Where enough light penetrates the water (up to a maximum depth of about 50m), one finds marine algae and plants, which use light to build up their own food. There are 3 main types of marine algae – brown, red and green.
The red algae are the ones to be found in the deepest water since they can use blue light which penetrates the most into the water column. Some of the red algae are rich in calcium and form rigid (non-soft) growths attached tightly to rocks and boulders. These encrusting (to cover or coat with, as if with a crust ) algae are known as coralline algae and it is impossible to detach such brightly-coloured algae from rocks with your bare hands.