杏吧原创

Blue blooded

Question: I have heard that some sea creatures such as horseshoe crabs have
blue, copper-based blood. Why is this, and what advantage does this kind of
blood give them over creatures with the more common red, iron-based blood, like
ourselves?

Do any creatures have blood that is based on metals other than iron or
copper?

Answer: Blood gets its colour from oxygen-carrying respiratory pigments, and
there are a number of different types. Their job is to bind oxygen in areas of
higher concentration (usually gas exchange surfaces such as lungs or gills) and
release it in areas of lower concentration (usually tissues).

The oxygen-carrying capacity of the various pigments varies with oxygen
concentration, temperature, pH and carbon dioxide concentration. It
depends on the nature of the protein part of the pigment as well as the metallic
component, and this differs from species to species.

The iron-containing pigments found in blood include haemoglobins (red),
myoglobins (red), chlorocruorins (green) and haemerythrins (violet). Haemocyanin
(blue), which is found in horseshoe crabs and other organisms, contains copper
not iron. The occurrence of these pigments does not appear to be strongly
related to organisms鈥� evolutionary relationships. Some organisms have no
oxygen-carrying respiratory pigments, some have one type, others more.

It is difficult to compare the efficiency of the respiratory pigments of
different species. For example, the diagram shows how much oxygen binds to the
pigments of different species at different concentrations, or partial pressures,
of oxygen. These oxygen saturation curves reveal how respiratory pigments bind
plenty of oxygen when the partial pressure of oxygen is relatively high (as in
lungs or gills) and release it when the partial pressure is low (as in muscles).
For simplicity, only one haemocyanin curve is shown, but its saturation curves
are as variable as those of haemoglobin.

Neither haemocyanin nor any of the other haemoglobins shown would work in the
low-oxygen environment for which the haemoglobin of the marine tube-dwelling
worm is adapted. Similarly, neither the haemocyanin of the horseshoe crab nor
the haemoglobin of the seal appear to be able to unload oxygen at the partial
pressures found in the bird. However, the seal haemoglobin and the crab
haemocyanin have similar saturation curves and (were such a thing possible)
might be interchangeable.

The effect of pH provides another example of the difficulty of
comparison. In most cases, a decrease in pH shifts the oxygen
saturation curve to the left. So as the amount of CO₂ in the tissues
increases, the pH decreases and more oxygen is unloaded from the
respiratory pigment.

Decreased pH at the oxygen uptake surface (lung) of an air breather
is seldom of significance. However, decreased pH at the oxygen uptake
surface (gill or skin) of an aquatic organism is not uncommon. Water often
becomes acidic, and as its pH decreases, oxygen uptake decreases.
Eventually, animals may die because their respiratory pigment is no longer able
to carry enough oxygen to support their metabolism.

To counteract this, organisms that live in environments in which the
pH varies usually have a respiratory pigment that is less sensitive to
pH change than the pigments found in animals which live in more stable
environments. Thus a haemocyanin that is very sensitive to pH would be
detrimental to an organism that lives in a pH-labile environment, even
if in other respects it is a 鈥渂etter鈥� oxygen carrier. Conversely, a p
H-stable haemoglobin might be a 鈥渂etter鈥� oxygen carrier, even though its
saturation curve is less efficient than a particular haemocyanin at a certain
pH.

However, this does not explain why crabs have haemocyanin rather than
haemoglobin. In some respects, the original question asking which respiratory
pigment is better is moot. Living organisms do not have the ability to swap one
respiratory pigment for another. If an organism has haemoglobin, it is stuck
with it.

Even if it were possible to change pigments, the required physiological
adjustments would probably be so far-reaching that the organism would no longer
qualify as the same species. In which case, you could find yourself asking the
same question again.

Peter Morgenroth

Retired lecturer in zoology

Royal Melbourne Institute of Technology

Answer: The blood pigment of sea cucumbers (Holothuroidea) is based on
vanadium and is yellow-green.

Nicholas Ashley

St Ives, Cambridgeshire

Answer: Respiratory pigments have a range of functions and occur鈥攐r
fail to occur鈥攊n such varied species that there is no universal rule
linking them.

In some groups, such as molluscs and crustaceans, both copper and iron-based
pigments are found, yet some small species often have none. Some pigments store
oxygen, some transport it, others scavenge traces of oxygen to protect
oxygen-sensitive systems. Yet others handle gases to be excreted, such as CO₂.

Iron-based pigments such as haemoglobins are found both in special cells such
as red blood cells, or free in the haemolymph (blood), while copper-based
haemocyanins are always dissolved in the blood.

Some haemocyanins have a higher affinity for oxygen than most haemoglobins
and probably function more as an oxygen storage medium than as an oxygen
transport medium.

The very pigment chemistry varies: red haemoglobins and green chlorocruorins
(found in certain worms) carry the iron in a porphyrin molecule, whereas
iron-based haemerythrin and copper-based haemocyanin carry the metal attached to
protein chains. Given such variety, it is difficult to discuss the relative
advantages of the different respiratory pigments. Sea squirts have vanadium-rich
clusters of blood cells, but it is unclear if these have any respiratory
function. One suggestion is that their function is to discourage predators.

Jon Richfield

Dennesig, South Africa

This week鈥檚 question

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Eden, North Carolina