Credit: Image courtesy of Michael Lesser

The daytime orange glow of Caribbean coral is due to a symbiotic relationship with cyanobacteria, according to a report in Science.

Corals are known to require a symbiotic relationship with endosymbiotic dinoflagellates (zooxanthellae), which provide the coral with a source of carbon in the form of glycerol, but now it seems that a cyanobacterium is also involved in a 'three-way' symbiosis.

Michael Lesser and co-workers studied the Caribbean coral Montastraea cavernosa, which fluoresces orange during the day. In vivo excitation/emission spectra of the coral were found to be characteristic of absorption by the cyanobacterial protein phycoerythrin (a phycobilin protein that acts as a light-harvesting device in cyanobacterial photosynthesis), and the presence of this protein was confirmed by immunoblots of coral homogenates that were positive for the β-polypeptide of phycoerythrin.

The authors then used epifluorescent microscopy to identify many small orange-fluorescing cyanobacterial cells, and transmission electron microscopy showed that these cells are present in the epithelial cells of M. cavernosa. Furthermore, immunogold staining was used to demonstrate the presence of phycoerythrin in these cells, and 16S rDNA analysis identified a sequence characteristic of cyanobacteria, all of which point to the presence of cyanobacteria within the coral.

Life-time analyses of the fluorescence indicate that the phycoerythrin of these cyanobacteria is uncoupled from their primary photosynthetic apparatus. Phycoerythrin has been proposed to function as a storage pool for nitrogen in some cyanobacteria and, in attempts to determine whether these cyanobacteria provide a source of nitrogen for the coral, the authors used a polyclonal antibody to the 32-kDa subunit of the nitrogen-fixing enzyme nitrogenase, which gave a strong positive cross-reaction, showing that the nitrogenase enzyme is expressed in the coral.

Interestingly, detachment of phycoerythrin from the photosynthetic apparatus can be caused by glycerol — produced by the zooxanthellae. Taken together, these data indicate the existence of a three-way symbiosis in which corals, zooxanthellae and nitrogen-fixing cyanobacteria all have important roles.

Corals are found in nutrient-poor waters, so an important question in coral biology is how do they obtain nutrients, specifically nitrogen? The discovery of a 'three-way' symbiosis in this coral might provide the answer.