Functionality of different Symbiodinium clades

Functionality of different Symbiodinium clades

Coral reefs are founded on the symbiotic relationship between the coral animals and photosynthetic algae (Symbiodinium a.k.a. zooxanthellae). Symbiodinium is a genus of dinoflagellate which contains many different edosymbiotic algae found in a diverse group of host organisms, primarily tropical cnidarians, but also other groups such as molluscs and sponges. Within the Symbiodinium genus there are numerous clades, it has been reported that single organisms can harbour one or more clades at any one time. Whilst the molecular diversity of the Symbiodinium is well documented, the functional significance of such diversity has received less attention. In the paper reviewed, Loram et al. investigated the functional significance of different zooxanthellae clades in the giant anemone Condylactis gigantean. It is important to increase our understanding on the physiological functioning of zooxanthellae in order to learn more about bleaching. This directly relates to the Adaptive Bleaching Hypothesis (ABH) proposed by Buddheimer & Fautin, (1993) which says that when environmental conditions change the loss of one or more kinds of zooxanthellae is rapidly followed by the formation of a new consortium to adapt. In other words coral, or in this case the closely related anemone, can pick and choose their zooxanthellae to help them adapt to different environmental situation. The ABH relies heavily on assumptions that different zooxanthellae clades are functionally diverse and have varying ability to cope with environmental parameters such as irradiance and temperature.

Loram et al. (2007) genetically identified Symbiodinium clades A and B in their test organisms; 32% of organisms contained clade A, 20% contained clade B and 48% contained a mixture of both A and B. An experiment was sent up to have two environmental situations, 25 °C represented a control and 30°C represented elevated temperature. As a measure of nutritional function, the photosynthetically fixed carbon in the algae was measured as well as the translocation of fixed carbon into the animal tissues. This parameter was measured by radio isotope analysis of ¹⁴C. The paper presents its results in an overcomplicated way, I feel to disguise the lack of biologically relevant statistical significance. Loram et al. make statements such as “carbon fixation rates per algal cell differed among the clades, with total fixation rates elevated at 30 °C for symbioses bearing clade A and depressed for symbioses bearing B” although close inspection of the results table reveal that this difference is not statistically significant. Overall, and most importantly, the results do indicate that clades A and B are functionally different, although the biological significance of this is perhaps over interpreted in the discussion section.

This paper fits nicely into one of our lectures with Emma “Coral Health and The Environment” and builds on work presented in the lecture by Rowan et al. (1997) and Rowan (2004). I decided to review the paper as it focuses is on  an anemone rather than the reef building corals, and although the corals are vitally important and should never be underestimated, I feel that other interesting reef species get overlooked. I think the way the paper has used genetic techniques for I.D. and more traditional experimental method to assay functionality works really well. Often too much emphasis is put onto the genes present and genomic potential without any physiological evidence of function. It is important to note that this study was done five (ish) years ago now in a time when "holobiont" research increased rapidly. It would be interesting to review more recent work to see how the field has progressed.

Loram, J. E., Trapido-Rosenthal, H. G., & Douglas, A. E. (2007). Functional significance of genetically different symbiotic algae Symbiodinium in a coral reef symbiosis. Molecular ecology, 16(22), 4849–57.

 

http://www.ncbi.nlm.nih.gov/pubmed/17868294