Zooxanthellae and Their Relationship with Corals

What are Zooxanthellae?

“Zooxanthella” (plural: “zooxanthellae”) is the colloquial term used to describe the symbiotic golden-colored dinoflagellates that reside within animals (such as corals, anemones, clams, etc.). On a taxonomic level, “zooxanthellae” is synonymous to members of the family Symbiodiniaceae. Zooxanthellae associated with scleractinian (i.e., stony) corals are typically 5 - 20 µm (with most being <11 µm) in diameter and specifically reside within membrane-bound vacuoles located within cells of the gastrodermis of the coral polyp. While members of the family Symbiodiniaceae share many similarities, they are also very diverse, with species varying significantly in their morphology, adaptations, environmental tolerances, host preferences, and regional distribution, among other factors (LaJeunesse et al., 2018; Muller-Parker et al., 2015; Toller et al., 2001).

Microscopic image of zooxanthellae (Cladocopium sp.)
​© Aquaholic Aquaculture

Zooxanthella and Coral Symbiosis

​Zooxanthellae form a symbiotic relationship with corals in a partnership known as mutualism, in which the phototrophic zooxanthellae partner with the heterotrophic host coral. Through photosynthesis the zooxanthellae convert sunlight, inorganic nutrients (derived from either the coral’s metabolic waste products or through uptake of dissolved inorganic compounds from the water column), and carbon dioxide into carbon and energy sources for the coral, and, in return, the coral provides a safe habitat for the zooxanthellae. The zooxanthellae consume some of their photosynthetically fixed carbon for their own respiratory and growth requirements, but the rest of the carbon is made available for utilization by the host coral. While corals are able to acquire some nutrition through holozoic feeding (e.g., capturing and consuming zooplankton), the rest of the coral’s nutrition is dependent upon the photosynthetic products derived from its zooxanthellae (Muller-Parker et al., 2015; Toller et al., 2001). Through photosynthesis, zooxanthellae provide up to 90% of a coral’s energy demand, playing a vital role in coral nutrition and health (Mashini et al., 2015).
 
Recent research by LaJeunesse et al. (2018) estimates that this mutualistic relationship between corals and their microalgal symbionts has existed for over 140 million years and has survived multiple extinction events, with rDNA sequencing tracing ancestors of modern-day corals and their symbionts back to the middle to late Jurassic period of the Mesozoic Era. It is theorized that this algal-animal relationship originally evolved in response to corals being unable to attain sufficient nutrition solely from holozoic feeding, especially in harsh environments (Muller-Parker et al., 2015).

How do Corals Get their Zooxanthellae?

How corals acquire their initial population of zooxanthellae depends on whether the coral is reproduced asexually or sexually.
 
For asexually produced corals (i.e., produced via clonal fragmentation), the new coral fragment already contains zooxanthellae from the parent colony. As the coral fragment grows, these zooxanthellae multiply asexually, densely populating the coral tissues. This mechanism for acquisition of zooxanthellae is known as direct transmission. In direct transmission the zooxanthellae acquired are identical clones to those of the parent colony (Muller-Parker et al., 2015).

Microscopic image of the zooxanthellae of an Acanthophyllia polyp
​© Aquaholic Aquaculture

For sexually produced corals, they can either acquire zooxanthellae directly from the parent colony or indirectly from their environment. In direct transmission via sexual reproduction, the new coral is established from either an egg or larvae containing zooxanthellae from the parent colony. However, most coral eggs do not have zooxanthellae in them. Rather, most sexually produced corals obtain zooxanthellae indirectly from their environment (i.e., indirect transmission). In indirect transmission, corals can acquire zooxanthellae in one of two ways: (1) Via motile zooxanthellae in the surrounding seawater (through chemotaxis), or (2) Via the ingestion of fecal matter that contains zooxanthellae (from predators that have consumed prey containing zooxanthellae). Indirect transmission enables the new coral to establish a population of zooxanthellae that is genetically distinct from its parents (Muller-Parker et al., 2015).
 
While direct transmission can only occur at the ‘birth’ of the new coral, research has shown that corals are able to indirectly acquire zooxanthellae throughout their lives. Further, a coral can acquire zooxanthellae of different species that can reside within the same host coral simultaneously (Muller-Parker et al., 2015). Many corals contain multiple species of zooxanthellae at any given time, and are flexible in the species of Symbiodiniaceae that they contain (Baker et al., 2004), with corals “shuffling” (increasing the population of an already present background symbiont) or “switching” (uptaking a new species of symbiont from the environment) species of zooxanthellae as environmental conditions require (LaJeunesse et al., 2009; Muller-Parker et al., 2015).

What is Coral Bleaching,
and How Does it Affect the Coral and its Zooxanthellae?

​Coral bleaching is a phenomenon in which a host dissociates from its symbionts when stressed. When corals bleach, they lose the zooxanthellae that give them their color, and they turn white. Coral bleaching is an indicator that the coral is under significant stress, and it is correlated with coral death (LaJeunesse et al., 2018; Muller-Parker et al., 2015).
 
In order for corals and zooxanthellae to maintain their symbioses, the benefits from the zooxanthellae’s photosynthetic production must outweigh the metabolic cost of maintaining the zooxanthellae. When the cost of sustaining the zooxanthellae becomes too great and the mutualistic relationship is no longer advantageous to both parties, the mutualism is disrupted, and the coral expels its zooxanthellae (LaJeunesse et al., 2018; Muller-Parker et al., 2015). There are many factors that can influence the balance of this symbiotic relationship and thus affect coral bleaching, including but not limited to: Fluctuations in temperature, salinity, nutrients, sediments, turbidity, air exposure, rainfall, and light intensity, as well as respective growth rates of the host coral and its symbionts. Additionally, both the severity of the stressor as well as the duration of the stressor affect whether or not a coral succumbs to bleaching (LaJeunesse et al., 2018; Muller-Parker et al., 2015; Toller et al., 2001).

Can Corals Recover from Bleaching?

​Yes, corals can recover from bleaching. Bleaching alone is not a death sentence. When the stressor that caused the bleaching event is removed and favorable conditions return, corals can reestablish symbioses with zooxanthellae and get their colors back (i.e., re-brown). Corals can reestablish symbioses with zooxanthellae in one of two ways: (1) Residual background zooxanthellae populations that survived through the bleaching event can multiply and/or (2) The coral can establish new symbiotic relationships with free-living zooxanthellae from the water column (i.e., indirect transmission) (Muller-Parker et al., 2015). Once symbioses are reestablished, it can take some time for the coral to re-brown and appear healthy again; augmentation of zooxanthella populations to standard healthy coral densities usually takes 1.5 – 3 months (LaJeunesse et al., 2009).
 
Recent research has demonstrated that the unique adaptations of different species of Symbiodiniaceae affect a coral host’s ability to persevere through and/or recover from bleaching events, with specific species of Symbiodiniaceae being better suited to survive and thrive in particular situations. As such, a coral may “shuffle” or “switch” symbionts to meet its needs within its specific environment (LaJeunesse et al., 2009; Muller-Parker et al., 2015).

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References

[1] Baker, A. C., Starger, C. J., McClanahan, T. R., & Glynn, P. W. (2004). Corals' adaptive response to climate change. Nature.
 
[2] LaJeunesse, T. C., Parkinson, J. E., Gabrielson, P. W., Jeong, H. J., Reimer, J. D., Voolstra, C. R., & Santos, S. R. (2018). Systematic revision of Symbiodiniaceae highlights the antiquity and diversity of coral endosymbionts. Current Biology, 28(16).
 
[3] LaJeunesse, T. C., Smith, R. T., Finney, J., & Oxenford, H. (2009). Outbreak and persistence of opportunistic symbiotic dinoflagellates during the 2005 Caribbean mass coral 'bleaching' event. Proceedings of The Royal Society. 276(1676).
 
[4] Mashini, A. G., Parsa, S., & Mostafavi, P. G. (2015). Comparison of Symbiodinium populations in corals from subtidal region and tidal pools of northern coasts of Hengam Island, Iran. Journal of Experimental Marine Biology and Ecology, 473.
 
[5] Muller-Parker, G., D’Elia, C.F., & Cook, C.B. (2015). Interactions between corals and their symbiotic algae. In: Birkeland, C. (eds) Coral Reefs in the Anthropocene.
 
[6] Toller, W. W., Rowan, R., & Knowlton, N. (2001). Zooxanthellae of the Montastraea annularis species complex: patterns of distribution of four taxa of Symbiodinium on different reefs and across depths. Biological Bulletin, 201(3).