Last October I had the opportunity to chat with Dick Hill, who is the author of one of the most popular animal physiology textbooks in circulation right now and a pretty darn successful ecophysiologist. He used to work with mammals and birds, but now he's moved on to marine invertebrates, and he's currently studying betaines as a possible defense mechanism against coral reef bleaching.
Hill got into studying coral bleaching as a bit of a happy accident. He was doing work on giant clams in the south Pacific, investigating why the giant clams took on a foul odor a few hours after being killed. The locals had undertaken a major operation to farm and sell the giant clams around the world as delicacies (apparently they're very delicious), but even when frozen, the clams were inedible after a few hours because of the breakdown of a chemical called dimethylsulfoniopropionate (DMSP) (which Hannah blogged about recently over at Culturing Science). The clams are symbionts with a species of algae that produce very large quantities of DMSP, and as a result, the DMSP settles in their bodies in quantities that are orders of magnitude larger than similar marine animals. After the animal dies, the DMSP breaks down, and one of the by-products is dimethylsulfide, which is responsible for the rank odor of the clams. The happy accident happened when Hill began using mass spectrometry to confirm the presence and by-products of DMSP in the clams. One of the mass spectrometry specialists that he worked with commented on the fact that there were multiple types of betaines in the clams as well.
Betaines are well-known to exist in various plants, especially crop plants, many of which have been genetically engineered to express more betaines. From my understanding, betaines can act as osmolytes (compounds that affect the diffusion of water through membranes) and help protect membranes and proteins from various stressors. In plants, this generally means protecting photosynthetic pathways from high temperatures and high levels of radiation from the sun during the peak of the daylight cycle. You would think that more sunlight would be good for photosynthesis, but apparently it is the opposite because of the negative effect of irradiation. As a result, there is often a dip in photosynthetic productivity in the afternoon (a phenomenon called photoinhibition), unless the plant can shield itself from harmful irradiation using betaines.
This information is relevant to corals because of the decline in coral populations due to bleaching. 'Bleaching' occurs when corals lose their algal symbionts (which are what give corals their brilliant colors) because of high water temperatures and high light intensity. When the symbionts leave or die due to photoinhibition, it is more difficult for the corals to survive. While corals were known to contain betaines for osmotic protection, the potential role of betaines in the stabilization of coral photosynthetic pathways had been largely ignored until recently. With this in mind, Hill and colleagues set out to determine the ecological patterns in coral betaine concentrations, which would be quite valuable for informing future conservation efforts.
Hill found that colonies of corals at shallower depths had higher concentrations of betaines than colonies in deeper waters and that colonies living in exposed areas had higher concentrations of betaines than shaded colonies. This is good supporting evidence that betaines are suppressing photoinhibition in corals, because exposed colonies and colonies in shallower water experience higher water temperatures and higher levels of radiation than their deeper or shaded counterparts. In addition, they found that betaine levels were higher in the afternoon than in the morning, suggesting that betaine levels were susceptible to phenotypic plasticity in response to more direct sunlight.
Hill is currently investigating the function of the betaines in these corals, to determine whether or not they are actually stabilizing photosynthesis in corals as they do in vascular plants. He is also investigating whether or not the betaines are being produced by the corals themselves, or by the algal symbionts, or both.
Hill, R., Dacey, J., Hill, S., Edward, A., & Hicks, W. (2004). Dimethylsulfoniopropionate in six species of giant clams and the evolution of dimethylsulfide after death Canadian Journal of Fisheries and Aquatic Sciences, 61 (5), 758-764 DOI: 10.1139/f04-029
Hill, R., Li, C., Jones, A., Gunn, J., & Frade, P. (2010). Abundant betaines in reef-building corals and ecological indicators of a photoprotective role Coral Reefs, 29 (4), 869-880 DOI: 10.1007/s00338-010-0662-x