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IV. Tolerance Adaptations

c. adaptation to temperature

If we purify a given type of protein from species with different body temperatures and measure the stability of the protein, what we typically find is what's shown for the muscle protein actin shown on this figure. This work, done by Dr. Robert Swezey, shows how the stability of actin varies among warm- and cold-adapted species. Actin from the warm-adapted desert iguana, whose core temperature might reach 47 degrees Celsius, is much more stable than actins from cold-adapted Antarctic fishes that die of heat death near 4 degrees Celsius.

When we look at deep-sea fishes, most of which live at temperatures of 1-3 degrees above zero, we would expect that if pressure had no effect on protein stability--if there were no selection for tougher proteins in the deep sea--then deep-sea proteins would show relatively low stability, and would be very similar to those of polar fishes. What we find instead is that for actin, from three different deep sea fishes, the protein is tougher than even the protein from a bird or a mammal. So, even though this is a very cold-adapted group of organisms, this one protein turns out to have a very, tough structure. We think this is a reflection of the necessity of having a more rigid protein to be able to withstand the crushing pressures of the deep sea. Some of these fishes do get down to depths of 3-5 kilometers.


We typically think of tolerance adaptations in the deep sea in terms of pressure when we're dealing with physical factors. But when we look at the hydrothermal vents, we find a very interesting situation in terms of an extraordinarily large and very steep thermal gradient. I want to look at a few data that concern the way in which different hydrothermal vent species are distributed within this incredible thermal field, and then look at some of the mechanisms that might be conferring adaptations not just to high pressure but also to different temperatures in the vents field. One of my points here is to illustrate how important temperature is as an environmental variable in governing where organisms occur. We find that through the vent ecosystem, there are characteristic differences in habitat temperature that correlate very well with physiological properties of the organisms. So physiological adaptations play major roles in the structuring of ecosystems.

I've drawn this cartoon to illustrate the types of water masses that are present at the hydrothermal vents. As I said before, the ambient bottom water is very cold. Around the hydrothermal vents near the East Pacific rise and the Galapagos spreading center most of the water is cold, relatively rich in oxygen and lacking in hydrogen sulfide. I'll come back to sulfide in just a moment to talk about the symbiotic systems that thrive at the vents.

There are two extremes of water: the cold extreme and the warm extreme. The warm extreme is water that is exiting from the black smoker chimneys. The black smoker chimneys are putting out what's called "end member" water. What "end member" refers to is that it is water that has not been diluted with any of this cold bottom water. The end member water remains liquid because it's under a pressure of about 250 atmospheres at the Galapagos and East Pacific rise sites.

The water stays in a liquid state even though it's super heated. The black smoker water is also depleted in oxygen but it can have very high concentrations of hydrogen sulfide, concentrations in the millimole range as high as concentrations that you find in some smelly mud flats. Sulfide, of course, is what gives the rotten egg smell to mud flats. Biologically, the most interesting waters at the vents are what are called the warm water vent waters. This is where there occurs a mixing of the very cold bottom water and this ultra-hot black smoker water. As you'd expect, there is quite a temperature gradient in the warm water vents. Temperatures can go from 2 degrees Celsius up to about 20 degrees Celsius with pulses perhaps of 40 degrees Celsius or 50 degrees Celsius. It's very hard to get an upper limit on this number because of the very rapidly changing temperature over distance and over time. Oxygen is variable. Sulfide is variable.


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