Algae to form coral reefs.
green
fire
brown
red

RED algae help to form coral reefs.

Red algae contains the pigment phycoerythrin which gives out the red color of the algae; it absorbs blue light and reflects red light.

The Red algae that help to form coral reefs is the coralline algae. The coralline algae secretes a hard shell of carbonate around themselves, which in turn becomes a part of the coral reefs.

Giga is the second highest prefix on the normal scale, the first is terra.