Which of the following is associated with biological nitrogen fixation?

1. A

   Red algae

   (a) Red algae. Rhodophyta are not the textbook biological nitrogen-fixers.

2. B

   Brown algae

   (b) Brown algae. Phaeophyceae (kelps) do not fix N2 as their defining ecology.

3. C

   Green algae

   (c) Green algae. Chlorophyta are photosynthetic eukaryotes, not the N-fixing group named here.

4. D

   Blue-green algae

   (d) Blue-green algae. Cyanobacteria (Anabaena, Nostoc, Trichodesmium) fix N2 in heterocysts or analogous cells. (d) Official key.

Summary. Official key is (d). Biological nitrogen fixation among the algae options is the cyanobacteria (blue-green algae). Red, brown and green algae are not that guild.