Penicillium is a:

Penicillium is a genus of fungi commonly known as blue or green mould. It grows on decaying fruits, bread, cheese and other organic matter. A saprophyte is an organism that gets its food from dead and decaying organic matter. It releases digestive enzymes into the material, breaks it down outside its body and absorbs the simpler substances. Most fungi, including Penicillium, Mucor and Rhizopus (bread mould), are saprophytes. In this way they act as decomposers, returning nutrients to the soil.

Penicillium has a body made of thread-like structures called hyphae, which together form a mycelium. It reproduces asexually by spores called conidia, which are produced in brush-like chains (the name comes from the Latin penicillus, meaning "painter's brush").

Penicillium is famous for giving the world its first true antibiotic. In 1928, the Scottish scientist Alexander Fleming noticed that a mould, Penicillium notatum, growing on a culture plate of Staphylococcus bacteria had killed the bacteria around it. He named the active substance penicillin. Later, Howard Florey and Ernst Chain developed methods to purify and produce it in large amounts during the Second World War. Fleming, Florey and Chain shared the Nobel Prize in Physiology or Medicine in 1945. Penicillin works by stopping bacteria from building their cell walls.

Some species of Penicillium are used to make cheeses: Penicillium roqueforti for Roquefort and other blue cheeses, and Penicillium camemberti for Camembert and Brie.

Why not the other options?

• ✗Penicillium is not a bacterium; it is a eukaryotic fungus.
• ✗Green algae, such as Chlamydomonas and Spirogyra, make their own food by photosynthesis.
• ✗Lichens are a symbiotic association of an alga and a fungus.