Impact of nutrient deficiency on the proliferation and digestion of photosynthetic symbionts in corals.
Supervisor: Joerg Wiedenmann (SOTON, UK)
Coral reefs are one of the most diverse ecosystems on Earth despite being restricted to nutrient-poor waters. The answer to this conundrum, known as Darwin’s paradox, lies in the symbiotic association between the coral animal and the dinoflagellate algae, which re- quires intricate and coordinated growth. The production of carbon-rich compounds by the symbiotic partner and their translocation to the coral host can only partially explain the suc- cess of corals in nutrient-depleted waters, as these compounds are not sufficient to entirely supply the nutritional demands of the coral host. Here we show that symbiont digestion is a crucial mechanism by which corals regulate algal growth and fulfill their nutritional re- quirements. We found that both the division and digestion of symbionts exhibit dampened diel patterns. Furthermore, we observed that depending on the coral species, nutrient defi- ciency can significantly impact either the rate of symbiont division or the rate of symbiont digestion, destabilizing the equilibrium between both processes and causing the corals to bleach. These results demonstrate that symbiont digestion is critical for nutrient cycling as it allows the coral animal to incorporate both organic and inorganic nutrient sources. In ad- dition, in conditions of nutrient deficiency, the coral host can meet its nutritional demands by relying on symbiont digestion. This study helps to narrow the gap in the understanding of how coral reefs will respond to future changes.