Practice question

Reef-building corals live in an intimate nutritional mutualism with intracellular dinoflagellates of the family Symbiodiniaceae, commonly called zooxanthellae. The algae photosynthesize and transfer a substantial fraction of fixed carbon to the coral host, supporting respiration, growth, and rapid calcium-carbonate deposition in nutrient-poor tropical water. Corals supply carbon dioxide, nitrogenous wastes, phosphorus, and a protected, well-lit cellular habitat. This reciprocal metabolic exchange explains the high productivity of reefs despite low surrounding nutrient concentrations. Many reef corals depend strongly on the association, so it is conventionally described as obligate mutualism, although dependence varies among coral and symbiont species and some corals can feed heterotrophically. Heat or other stress can disrupt the partnership, causing bleaching through symbiont loss; prolonged bleaching often leads to starvation and mortality. Commensalism ignores the algal contribution, parasitism misstates the usual net benefit, and amensalism predicts unilateral harm. “Obligate” here means essential under normal ecological conditions, not that every participant is incapable of any independent existence.