VII. Mitochondrial Activity
One of the systems that has proven to useful to gain an index of thermal tolerance
is respiration of mitochondria. The mitochondria, as you all know, are the
energy factories of cells, the primary part of the cell in which ATP, the
cell's energy currency, is generated. It turns out that even though you
can't bring animals up alive from the vents in all cases, you can bring
up the animals in good enough shape to allow isolation and can then isolate
mitochondria for physiological studies, for example, studies of mitochondria
consuming oxygen at different temperatures.
One of the things that we have discovered in looking at a variety of different
organisms is that there is a certain temperature at which the oxygen consuming
activity of the mitochondria crashes. We call this the "break temperature".
The break temperature gives us a very good basis for answering the question:
"How does the resistance of mitochondrial respiration to temperature
differ among hydrothermal vent organisms?" The experiment has been
to bring up a number of different hydrothermal vent species, isolate their
mitochondria with centrifugation, and measure their respiration at different
temperatures. The way in which data of this sort are usually plotted is
called an Arrhenius plot named after Svante Arrhenius, Nobel Laureate at
the beginning part of the century. What you're looking at is a set of axes
where high temperature is to the left, cold temperature on the right. So,
if we're measuring the rate of oxygen consumption and express this on a
logarithmic basis, as you begin to raise the temperature, you see the nice
linear increase in the rate of activity up to a certain temperature. Then
bam, the mitochondria simply crash. These are the "break temperatures".
As you'll see, the break temperatures give you a nice signal as to what the
organism's operating temperature seems to be. This figure shows the relationship
of Arrhenius break temperatures, the ABTs, to the maximal habitat temperature,
which we either know from direct measurement, or we estimate to get ball park
range values.
Here is Calyptogena magnifica the big white clam. Here is Calyptogena
elongata, a clam that occurs in cold waters, for example, in the Santa
Barbara channel. Calyptogena magnifica lives in tepid vent waters.
There is still enough sulfide coming up to fuel symbiosis but the waters
are relatively cool. Bathymodiolus thermophilous is kind of a misnomer.
When people first discovered this mussel at the vent, they thought, "Boy,
this must be a really high temperature mussel." Well, it turns out
that based on our mitochondrial data and some independent data that we've
gathered looking at soluble enzymes, Bathymodiolus is not really
a high temperature species. Here is Mytilus, the mussel that we see
living in the intertidal. It's found at temperatures up to around 2025 degrees.
Here is Riftia. Here is Bythograea thermydron, a brachyuran
crab that's found crawling on the outside of the black smokers. Bythograea
is in fact seeing some pretty high temperatures. The alvinellids are seeing
even higher temperatures, probably body temperatures higher than the ones
that we experience ourselves. So this is some initial evidence that we're
looking at a very high temperature for a deep sea animal.
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