Background
The authors write that "zooxanthellae (symbiotic dinoflagellates of the genus Symbiodinium) are critical to the survival of reef-building corals, providing a major source of energy from photosynthesis for cell maintenance, growth and reproduction of their coral hosts," which services, as we have long noted (Idso et al., 2000), include the energetically-expensive process of calcification.

What was done
Cantin et al. studied the amount of photosynthetic "rent" paid by two different clades of Symbiodinium (C1 and D) to their coral hosts (juvenile Acropora millepora) for the privilege of living within the latter's calcareous "houses." This was done by measuring the "financial transfer" to nine-month-old corals that had been developed "from crosses involving the same parent corals," which "planned parenthood" minimized any host genetic differences that might otherwise have influenced the physiology of the host-symbiont "lease agreement."

What was learned
The five researchers report that "Symbiodinium C1 exhibited a 121% greater capacity for translocation of photosynthate to A. millepora juveniles along with 87% greater relative electron transport through photosystem II under identical environmental conditions." In addition, they note that "A. tenuis and A. millepora juveniles in a previous study exhibited 2 to 3 times faster growth rates when associated with Symbiodinium C1 compared to those associated with Symbiodinium D (Little et al., 2004) at the same field site where juveniles were reared in the present study."

What it means
Cantin et al. conclude that "the differences in carbon-based energy transfer between symbiont types may provide a competitive advantage to corals associating with Symbiodinium C1, particularly during their early life histories, when greater energy investment into rapid tissue and skeletal growth can prevent overgrowth of juveniles by competitors and mortality from grazers." Likewise, they opine that "as the community structure of coral reefs shift in response to global climate change [global warming?] and water quality impacts [ocean acidification?], opportunistic corals harboring symbionts that enable maximum rates of growth may similarly gain a competitive advantage." Consequently, it would appear that in the economy of nature ample provision has been made for earth's corals to weather all sorts of environmental challenges that may come their way, including those that climate alarmists contend will be driven by rising atmospheric CO2 concentrations.

References
Idso, S.B., Idso, C.D. and Idso, K.E. 2000. CO2, global warming and coral reefs: Prospects for the future. Technology 7S: 71-94.

Little, A.F., van Oppen, M.J.H. and Willis, B.L. 2004. Flexibility in algal endosymbioses shapes growth in reef corals. Science 304: 1492-1494.