Rubisco of Bacterial Endosymbionts of Hydrothermal Vent Animals Undersea
hydrothermal vents support remarkable ecosystems. At these extreme depths
there is no light to support photosynthesis, yet thriving vent communities are
found. Much of their primary productivity occurs through chemosynthesis
carried out by bacterial symbionts that live in specialized organs
(trophosomes) of certain ventanimals.
Chemosynthesis in these bacteria involves a process that is virtually
identical to photosynthesis. Carbon dioxide is fixed by rubisco and reduced to
glucose, and the necessary ATP and NADPH are produced by electron-transfer
processes similar to those of the light-dependent reactions of photosynthesis.
The key difference is that in chemosynthesis, the energy driving electron
transfer comes from a highly exergonic chemical reaction rather than from
light. Different chemosynthetic bacteria use different reactions for this
purpose. The
bacteria found in hydrothermal vent animals typically use the oxidation of
\(\mathrm{H}_{2} \mathrm{S}\) (abundant in the vent water) by \(\mathrm{O}_{2},\)
producing elemental sulfur. These bacteria also use the conversion of
\(\mathrm{H}_{2} \mathrm{S}\) to sulfur as a source of electrons for
chemosynthetic \(\mathrm{CO}_{2}\)
reduction.
(a) What is the overall reaction for chemosynthesis in these bacteria? You do
not need to write a balanced equation;
just give the starting materials and products.
(b) Ultimately, these endosymbiotic bacteria obtain their energy from
sunlight. Explain how this occurs.
Robinson and colleagues (2003) explored the properties of rubisco from the
bacterial endosymbiont of the giant tube worm Riftia pachyptila. Rubisco, from
any source, catalyzes
the reaction of either \(\mathrm{CO}_{2}\left(\text { Fig. } 20-7 \text { ) or
} \mathrm{O}_{2} \text { (Fig. } 20-20\) ) with \right. ribulose 1,5
-bisphosphate. In general, rubisco reacts more readily with \(\mathrm{CO}_{2}\)
than \(\mathrm{O}_{2}\). The degree of selectivity \((\Omega)\) can be expressed
in the equation
$$\frac{V_{\text {carboxylation }}}{V_{\text {oxygenation }}}=\Omega
\frac{\left[\mathrm{CO}_{2}\right]}{\left[\mathrm{O}_{2}\right]}$$
where \(V\) is the reaction velocity. Robinson and coworkers measured the
\(\Omega\) value for the rubisco of the bacterial endosymbionts. They purified
rubisco from tube-worm trophosomes, reacted it with mixtures of different
ratios of \(\mathrm{O}_{2}\) and \(\mathrm{CO}_{2}\) in the presence of
\(\left[1-^{3} \mathrm{H}\right]\) ribulose 1,5 -bisphosphate, and measured the
ratio of \(\left[^{3} \mathrm{H}\right]\) phosphoglycerate to \(\left[^{3}
\mathrm{H}\right]\) phosphoglycolate
(c) The measured ratio of \(\left[^{3} \mathrm{H}\right]\) phosphoglycerate to
\(\left[^{3} \mathrm{H}\right]\) phosphoglycolate is equal to the ratio
\(V_{\text {carboxylation }} / V_{\text {oxygenation }}\). Explain why.
(d) Why would \(\left[5^{-3} \mathrm{H}\right]\) ribulose 1,5 -bisphosphate not
be a suitable substrate for this assay? The \(\Omega\) for the endosymbiont
rubisco had a value of \(8.6 \pm 0.9\)
(e) The atmospheric (molar) concentration of \(\mathrm{O}_{2}\) is \(20 \%\) and
that of \(\mathrm{CO}_{2}\) is about 380 parts per million. If the endosymbiont
were to carry out chemosynthesis under these atmospheric conditions, what
would be the value of \(V_{\text {cartoxylution }} / V_{\text {oxygeration? }}
?\)
(f) Based on your answer to (e), would you expect \(\Omega\) for the rubisco of
a terrestrial plant to be higher than, equal to, or lower than \(8.6 ?\) Explain
your reasoning.
Two stable isotopes of carbon are commonly found in the environment: the more
abundant \(^{12} \mathrm{C}\) and the rare \(^{13} \mathrm{C}\). All rubisco
enzymes catalyze the fixation of \(^{12} \mathrm{CO}_{2}\) faster than that of
\(^{13} \mathrm{CO}_{2}\). As a result, the carbon in glucose is slightly
enriched in \(^{12} \mathrm{C}\) compared with the isotopic composition of
\(\mathrm{CO}_{2}\) in the environment. Several factors are involved in this
"preferential" use of \(^{12} \mathrm{CO}_{2},\) but one factor is the
fundamental physics of gases. The temperature of a gas is related to the
kinetic energy of its molecules. Kinetic energy is given by \(1 / 2 m v^{2}\),
where \(m\) is molecular mass and \(v\) is velocity. Thus, at the same temperature
(same kinetic energy), the molecules of a lighter gas will be moving faster
than those of a heavier gas.
(g) How could this contribute to rubisco's "preference" for \(^{12}
\mathrm{CO}_{2}\) over \(^{13} \mathrm{CO}_{2} ?\) Some of the first convincing
evidence that the tube-worm hosts were obtaining their fixed carbon from the
endosymbionts was that the \(^{13} \mathrm{C} /^{12} \mathrm{C}\) ratio in the
animals was much closer to that of the bacteria than that of nonvent marine
animals.
(h) Why is this more convincing evidence for a symbiotic relationship than
earlier studies that simply showed the presence of rubisco in the bacteria
found in trophosomes?
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