Simplify the following text for a Grade 7 native English-speaking person.
Original: Five hundred species of marine algae or seaweed live on the reef , including thirteen species of genus Halimeda , which deposit calcareous mounds up to 100 m wide , creating mini-ecosystems on their surface which have been compared to rainforest cover .
Simplified: These deposit chalky ( calcareous ) mounds up to 100 metres wide . They have mini-ecosystems on their surface which have been compared to rainforest cover .