Fig. 4.

Micro-environmental gradients through a vertical cross-section of the skeleton of a massive coral species dominated by an Ostreobium algal band. During the day, sunlight gets depleted in the coral tissue with only small amounts of mostly far-red light reaching the skeleton. Oxygen levels are high near the tissue and in the endolithic algal zone due to photosynthesis, and this is also reflected in elevated pH in those zones. During the night, oxygen gets depleted from the skeleton by respiration and pH levels drop