What Is the Primary Benefit to Phototrophic Microorganisms Associated with Corals

What is the primary benefit to phototrophic microorganisms associated with corals?

A) Calcium and carbonate ions are released, which help buffer the pH and prevent dramatic pH shifts.
B) The coral skeleton is used as a source of bicarbonate for the autotrophic growth of the phototrophs.
C) The coral skeleton is an extremely efficient light-gathering structure that greatly enhances light harvesting for photosynthesis.
D) The water temperature in and around the coral skeleton is much warmer than elsewhere.