Thermally tolerant symbionts: adaptive saviors or opportunists?

Written by Tim Bateman

The symbiotic dinoflagellates (a type of microalgae) that live inside scleractinian corals (corals that secrete calcium carbonate) form the basis of the coral reef ecosystem and have allowed them to persist throughout geologic time. Some of these symbionts of the family Symbiodiniaceae (formerly Symbiodinium LaJeunesse et al. 2018) are specialists and only form symbioses with specific species while others are generalists that will associate with a variety of coral hosts. Symbiodiniaceae can vary markedly in their benefit to the coral host by translocating different amounts of energy under different environmental conditions. This is becoming increasingly relevant as climate change is causing stress events to become more frequent and has prompted further research on the taxonomic and functional disparities between strains of symbiont and their performance under climate change stresses.

Coral hosts rely on their symbionts for up to 90% of the carbon they require for daily metabolic needs. Physiological stress, such as that caused by increased temperature or ocean acidification, can reduce the carbon produced by symbionts and thus reduce the energy translocated to their coral hosts. In the algae, physiological stress manifests as the breakdown of the photosynthetic machinery, which prevents the continued production of energy from sunlight (photosynthesis). This reduction in available energy along with other stress-induced byproducts ultimately causes the breakdown of the symbiosis and coral bleaching.

Some symbionts, such as many of those from the genus Durusdinium (formerly clade D), have increased thermal tolerance and do not lose physiological functional at high temperatures. Indeed, after a bleaching event in the Gulf of Mexico, a study found an increase in the presence of Durusdinium in partially bleached corals indicating that these colonies had taken up more thermally tolerant symbionts. This could be interpreted as an advantage for these colonies, however, there was an unexpected factor that came with these thermally tolerant symbionts, and that was a reduction in carbon translocation from the algae. This means that although these symbionts can survive and maintain photosynthesis at higher temperatures, they translocate less carbon, and reduce growth at a variety of temperatures (Fig 1. Pettay et al. 2015). One hypothesis behind this phenomenon is that Durusdinium are relative newcomers to the Caribbean and thus have not adapted a relationship with the corals in this region as other resident symbionts have (Pettay et al. 2015).

Picture1
Fig. 1 Results of photosynthetic rates and instant calcification among different coral-symbiont combinations. (A) Orbicella spp. harboring Symbiodinium trenchii (Photo by Justin Kemp). (B) Photosynthetic rates for S. trenchii and three undescribed species (A3, B17, and C7). (C) Calcification rates at different temperatures (bar colors correspond to species as shown in (B)).

Thermally tolerant symbionts do not display lower carbon translocation at all times, however. In the Indo-Pacific, Durusdinium is relatively common and found in symbiosis with a large variety of corals and not only during thermal stress events, although they are more common in warmer habitats. In these warmer habitats, they can maintain photosynthetic function better than other thermally sensitive species but also translocate a similar amount of carbon to the host, eliminating the trade-off seen in the Caribbean. For example, a study conducted in Palau, Micronesia demonstrated that corals from a warmer inshore habitat hosting Durusdinium trenchii maintained photosynthesis and cell densities during a heat stress experiment better than offshore corals containing thermally sensitive symbionts (Figure 2, Warner LaJeunesse and Kemp 2016).

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The resilient relationship between Durisdinium trenchii and coral species in an adverse environment has prompted my lab, in collaboration with other institutions, to undertake a series of reciprocal transplant experiments to assess the trade-offs associated with hosting this thermally tolerant symbiont. Manipulating host-symbiont combinations in different environments allows us to examine the effects of key biotic influences on the physiological performance of stress tolerant symbiotic relationships. With the results from this research, we hope to provide information about how symbioses will respond to climate change and how different host-symbiont combinations will impact the ecological function and success of reefs in the future.

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Fig. 3 Dr. Dustin Kemp and Dr. Warner mount coral fragments on a platform at one of two transplant reef sites. These corals will later be collected for thermal stress experiments (Photo by Robin Smith).

References

Baker, A. C., et al. “Corals’ Adaptive Response to Climate Change.” Nature 430.7001 (2004): 741-41.

LaJeunesse, Todd C., et al. “Systematic Revision of Symbiodiniaceae Highlights Antiquity and Diversity of Coral Endosymbionts.” Current Biology 28 (2018): 2570-80.

Pettay, D. T., et al. “Microbial Invasion of the Caribbean by an Indo-Pacific Coral Zooxanthella.” Proceedings of the National Academy of Sciences of the United States of America 112.24 (2015): 7513-18.

Warner, Mark E., LaJeunesse, Todd C., Kemp, Dustin. NSF Proposal 1636022. (2016).

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