by Kimberly Coombs
Corals share a mutualistic relationship with algal symbionts, but with increasing sea temperatures, these symbionts become expelled from the coral. The loss of symbionts causes the corals to become bleached and there have been declines of coral cover worldwide. Recent research has shown that there may be symbionts that are thermotolerant, such as the genus Symbiodinium, which may help reduce the amount of bleaching episodes seen amongst corals. Symbiodinium is divided into nine subgeneric clades, A-I, and the Symbiodinium D1a has been documented showing thermotolerance.
Silverstein et al. (2015) explored the impacts of increased sea temperatures of corals that contain different Symbiodinium clades. They collected colonies of Montastaea cavernosa, which they exposed to either a heat stress of 32 or herbicide DCMU at 24 for ten days. After ten days, the corals entered a recovery period of three months before half of them were exposed to the same stressor for another ten days and then placed back into a three-month recovery. Before the corals underwent their bleaching treatments, they contained Symbiodinium clade C. In both treatments, corals lost the same proportion of symbionts, and corals with a higher abundance of symbionts before treatments lost a larger proportion of them. Symbiodinium clade D was discovered to dominate the type of symbionts observed in corals after the three-month recovery period.
When corals were exposed to a second bleaching treatment, Silverstein et al. found bleached corals that developed Symbiodinium clade D1a lost fewer symbionts than corals still only containing Symbiodinium clade C from the first treatment. Between the two exposures (heat stress and herbicide DCMU), the amount of Symbiondinium clade D1a lost was the same amongst the corals; however, corals that had a higher abundance of clade D1a symbionts lost a greater proportion of them. Corals who also contained more clade D1a symbionts lost less of their symbionts than corals who contained clade C symbionts. During the second recovery phase, Symbiondinium clade D dominated the symbionts observed in the corals.
Corals that remained unbleached were found to contain low amounts of Symbiodinium clade D1a, which caused them to experience more acute bleaching when exposed to heat stresses. Silverstein et al. discovered that corals acclimated to 29 also did not gain any thermoteralance when exposed to a higher temperature of 32. Thus, the corals that bleached in the first phase gained an advantage during the second phase of the bleaching treatment. Bleached corals gained a higher amount of Symbiodinium clade D1a that helped them be more resistant to bleaching and loss of symbionts in the second phase of bleaching.
Silverstein et al. observed that corals that had developed clade D symbionts and did not undergo the second phase of bleaching treatment, reverted back to clade C symbionts. Symbiodinium clade D may be thermotolerant, yet they cause the corals to grow more slowly, experience lower fecundity, and be less effective in translocating sugars.
Therefore, clade D symbionts may be useful in aiding the corals under high heat conditions, but when conditions are less stressful, clade C symbionts are more efficient for coral function.
Silverstein, R.N., Cunning, R., Baker, A.C., 2015. Change in algal symbiont communities after bleaching, not prior heat exposure, increases heat tolerance of reef corals. Global Change Biology, 21, 236-249.