2 Kingdom Protista — The Unicellular Eukaryotes

Kingdom Protista gathers all single-celled eukaryotes. Its boundaries are notoriously blurry — some members look like plants (they photosynthesise), others like animals (they hunt), and a few behave like fungi (they decompose). Protists thus form an evolutionary bridge linking the three higher kingdoms.

2.2.1 Chrysophytes — Diatoms and Golden Algae

This group includes the beautiful diatoms and the freshwater desmids. Most are photosynthetic. Each diatom is encased in a two-valved cell wall made of silica, fitting together like a soap-dish. When diatoms die, their indestructible shells accumulate on the sea floor as diatomaceous earth. Diatoms are the chief producers in the oceans — the base of the marine food chain.

2.2.2 Dinoflagellates — The Whirling Drifters

Mostly marine and photosynthetic, dinoflagellates appear yellow, green, brown, blue or red depending on their pigments. Each cell carries two flagella — one longitudinal and one transverse — whose spin gives the group its name. When red species such as Gonyaulax bloom suddenly, the ocean turns rust-red — a red tide — and the released toxins can kill fish and even humans who eat shellfish from the affected water.

2.2.3 Euglenoids

Most euglenoids live in stagnant fresh water. They have no cell wall; instead the cell is wrapped in a flexible protein layer called the pellicle. Two unequal flagella (one long, one short) propel them along. In sunlight Euglena is photosynthetic, but in the dark it switches to a heterotrophic diet — a "plant-animal" hybrid.

2.2.4 Slime Moulds

Slime moulds are saprophytic protists of the forest floor. Their cells stream together to form a giant multinucleate mass called the plasmodium, which may stretch several feet under favourable conditions. When the habitat turns harsh, the plasmodium halts and produces resistant, air-dispersible spores.

2.2.5 Protozoans — The Animal-Like Protists

All protozoans are heterotrophs, living as predators or parasites. They are believed to be the ancestors of the animal kingdom. Four groups are recognised:

• Amoeboid protozoans — crawl on pseudopodia (Amoeba); some are parasites such as Entamoeba (amoebic dysentery).
• Flagellated protozoans — swim on flagella; Trypanosoma causes African sleeping sickness.
• Ciliated protozoans — densely covered with cilia, swimming rapidly; Paramecium.
• Sporozoans — have an infectious spore-like stage in their life cycle; Plasmodium causes malaria.