With a little help from endosymbiotic algae

Scleractinian corals have been known to contain endosymbiotic dinoflagellate algae from the genus Symbiodinium, in return being provided with photosynthetic products, contributing to the corals carbon requirements. Photosynthesis in these photosynthetic algae also provides valuable amino acids, and also increases calcification rate in the skeletons of the corals. This has led to the belief that endosymbiotic algae may affect the growth rates in corals.   

Sulphur-containing compounds are found to be important in many organisms, but just how this sulphur is used in corals is yet to be studied in depth. It has been suggested however, that these compounds may be used for a sulphate and sulphur-containing amino acid transport system between corals and symbiotic algae.

In a previous study by Yayama et al (2011), their results including the identification of a gene encoding a sulphate transporter in Acropora tenuis led to the suggestion that sulphate utilization by the coral is enhanced by its symbiotic associations with algae. These findings have led to the current study by Yayama et al (2016), in which they aimed to look at the how corals and endosymbiotic algae use environmental sulphate ions.

This was done with autoradiography using ³⁵S-labelled sulphate ions (³⁵SO₄^2-). After the exposure to ³⁵SO₄^2-, the ³⁵S that had been incorporated were shown as brown dots in the coral sections. These sections exposed for 6 and 12 hours showed abundant ³⁵S grains in symbiotic algal cells, nematocytes and ectodermal cells. Incorporation of ³⁵S in endosymbiotic algae and coral skeletons increased during the incubation period, but the amount in coral soft tissue didn’t change significantly. Incorporation of ³⁵S concentrations in coral soft tissue were overall higher than in endosymbiotic algae, except after 2 days of non-exposure. The distribution of ³⁵S radioactivity varied significantly between the algae and coral tissue, and increased during times of incubation.

The incorporation of ³⁵S under dark (non-photosynthetic) and light (photosynthetic) conditions was also looked at. The radioactivity of ³⁵S in soft tissue and algae, chemical fractions and skeletons, was greater under light conditions than dark conditions. A significant difference between the two conditions however was only seen in the algal total fraction, with a significantly higher quantity of ³⁵S being incorporated under light conditions.

The results seen in this study give evidence to support the theory that there is a close relationship between transported sulphate and photosynthesis in symbiotic algae, and that endosymbiotic algae take up ³⁵SO₄^2- from the seawater, which is supported by the overall increase of incorporation rate of coral tissue under photosynthetic conditions. This suggests that the photosynthesis of algal endosymbionts contributes to the synthesis and utilization of sulphur compounds in corals.

In my opinion, this study was relatively easy to follow and stuck to their aims by using seemingly fitting methods. It must be taken into account however, that because of the lack of studies and therefore lack of understanding of the mechanisms of the coral-algae symbiotic relationship, the utilization and metabolism of sulphur in corals isn’t properly understood at present. I think that due to this the studies done by Yayama et al have provided a wide range of directions for future study, and that the applications of sulphur within this endosymbiotic relationship may have many more uses than have been explored already.

Reviewed paper: Yuyama, Ikuko., Higuchi, Tomihiko., and Takei, Yoshio. (2016). Sulfur utilization of corals is enhanced by endosymbiotic algae. Biology open. 5: 1299-1304. http://bio.biologists.org/content/biolopen/5/9/1299.full.pdf