Zooxanthellae Podcast

3.24.6 So i have just published my first podcast through talking reef (thanks Rob) and I am excited to hear how it is received. Coral people, aquarists and researchers alike, are all becoming increasingly aware of the importance of the coral-algae symbiosis in terms of coral husbandry and reef ecology. This episode is a nice distillation of the advances in our understanding of this relationship over the past few years. Many thanks to the Schmidt lab for hosting me recently and to Dr. Dan Thornhill for participating in the interview. As mentioned in the segment, I will be attending Symbiofest in Athens in about a month and I hope to record material for an additional segment soon. As always, love to hear what everyone thinks. coralite at gmail dot com.

Transcription of Part 1
Zooxanthellae is a group of unicellular algae which are paramount in providing a food source for important reef-building corals. Most zooxanthellae are classified as dinoflagellates but some zooxanthellae groups are actually chrysophytes or diatoms. Zooxanthellae is not a taxonomic term as much as descriptive term which means golden-brown animal. The genus that is of most importance to corals is Symbiodinium which is characterized by a round or ellispoidal shape which measures 2 to 17 microns across. In its pelagic form the cells have two flagellae for motility but when they live symbiotically they do not develop flagellae.

Zooxanthellae can be found in symbiotic relationships with sponges, flatworms, clams, anemones, jellyfish and ascidians (sea squirts). Zooxanthellae can be intracellular, growing within the cells, such as in corals or they can be extracellular, growing outside the cells, such as in giant clams of the genus Tridacna. In corals, zooxanthellae occur in special vacuoles with up to seven zoox cells per vacuole. They occur in all cells of the endoderm but they are concentrated in the cells which are exposed to the most light such as the oral disc, tentacles and coenosarc, the thin tissue covering the skeleton between polyps. The density of zooxanthellae can range from thousands to millions of cells per square centimeter.

Corals which contain zooxanthellae are called symbiotic, autotrophic or hermatypic. Symbiotic means they have a mutual relationship with another organism, autotrophic means that it can make its own food and hermatypic means that they are reef-building corals. Most of the corals that aquarists will be familiar with fall under most of these categories.Some symbiotic invertebrates are called facultative autotrophs which means that they can be found with or without zooxanthellae. Sometimes the association is seasonal but even when being symbiotic is “in season”, symbiotic and non-symbiotic colonies can be found growing right next to each other. In these species, the contribution of zooxanthellae to the overall biomass is marginal whereas in obligate symbiotic corals such as Pocillopora damicornis, the zooxanthellae can make up 45 to 60% of the coral's biomass.

For many years coral zooxanthellae was considered to be a single species, Gymnodinium microadriaticum. In time, researchers came to distinguish several types of zooxanthellae through painstaking and time consuming morphological examination. Thanks to molecular analysis techniques, we now know that there are about 100-150 types of zooxanthellae and most can be identified within a few days. The groups are broken down into clades designated by letter from A to G although clades A through D are the most frequently reported. It is important to note that although most corals contain a single dominant symbiont type, corals can harbor several types of symbionts in one colony. The distribution of symbionts can vary either from season to season or it can vary across the shape of a large individual colony with different symbionts being concentrated either in the most or the least illuminated parts of the coral.

I could get into the specifics of zooxanthellae genetics and characteristics of different clades but a recent article by Dana Riddle has already done a fabulous job of that. Mr Riddle also took the time to compile a database of symbiotic coral species and the zooxanthellae types they associate with. As we come to understand the subtle differences in how our corals behave based on the dominant zooxnthellae type, I believe this list will grow and become more widely considered when grouping different coral species. The article is available here and the database can be downloaded here.

The only addition I would like to make is about Mr Riddle's description of Clade D which will also touch on some of the prevailing views about coral bleaching and how it pertains to zooxanthellae in general. Coral bleaching is a phenomenon wherein during periods of abnormally high temperatures and reduced water flow, corals expel their golden brown symbionts and they appear very pale or white. There has been a hypothesis put forth called the adaptive bleaching hypothesis which suggests that during bleaching events, corals expel their symbionts in order to repopulate their tissues with new strains of symbiodinium which may be more resistant to high temperature stress. This hypothesis was supported by the fact that following bleaching, the zooxanthellae which repopulates coral tissues are often from clade D. Although it is true that this clade is more tolerant of high temperature this endurance comes at a metabolic cost and therefore, under normal environmental conditions this clade contributes less nutrition to it's host and in general, clade D symbionts will generally be succeeded by a reversion to the original symbiont type.