Anabaena is a:

Anabaena is a filamentous cyanobacterium. Cyanobacteria were traditionally called blue-green algae, because they contain the green pigment chlorophyll a and a blue pigment called phycocyanin, and they carry out photosynthesis like plants. However, they are prokaryotes, without a true nucleus, so in the five-kingdom classification of R.H. Whittaker (1969), they are placed in the Kingdom Monera, along with bacteria.

Anabaena forms long chains (filaments) of cells, looking like beads on a string. Some of the cells in the chain become larger, thick-walled cells called heterocysts. Heterocysts carry out nitrogen fixation, converting nitrogen gas from the air into ammonia, which plants can use. Nitrogen fixation needs the enzyme nitrogenase, which is destroyed by oxygen, so the heterocysts keep oxygen out.

Anabaena is important in agriculture:

It lives in a symbiotic relationship inside the leaves of Azolla, a small floating water fern. Azolla with Anabaena is grown in rice fields in India and other Asian countries as a biofertiliser, supplying nitrogen to the rice crop.

Free-living cyanobacteria such as Anabaena and Nostoc also fix nitrogen in paddy fields, reducing the need for chemical fertilisers.

Anabaena also lives with the roots of cycads.

Cyanobacteria were among the first organisms on Earth to produce oxygen by photosynthesis, more than 2.5 billion years ago, and helped create the oxygen-rich atmosphere. Some cyanobacteria, such as Microcystis, form harmful "blooms" in polluted water. Spirulina, another cyanobacterium, is used as a protein-rich food supplement.

Why not the other options?

• ✗Mosses are bryophytes, a type of plant.
• ✗Protozoans are unicellular animal-like protists, such as Amoeba.
• ✗Good to know
• ✗Fungi do not carry out photosynthesis.