Astute algae, conned corals

Coral reefs form as a partnership between sea anemone-like polyps and photosynthetic algae that provides nutrients for the former and safe, well-lit shelter for the latter. But this alliance might not start off as a true joint venture. New research published online earlier this month in__ linkurl:Molecular Ecology;http://www3.interscience.wiley.com/journal/122268905/abstract __indicates that symbiotic algae sneak inside coral cells in a stealth manner, rather than being actively welcomed by their coral host.

Elkhorn coral (Acropora palmata)
Image: flickr/species_snob

Early in development, juvenile corals acquire a wide range of different algal strains that are later winnowed down as the coral matures. This process is important to weed out all but the select few algae that form long-term symbiotic partnerships into adulthood. But what drives the coral's initial acceptance and later purging of algae was unclear. Now, a team led by Mónica Medina of the University of California, Merced, has shown that the coral host often takes a backseat role in this process. Medina and her colleagues tracked gene expression profiles in two distantly-related coral species from Mexico and Florida after exposing juveniles to various algal strains that differed in their ability to form symbiotic relationships. Microarray analyses revealed that the coral host transcriptome was drastically altered by "bad" algae that didn't make good partners but remained largely unaltered after infection by "good" algae, suggesting that corals don't actively respond to the arrival of good algae. The hundreds of genes switched on or off by bad algae probably represent an immune-related response to unwanted visitors, Medina said. Good algae, however, somehow cloak themselves from immune detection or manipulate the coral's response in some way. "It seems like the host is just ignoring this thing that's in there," Medina told __The Scientist__. Now, Medina's team is testing the algae's gene expression profiles to understand how the symbiont brings about this sneaky seal of approval.

Mountainous star coral
(Montastraea faveolata)
Image: M. Medina/BMC Genomics

"It's a nice conundrum," said linkurl:Adam Marsh,;http://www.ocean.udel.edu/people/profile.aspx?amarsh a marine biologist at the University of Delaware in Lewes who was not involved in the study. Most researchers had assumed that the interspecies interaction would be much more complex, involving active gene expression in both the coral host and algal symbiont. But "it looks to be a more passive process on the side of the host," he said. The study has important implications for how coral reefs will fare in the face of climate change, noted linkurl:Mikhail Matz,;http://www.bio.utexas.edu/research/matz_lab/matzlab/Welcome.html a geneticist at the University of Texas at Austin who studies coral adaptation and was not involved in the research. If symbiotic algae evolve higher thermal tolerance, they might only be accepted by the corals if "there's some process of coevolution that allows the coral to relax its guard further and to allow in more strains." Since algal uptake is largely passive, one would expect evolution to occur at surface receptors, for example, rather than at actively expressed genes, he said. "This [finding] narrows the number of options for how that evolution can happen." "The conclusions are probably right," said linkurl:Ruth Gates,;http://www.hawaii.edu/HIMB/Faculty/gates.html a molecular ecologist at the Hawaii Institute of Marine Biology in Kaneohe who did not contribute to the research findings. But Gates noted that the corals don't actually incorporate the bad algae into their cells. Thus, rather than measuring corals with two types of symbiotic algae, the researchers "are comparing the transcriptomes of an organism that has no symbionts with one that has symbionts," she said. Gates suspects that transcriptional changes might still be taking place with the good algae, but that "whatever is going on here is probably much more subtle." Medina agrees this is possible. "We may have missed those few key pathways," she said, but the response is clearly "not at a whole transcriptome level."
**__Related stories:__***linkurl:Ocean thermostat protects corals?;http://www.the-scientist.com/blog/display/54291/
[7th February 2008]*linkurl:Compromising coral immunity;http://www.the-scientist.com/article/display/54068/
[January 2008]*linkurl:Climate change and corals;http://www.the-scientist.com/article/daily/54070/
[January 2008]