Studies on the haemoglobins of the small Pogonophora

Haemoglobin is present, often at high concentration, dissolved in the blood of all small Pogonophora so far examined.The haemoblobin ofSiboglinum fiordicumresembles annelid and vestimentiferan haemoglobins in having subunits with molecular weights of 15000–16700 plus traces of 35000–40000 MW material. 5.fiordicumhaemoglobin has 1 mol haem per 21500 g protein, and a similar lack of correspondence between subunits and mols of haem is shown by annelid and vestimentiferan haemoglobins. However,S. fiordicumhaemoglobin differs considerably from annelid and vestimentiferan haemoglobins in other respects, even though the three groups are closely linked taxonomically. The haemoglobin ofS. fiordicumhas a lower apparent molecular weight (Mr= 3.5–4.0 x 105) than that of annelid haemoglobins (Mr= 3–4 x 108) when measured under the same conditions. Pogonophore blood examined with the electron microscope does not show the two-tiered hexagonal structure found in annelid and vestimentiferan haemoglobins.S. fiordicumhaemoglobin also differs from annelid and vestimentiferan haemoglobins in amino acid composition.The haemoglobins of the small pogonophores examined have a very high affinity for oxygen. The P50ranges from 0.36 Torr in diluted blood ofS. fiordicum(15 °C) to 0.5–1.0 Torr in undiluted blood ofS. atlanticum(10 °C) ands. fiordicum(15 °C). Only a slight Bohr effect was found. The high oxygen affinity can be related to the wide range of Po2values experienced in the habitat and the need to transport oxygen to the deeply buried posterior end of the body that contains autotrophic endosymbiotic bacteria.