Moderate Thermal Stress Causes Active and Immediate Expulsion of Photosynthetically Damaged Zooxanthellae (Symbiodinium) from Corals

Abstract

The foundation of coral reef biology is the symbiosis between corals and zooxanthellae (dinoflagellate genus Symbiodinium). Recently, coral bleaching, which often results in mass mortality of corals and the collapse of coral reef ecosystems, has become an important issue around the world as coral reefs decrease in number year after year. To understand the mechanisms underlying coral bleaching, we maintained two species of scleractinian corals (Acroporidae) in aquaria under non-thermal stress (27°C) and moderate thermal stress conditions (30°C), and we compared the numbers and conditions of the expelled Symbiodinium from these corals. Under non-thermal stress conditions corals actively expel a degraded form of Symbiodinium, which are thought to be digested by their host coral. This response was also observed at 30°C. However, while the expulsion rates of Symbiodinium cells remained constant, the proportion of degraded cells significantly increased at 30°C. This result indicates that corals more actively digest and expel damaged Symbiodinium under thermal stress conditions, likely as a mechanism for coping with environmental change. However, the increase in digested Symbiodinium expulsion under thermal stress may not fully keep up with accumulation of the damaged cells. There are more photosynthetically damaged Symbiodinium upon prolonged exposure to thermal stress, and corals release them without digestion to prevent their accumulation. This response may be an adaptive strategy to moderate stress to ensure survival, but the accumulation of damaged Symbiodinium, which causes subsequent coral deterioration, may occur when the response cannot cope with the magnitude or duration of environmental stress, and this might be a possible mechanism underlying coral bleaching during prolonged moderate thermal stress.

The first case of disease of the sponge Lubomirskia baicalensis: Investigation of its microbiome

“…Fluctuations of water temperature, light intensity, salinity, and infec tions are listed among the causes of imbalance in sym biotic communities of corals and sponges (Webster et al, 2008;Luter et al, 2010;Fan et al, 2013;Fujise et al, 2014). It is known that environmental factors control growth and development of the main group of sponge endosymbionts-unicellular algae (Sand Jensen and Pedersen, 1994), and microbial communi ties are most sensitive to changes in habitats including stress (Webster et al, 2008).…”

Cell Cycle Dynamics of Cultured Coral Endosymbiotic Microalgae (Symbiodinium) Across Different Types (Species) Under Alternate Light and Temperature Conditions

“…) to regulate symbiont density in hospite (Dimond and Carrington ; Fujise et al. ; Jones and Yellowlees ). However, premitotic control of individual cells has not been well‐described and hence it is not clear whether and if so how this control mechanism contributes to the maintenance of a functional equilibrium between symbionts and host cells.…”

Developmental symbiosis facilitates the multiple origins of herbivory

“…Developmental constraint and facilitation in holobionts are entangled to a degree where it is difficult to separate them. Similarly, in the coral-algal symbiosis, increased water temperatures cause the death of the coral by facilitating the release of its plant symbiont (Fujise et al, 2014;Gates, Baghdasarian, & Muscatine, 1992). Just as the nutritional symbiosis between the pea aphid and Buchnera bacteria has allowed the aphid to use an abundant and under-utilized resource-plant sap, it made the aphid dependent upon that mode of feeding.…”