Microorganisms

Microorganism: An organism which cannot be seen by naked eyes is called microorganism or microbe. We need a microscope to see a microbe.

Types of Microorganisms

Bacteria: Bacteria are microscopic organisms which have cell walls but do not have a nucleus. Some bacteria are autotrophs but most are heterotrophs. Bacteria are found in different shapes; like rod-shaped, spiral, spherical and comma-shaped. Examples: Rhizobium, Pseudomonas, Streptococcus, etc.
Algae: Algae are green plants which are simple in structure. A cell of algae contains cell wall and chloroplast. Algae are autotrophs. Some algae are microscopic. Examples: Chlamydomonas, Spirogyra, etc.
Fungi: Fungi have cell wall and nucleus in their cells but do not have chloroplasts. Fungi are saprotrophs, i.e. they feed on dead remains of plants and animals. Almost all fungi are microscopic, except mushrooms. Examples: yeast, Rhizopus (bread mould), Aspergillus, Penicillium, etc.
Protozoa: Protozoa do not have cell wall and chloroplast but they do have a nucleus. Amoeba and Plasmodium are examples of protozoa.