In terms of the demand for CO2(aq), an enlarged endosymbiont population increases the likelihood of CO2(aq) becoming a limiting internal substrate during periods of peak photosynthesis [18, 19]. Several environmental factors favour increased zooxanthellae densities (particularly on a per host cell basis), including: (i) elevated nutrient levels (e.g. dissolved inorganic nitrogen, DIN) in the surrounding sea water , elevation of the CO2 partial pressure (pCO2) in the surrounding sea water , and diffusive (i.e. branching) coral colony morphologies . Experimental manipulations confirm the higher expulsion rate of zooxanthellae during periods of high irradiance in branching corals  and in corals exposed to DIN and pCO2 enrichment [24, 25].