Symbiosis of the Coral Host and its Dinoflagellates

Coral reefs are largely found in oligotrophic waters. For sessile organisms, meeting energy demands often requires some biological ingenuity. Corals have achieved this by evolving to have a life-long cooperation with single-celled dinoflagellate algae living in their cells, a unique type of mutualism called symbiosis. These algae (in the family Symbiodiniaceae) supply the coral host with nearly all of the nutrients it needs in exchange for a protected environment in which to live and reproduce. When corals are subject to prolonged thermal stress, the symbiotic relationship may break down, resulting in the phenomenon known as coral bleaching.

Corals can acquire their symbionts in two ways: vertical and horizontal transmission. Vertical transmission occurs when the coral receives its Symbiodiniaceae from its parent. Horizontal transmission is the case where young corals take up Symbiodiniaceae from the environment where they settle. When a coral settles in an environment with strong selective pressures, taking up the local symbiont may be advantageous if it is locally adapted.

Most research suggests that adult corals do not take up new symbiont types after acquiring the initial community. However, the relative abundances of different Symbiodiniaceae species can change over time—often in response to stress. Different subtypes, known as clades, exhibit high levels of genetic and physiological diversity. Some types appear to provide more nutritional benefit to the coral host while others are noted for their thermotolerance.

The relative contributions of the host and symbiont to the coral’s ability to withstand any number of stressors are poorly characterized. Symbiont shuffling in response to stress suggests that there is some cost to certain subtypes under a given set of environmental conditions, which has implications for coral health and survival. Our lab seeks to understand the role of the coral-algae symbiosis in determining coral responses to stress by examining which symbiont types dominate which corals under different conditions and across spatial gradients.