The Microbes That Turn Cow Dung Into Cooking Gas

For a long time, living things were sorted into just two groups — plants and animals. But bacteria, mushrooms and a single-celled Euglena that both photosynthesises and swims simply do not fit that split.

A better system uses several features at once to group organisms, and it begins with the simplest cells.

This part covers the five-kingdom system, Kingdom Monera, Mycoplasma and bacterial reproduction, and Kingdom Protista.

The two-kingdom system lumped together organisms that differ fundamentally — prokaryotes with eukaryotes, unicellular with multicellular, and photosynthetic with non-photosynthetic — so Whittaker's five-kingdom system used five criteria: cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships.

Problems with two kingdoms:

- Bacteria (prokaryotes) were placed with plants alongside eukaryotic trees
- Fungi, which have no chlorophyll and have chitin in their walls, were counted as plants
- Unicellular and multicellular organisms sat in the same kingdom

The five criteria:

- Cell structure — prokaryotic or eukaryotic
- Body organisation — unicellular or multicellular
- Mode of nutrition — autotrophic or heterotrophic
- Reproduction
- Phylogenetic relationships

The five kingdoms: Monera, Protista, Fungi, Plantae and Animalia.

Worked example — placing organisms.

- A bacterium: prokaryotic, unicellular — Monera
- Amoeba: eukaryotic, unicellular — Protista
- A mushroom: eukaryotic, heterotrophic, chitin walls — Fungi
- A mango tree: eukaryotic, multicellular, autotrophic — Plantae
- A dog: eukaryotic, multicellular, heterotrophic, no cell wall — Animalia

An everyday example. A wardrobe with only two shelves — one for clothes, one for books — leaves shoes and school bags jammed in wherever they fit; more shelves sort them properly.

The substance. No single feature decides a kingdom — nutrition, cell type and body organisation are used together.

Monera contains prokaryotes, divided into archaebacteria, which live in extreme habitats, and eubacteria, the true bacteria, which are grouped by shape as coccus, bacillus, vibrio and spirillum and by nutrition as photosynthetic, chemosynthetic or heterotrophic.

Archaebacteria:

- Halophiles — very salty places
- Thermoacidophiles — hot springs
- Methanogens — marshy areas and the guts of cattle, where they produce methane

Their cell wall structure differs from that of eubacteria, helping them survive such conditions.

Eubacteria by shape: coccus (spherical), bacillus (rod-shaped), vibrio (comma-shaped), spirillum (spiral).

Eubacteria by nutrition:

- Photosynthetic autotrophs — cyanobacteria such as Nostoc and Anabaena, with chlorophyll a; some fix nitrogen in special cells called heterocysts, and they can form blooms in polluted water
- Chemosynthetic autotrophs — oxidise inorganic substances such as nitrates, nitrites and ammonia to obtain energy, helping recycle nitrogen, phosphorus, iron and sulphur
- Heterotrophic bacteria — the most abundant; decomposers, curd formation from milk, antibiotic production and nitrogen fixation in legume roots, but also diseases such as cholera, typhoid and tetanus

An everyday example. Dahi sets overnight because bacteria in a spoonful of old curd multiply and turn milk sugar into acid.

The substance. Cyanobacteria are bacteria, not true algae, even though they are often called blue-green algae.

Mycoplasma are the smallest known living cells and lack a cell wall entirely; bacteria in general multiply by fission, form resistant spores in bad conditions, and can transfer DNA from one cell to another, which together explain their huge success.

Mycoplasma:

- No cell wall at all
- The smallest living cells known
- Can survive without oxygen
- Many cause diseases in plants and animals

Reproduction in bacteria:

- Binary fission — one cell splits into two
- Spores — formed when conditions are unfavourable
- DNA transfer — a primitive, sexual-like exchange of DNA between cells

Worked example — the power of fission. Suppose a bacterium divides every $20$ minutes. In $4$ hours it divides $12$ times:

$$2^{12}=4096\text{ cells from one}$$

With enough food, numbers climb so fast that bacteria can colonise almost any habitat.

Why DNA transfer matters. Passing DNA between cells can spread useful traits, such as resistance to an antibiotic, through a population.

An everyday example. Cooked dal left out on a hot summer day can spoil by the next morning, as bacteria multiply by fission into huge numbers.

The substance. DNA transfer in bacteria is not true sexual reproduction — no gametes are formed and no fertilisation occurs.

Protista are single-celled eukaryotes, mostly aquatic, and their main groups differ in cell covering, movement and nutrition — from photosynthetic diatoms and dinoflagellates to wall-less euglenoids, saprophytic slime moulds and heterotrophic protozoans.

- Chrysophytes (diatoms and desmids) — photosynthetic; diatom walls contain silica and fit together like two halves of a soapbox; their deposits form diatomaceous earth used in polishing and filtration; major producers in the oceans
- Dinoflagellates — mostly marine and photosynthetic; stiff cellulose plates on the cell wall; two flagella, one along the body and one in a groove around it; some, such as Gonyaulax, multiply rapidly to cause red tides whose toxins can kill fish
- Euglenoids — freshwater; no cell wall but a flexible, protein-rich pellicle; two flagella; Euglena photosynthesises in light but feeds on other organisms in the dark
- Slime moulds — saprophytic; cells gather into a mass called a plasmodium, which forms fruiting bodies with tough-walled spores that survive long periods
- Protozoans — heterotrophs: amoeboid (Amoeba, Entamoeba), flagellated (Trypanosoma, sleeping sickness), ciliated (Paramoecium), and sporozoans (Plasmodium, malaria)

An everyday example. Malaria spread by mosquito bites is caused by Plasmodium, and amoebic dysentery from contaminated water by Entamoeba — both protozoans.

The substance. ***Euglena behaves like a plant in light and like an animal in darkness*, which is why protists are seen as a bridge between kingdoms.