A review of: Wagner D., Pochon, X., Irwin, L., Toonen, R.J., and Gates, R.D., 2011, Azooxanthellate? Most Hawaiin black corals contain Symbiodinium, Proceeds of the Royal Society B, 278, 1323-1320.
A mesophotic coral reef ecosystem (MCE) can be defined as a reef that is located below the limits of normal SCUBA diving (40m) and includes the deepest portion of the photic zone at 150m. These ecosystems are away from anthropogenic and natural stresses and both photosynthetic and non-photosynthetic organisms are found here. This paper concentrates on the gorgonian and antipatharian coral groups (hexacorallia), which were previously thought to be azooxanthellate corals. Antipatharian corals are more commonly known as the black corals and 75% of these occur deeper than 50m, and the other 25% occur in shallower waters but still occupy locations such as crevices where the light levels are limited. Due to the locations of these species, it has historically been inferred that they do not contain the photosynthetic dinoflagellate Symbiodinium although this has never been demonstrated. This paper aims to use both histological and molecular analyses to check for the presence of Symbiodinium in a range of antipatharian corals around Hawaii.
53 samples were taken, from14 species, 9 genera and 5 families of black corals from a range of depths from 10 to 396m. These samples came from both museum specimens and recently collected samples using SCUBA and manned submersibles. Three different analysis methods were used: histological, molecular and chlorophyll autofluorescence and the samples were preserved differently depending on the method to be used. The molecular data was found by precipitating and amplifying the DNA using PCR methods. This DNA was then sequenced and checked for the presence of Symbiodinium sequences. The histological data was found by removing coral tissues from the skeleton. It was then infiltrated with paraffin wax and poured into a mould. These moulds were then cut into sections, stained and placed under a microscope with a camera attachment. The checks for chlorophyll autofluorescence were carried out by placing tentacle tissue under a con-focal microscope and using blue light excitation to make the chlorophyll apparent.
Using the DNA analyses, Symbiodinium were found in 43% of the samples, and 71% of the different species and these were found in the full range of depths down to 396m. The histological and chlorophyll autofluorescence evidence showed that the Symbiodinium was present in the gastrodermal tissue indicating that there is endosymbiosis occurring, not just associations with the surface or gut. The Symbiodinium found was from clades D and C, and were either identical or closely related to those commonly found in Hawaii and the Pacific. There was no relationship between the species of coral and clade of Symbiodinium showing that the acquisition of symbionts is opportunistic and not specific. The symbionts were founds to be in low densities in the tissues, which indicates that they don't fix carbon or play an important role in coral nutrition. This combined with the lack of light, shows that corals must rely more on heterotrophy than autotrophy as the depth of their location increases. This evidence suggests that this symbiosis is not based on nutrition and further research is required to find out why a symbiosis still exists. Could it be that it is present purely for historical reasons and has just been conserved by evolution?
Despite all of these findings, the most surprising is that of how deep Symbiodinium can be found. This discovery of Symbiodinium present at 396m is the deepest record to date. At these depths there is very little to no light, so photosynthesis cannot occur. So how does it survive? Could it be that the carbon demand is reduced due to dormancy at extreme depths or is it met by other means? What is shown, is that this dinoflagellate has an extremely diverse habitat preference and that we know very little about coral symbiosis at depths that are not accessible to normal SCUBA.
4 comments:
Wow, this seems areally fascinating discovery. Do the authors say anything about how these corals acquire their symbionts? Evidence from other corals shows that broadcast spawners pick up their zoox in the larval stage.
It seems very odd that a relationship that confers no mutual benefits would be maintained. If zoox are not providing carbon at great depths, perhaps they are contributing some important factor of benefit to the coral? Amicro-nutrient maybe?
I really enjoyed reading this paper. Very fasinating.
Reading through the discussion its seems to suggest that there is a possibility that the Symbiodinium could have a parasitic relationship with the coral. Although this is only a theory it seems plausible that the symbiotic relationship would become parasitic as the lack of light would leave the Symbiodinium with nothing to give.
Hi Natasha,
Very interesting article! I have had the privilege to dive with these black corals in Balicasag Island, Philippines. The dive site is aptly named black forest and the Antipatharia can be seen at a shallow depth of 20-25 metres,although I could only look from 17 metres as I am but a novice and do not hold an advanced PADI licence. It is rare to see them in such shallow waters but the island's shape and steep wall (which is great diving) shades the sun most of the time, thereby emulating light conditions (or lack of) at farther depths were they are typically found. They aren't exactly as colourful as their shallow depth counterparts but certainly a novel experience nonetheless.
It is very strange that they have zooxanthellea. A commensalism relationship perhaps?
Hi,
It seems very strange to me to! The authors made a definite point that they consider the symbiosis to be maintained purely from an evolutionary sense and haven't lost this symbiosis yet. However, if this is the case, we might expect to find some mutations that can survive without their symbionts, and in the future this symbiosis would surely die out. I think there must be some other function of this symbiosis but the authors don't seem to offer any ideas! Further research needed!
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