Oxygenation properties of the two co-occurring hemoglobins of the tube worm Riftia pachyptila

Comparative Oxygen Consumption of Gastropod Holobionts from Deep-Sea Hydrothermal Vents in the Indian Ocean

Physiological traits are the foundation of an organism's success in a dynamic environment, yet basic measurements are unavailable for many taxa and even ecosystems. We measured routine metabolism in two hydrothermal vent gastropods, Alviniconcha marisindica (n = 40) and the scaly-foot gastropod Chrysomallon squamiferum (n = 18), from Kairei and Edmond vent fields on the Central Indian Ridge (23-25°S, about 3000 meter depth). No previous studies have measured metabolism in any Indian Ocean vent animals. After recovering healthy animals to the surface, we performed shipboard closed-chamber respirometry experiments to compare oxygen uptake at different temperatures (10, 16, and 25 °C) at surface pressure (1 atm). The physiology of these species is driven by the demands of their chemoautotrophic symbionts. Chrysomallon has very enlarged respiratory and circulatory systems, and endosymbionts are housed in its trophosome-like internal esophageal gland. By contrast, Alviniconcha has chemoautotrophic bacteria within the gill and less extensive associated anatomical adaptations. Thus, we predicted that routine oxygen consumption of Chrysomallon might be higher than that of Alviniconcha. However, oxygen consumption of Chrysomallon was not higher than that of Alviniconcha, and, further, Chrysomallon maintained a steady metabolic demand in two widely separated experimental temperatures, while Alviniconcha did not. We interpret that these findings indicate that (1) the "trophosome" does not fundamentally increase oxygen requirement compared to other gastropod holobionts, and (2) cold temperatures (10 °C) induce a stress response in Alviniconcha, resulting in aberrantly high uptake. While these two large gastropod species co-occur, differences in oxygen consumption may reflect the separate niches they occupy in the vent ecosystem.

Structural basis for cooperative oxygen binding and bracelet-assisted assembly of Lumbricus terrestris hemoglobin

The iron-containing hemoglobins (Hbs) are essential proteins to serve as oxygen transporters in the blood. Among various kinds of Hbs, the earthworm Hbs are the champions in carrying oxygen due to not only their large size but also the unusually high cooperativity of ligand binding. However, the cooperative oxygen binding mechanisms are still mostly unknown. Here we report the cryo-electron microscopy structure of Lumbricus terrestris Hb in its native, oxygenated state at 9.1 Å resolution, showing remarkable differences from the carbon monoxide-binding X-ray structure. Our structural analysis first indicates that the cooperative ligand binding of L. terrestris Hb requires tertiary and quaternary transitions in the heme pocket and a global subunit movement facilitated by intra-ring and inter-ring contacts. Moreover, the additional sinusoidal bracelet provides the confirmation for the long-standing debate about the additional electron densities absent in the X-ray crystal structure.

The metabolic demands of endosymbiotic chemoautotrophic metabolism on host physiological capacities

While chemoautotrophic endosymbioses of hydrothermal vents and other reducing environments have been well studied, little attention has been paid to the magnitude of the metabolic demands placed upon the host by symbiont metabolism and the adaptations necessary to meet such demands. Here we make the first attempt at such an evaluation, and show that moderate to high rates of chemoautotrophic or methanotrophic metabolism impose oxygen uptake and proton equivalent elimination demands upon the hosts that are much higher than is typical for the non-symbiotic annelid, bivalve and gastropod lineages to which they are related. The properties of the hosts are described and compared to determine which properties are associated with and predictive of the highest rates. We suggest that the high oxygen demand of these symbionts is perhaps the most limiting flux for the symbioses. Among the consequences of such demands has been the widespread presence of circulating and/or tissue hemoglobins in these symbioses that are necessary to support high metabolic rates in thioautotrophic endosymbioses. We also compare photoautotrophic with chemoautotrophic and methanotrophic endosymbioses to evaluate the differences and similarities in physiologies. These analyses suggest that the high demand for oxygen by chemoautotrophic and methanotrophic symbionts is likely a major factor precluding their endosymbiosis with cnidarians.

Structural characterization of hemoglobins from Monilifera and Frenulata tubeworms (Siboglinids): first discovery of giant hexagonal-bilayer hemoglobin in the former "Pogonophora" group

Siboglinids are symbiotic polychete annelids having hemoglobins as essential oxygen- and sulfide-carriers for their endosymbiotic bacteria. We analyzed the structure of the hemoglobins from two species of siboglinids: the monilifera Sclerolinum contortum and the frenulata Oligobrachia webbi (i.e. haakonmosbiensis) from Norwegian cold seeps. Measured by Multi-Angle Laser Light Scattering (MALLS), Sclerolinum shows a 3190+/-50 kDa hexagonal bilayer hemoglobin (HBL-Hb) and a 461+/-46 kDa ring-Hb, just as vestimentifera, whereas Oligobrachia has a 409+/-3.7 kDa ring-Hb only. Electrospray Ionization-Mass Spectrometry (ESI-MS) showed Sclerolinum HBL-Hb composed of seven monomeric globins (15-16 kDa), three disulfide-bonded globin heterodimers and three linkers. The heterodimers always contain globin-b (15814.4+/-1.5 Da). Sclerolinum ring-Hb is composed of globins and dimers with identical masses as its HBL-Hb, but lacks linkers. Oligobrachia ring-Hb has three globin monomers (14-15 kDa) only, with no disulfide-bonded dimers. Comparison of Sclerolinum hemoglobins between Storegga and Haakon Mosby Mud Volcano, using the normalized height of deconvoluted ESI-MS peaks, shows differences in globin monomers abundances that could reflect genetic differences or differential gene expression between distinct seep populations. The discovery of HBL-Hb in Sclerolinum is a new element supporting the hypothesis of monilifera being phylogenetically more closely related to vestimentifera, than to frenulata.

Oxygenation properties of extracellular giant hemoglobin from Oligobrachia mashikoi

Oxygenation properties of hemoglobin (Hb) from Oligobrachia mashikoi were extensively investigated. Compared to human Hb, Oligobrachia Hb showed a high oxygen affinity (P(50)=1.4 mmHg), low cooperativity (n =1.4), and a small Bohr effect (deltaH(+)=-0.28) at pH 7.4 in the presence of minimum salts. Addition of NaCl caused no change in the oxygenation properties of Oligobrachia Hb, indicating that Na(+) and Cl(-) had no effect. Mg(2+) and Ca(2+) remarkably increased the oxygen affinity and cooperativity. The dependence of the oxygen affinity on Ca(2+) concentration indicated that ca. 0.6 Ca(2+) per heme is bound to the protein moiety upon oxygen binding. CO(2) and a polyanion, inositol hexaphosphate, showed a null effect on the oxygenation properties. Thus, unlike the vertebrate Hbs, but like the annelid extracellular Hbs, the oxygen binding properties of Oligobrachia Hb are regulated by divalent cations which preferentially bind to the oxy form.

Environmental differences in hemoglobin gene expression in the hydrothermal vent tubeworm, Ridgeia piscesae

Ridgeia piscesae, the siboglinid tubeworm inhabiting the hydrothermal vents of the northeast Pacific Juan de Fuca Ridge, displays a wide range of microhabitat-specific, genetically indistinguishable phenotypes. Local microhabitat conditions are hypothesized to play a role in the differentiation of R. piscesae phenotypes. Extracellular hemoglobins serve to connect the tubeworm and the surrounding vent fluid, binding environmental sulfide and oxygen for transport to endosymbionts that use the chemical energy for carbon fixation. Because hemoglobin is essential for this symbiosis, we examined its expression in two of the most extreme R. piscesae phenotypes at two levels: the mRNA encoding the globin subunits and the whole molecules in coelomic and vascular fluids. Levels of gene expression were up to 12 times greater in short-fat R. piscesae from higher temperature, sulfide chimney environments compared to long-skinny animals from a low temperature, diffuse flow basalt habitat. Gene expression levels were consistent with the relative concentrations of hemoglobin molecules in the vascular and coelomic fluids. Up to a 20-fold variation in globin gene expression was detected between the same phenotype from different sites. These data demonstrate that local environmental factors influence not only phenotype but gene expression and its resulting physiological outcome within this unique species.

Lumbricus Erythrocruorin at 3.5 Å Resolution: Architecture of a Megadalton Respiratory Complex

Annelid erythrocruorins are highly cooperative extracellular respiratory proteins with molecular masses on the order of 3.6 million Daltons. We report here the 3.5 A crystal structure of erythrocruorin from the earthworm Lumbricus terrestris. This structure reveals details of symmetrical and quasi-symmetrical interactions that dictate the self-limited assembly of 144 hemoglobin and 36 linker subunits. The linker subunits assemble into a core complex with D(6) symmetry onto which 12 hemoglobin dodecamers bind to form the entire complex. Although the three unique linker subunits share structural similarity, their interactions with each other and the hemoglobin subunits display striking diversity. The observed diversity includes design features that have been incorporated into the linker subunits and may be critical for efficient assembly of large quantities of this complex respiratory protein.

Purification, characterization and sequence analyses of the extracellular giant hemoglobin from Oligobrachia mashikoi

We purified an extracellular hemoglobin with the molecular mass of ca. 440 kDa from the whole homogenates of Oligobrachia mashikoi (phylum Pogonophora) by a one-step gel-filtration. The preparation was pure to be crystallized. The P50 values of the hemoglobin and the fresh blood prepared from O. mashikoi were about 0.82 Torr and 0.9 Torr, respectively, which were much lower than the P50 value of human hemoglobin. However, the n values of the hemoglobin and the blood were about 1.2 and 1.1, respectively. Using the improved tricine SDS-PAGE, we could separate O. mashikoi hemoglobin into four kinds of the globin chains, A1, A2, B1 and B2, and succeeded for the first time in cloning and sequencing of the complete cDNA encoding B1 globin gene, in addition to A1, A2 and B2 globin genes in full length. We found that all globin genes have the extracellular signal sequences in each molecule and the distal His of the B1 globin chain is replaced to Gln. Finally, we constructed phylogenetic trees of the hemoglobins from Pogonophora, Vestimentifera and Annelida.

Sulfide binding is mediated by zinc ions discovered in the crystal structure of a hydrothermal vent tubeworm hemoglobin

Key to the remarkable ability of vestimentiferan tubeworms to thrive in the harsh conditions of hydrothermal vents are hemoglobins that permit the sequestration and delivery of hydrogen sulfide and oxygen to chemoautotrophic bacteria. Here, we demonstrate that zinc ions, not free cysteine residues, bind sulfide in vestimentiferan hemoglobins. The crystal structure of the C1 hemoglobin from the hydrothermal vent tubeworm Riftia pachyptila has been determined to 3.15 A and revealed the unexpected presence of 12 tightly bound Zn(2+) ions near the threefold axes of this D(3) symmetric hollow sphere. Chelation experiments on R. pachyptila whole-coelomic fluid and purified hemoglobins reveal a role for Zn(2+) ions in sulfide binding. Free cysteine residues, previously proposed as sulfide-binding sites in vestimentiferan hemoglobins, are found buried in surprisingly hydrophobic pockets below the surface of the R. pachyptila C1 molecule, suggesting that access of these residues to environmental sulfide is restricted. Attempts to reduce the sulfide-binding capacities of R. pachyptila hemoglobins by addition of a thiol inhibitor were also unsuccessful. These findings challenge the currently accepted paradigm of annelid hemoglobin evolution and adaptation to reducing environments.

Metagenomics: application of genomics to uncultured microorganisms

Metagenomics (also referred to as environmental and community genomics) is the genomic analysis of microorganisms by direct extraction and cloning of DNA from an assemblage of microorganisms. The development of metagenomics stemmed from the ineluctable evidence that as-yet-uncultured microorganisms represent the vast majority of organisms in most environments on earth. This evidence was derived from analyses of 16S rRNA gene sequences amplified directly from the environment, an approach that avoided the bias imposed by culturing and led to the discovery of vast new lineages of microbial life. Although the portrait of the microbial world was revolutionized by analysis of 16S rRNA genes, such studies yielded only a phylogenetic description of community membership, providing little insight into the genetics, physiology, and biochemistry of the members. Metagenomics provides a second tier of technical innovation that facilitates study of the physiology and ecology of environmental microorganisms. Novel genes and gene products discovered through metagenomics include the first bacteriorhodopsin of bacterial origin; novel small molecules with antimicrobial activity; and new members of families of known proteins, such as an Na(+)(Li(+))/H(+) antiporter, RecA, DNA polymerase, and antibiotic resistance determinants. Reassembly of multiple genomes has provided insight into energy and nutrient cycling within the community, genome structure, gene function, population genetics and microheterogeneity, and lateral gene transfer among members of an uncultured community. The application of metagenomic sequence information will facilitate the design of better culturing strategies to link genomic analysis with pure culture studies.

Biochemical and enzymological aspects of the symbiosis between the deep-sea tubeworm Riftia pachyptila and its bacterial endosymbiont

Riftia pachyptila (Vestimentifera) is a giant tubeworm living around the volcanic deep-sea vents of the East Pacific Rise. This animal is devoid of a digestive tract and lives in an intimate symbiosis with a sulfur-oxidizing chemoautotrophic bacterium. This bacterial endosymbiont is localized in the cells of a richly vascularized organ of the worm: the trophosome. These organisms are adapted to their extreme environment and take advantage of the particular composition of the mixed volcanic and sea waters to extract and assimilate inorganic metabolites, especially carbon, nitrogen, oxygen and sulfur. The high molecular mass hemoglobin of the worm is the transporter for both oxygen and sulfide. This last compound is delivered to the bacterium which possesses the sulfur oxidizing respiratory system, which produces the metabolic energy for the two partners. CO2 is also delivered to the bacterium where it enters the Calvin-Benson cycle. Some of the resulting small carbonated organic molecules are thus provided to the worm for its own metabolism. As far as nitrogen assimilation is concerned, NH3 can be used by the two partners but nitrate can be used only by the bacterium. This very intimate symbiosis applies also to the organization of metabolic pathways such as those of pyrimidine nucleotides and arginine. In particular, the worm lacks the first three enzymes of the de novo pyrimidine biosynthetic pathways as well as some enzymes involved in the biosynthesis of polyamines. The bacterium lacks the enzymes of the pyrimidine salvage pathway. This symbiotic organization constitutes a very interesting system to study the molecular and metabolic basis of biological adaptation.

Control of oxygen transport in the microcrustacean Daphnia: regulation of haemoglobin expression as central mechanism of adaptation to different oxygen and temperature conditions

The pathway for oxygen, the control of oxygen transport and the role of haemoglobin expression for the physiological adaptation to different oxygen and temperature conditions were studied in the ecological model organism Daphnia magna. Ventilation of the inner walls of the carapace as the main gas exchange area as well as of the embryos in the brood pouch are controlled, oxygen-dependent processes. The P(O2)-dependent increase of heart rate as well as perfusion rate during short-term, progressive hypoxia improves the circulatory oxygen transport within the body. The regulation of haemoglobin (Hb) expression is the central mechanism for a medium-term adaptation to hypoxia. Genetic control elements and oxygen conditions near the two Hb synthesis sites (fat cells, epipodite epithelial cells) determine, which types of Hb subunits and, accordingly, hetero-multimeric Hb macromolecules are produced. One synthesis site may respond mainly to internal, the other one to external oxygen conditions. Depending on environmental condition, either higher quantities of macromolecules of unchanged functionality (P50) or increasing amounts of macromolecules with higher oxygen affinity are synthesized. The Hb subunit DmHbA is probably of considerable importance for this functional change. The physiological benefits of haemoglobin in Daphnia are discussed. Physiological adaptation of Daphnia to different temperatures is also related to the control of oxygen transport processes with the regulation of haemoglobin expression again as a central mechanism.

Hemoglobin function in deep-sea and hydrothermal-vent endemic fish: Symenchelis parasitica (Anguillidae) and Thermarces cerberus (Zoarcidae)

Deep-sea hydrothermal vents probably provide the harshest physico-chemical conditions confronting metazoan animals in nature. Given the absence of information on hemoglobin (Hb) function in hydrothermal-vent vertebrates, and the complex molecular and functional adaptations observed in hydrothermal-vent invertebrates, we investigated the oxygenation reactions of Hbs from the vent-endemic zoarcid Thermarces cerberus and the deep-sea anguillid Symenchelis parasitica from adjacent habitats. Electrophoretically cathodic and anodic isoHbs from S. parasitica exhibit radical differences in O(2) affinity and pH and organic phosphate (ATP) sensitivities, reflecting a division of labor as in other 'class II' fish that express both Hb types. Remarkably, the cathodic Hb (I) lacks chloride sensitivity, and the anodic Hb (II) shows anticooperativity near half-saturation at low temperature. T. cerberus isoHbs exhibit similar affinities and pH sensitivities ('class I' pattern) but much higher O(2) affinities than those observed in Hbs of the temperate, shallow-water zoarcid Zoarces viviparus, which, unless compensated, reveals markedly higher blood O(2) affinities in the former species. The temperature sensitivity of O(2) binding to T. cerberus Hbs and the anodic S. parasitica Hb, which have normal Bohr effects, is decreased by endothermic proton dissociation, which reduces the effects of ambient temperature variations on O(2) affinity. In the cathodic S. parasitica Hb, similar reduction appears to be associated with endothermic conformational changes that accompany the oxygenation reaction.

Respiratory adaptations in a deep-sea orbiniid polychaete from Gulf of Mexico brine pool NR-1: metabolic rates and hemoglobin structure/function relationships

Methanoaricia dendrobranchiata Blake (Polychaeta; Orbiniidae)occurs in large numbers in association with communities of the mussel Bathymodiolus childressi at hydrocarbon seeps on the Louisiana Slope of the Gulf of Mexico. Its microhabitat can be strongly hypoxic (oxygen is often undetectable) and sulfidic (sulfide concentrations can reach millimolar levels), which may seriously challenge aerobic metabolism. We describe a suite of adaptations to its low-oxygen environment. The worms are capable of regulating their rate of oxygen consumption down to partial pressures of approximately 870 Pa oxygen. This capability correlates with a large gill surface area, a small diffusion distance from sea water to blood, a very high hemoglobin oxygen-affinity (P50=27.8 Pa at 10°C and pH 7.6) and a Bohr effect that is pronounced at high oxygen saturations. When fully saturated, the hemoglobin binds sufficient oxygen for only 31 min of aerobic metabolism. However, these polychaetes can withstand extended periods of anoxia both in the absence and presence of 1 mmoll-1 sulfide(TL50=approx. 5.5 and 4 days, respectively).

Nonvertebrate hemoglobins: functions and molecular adaptations

Hemoglobin (Hb) occurs in all the kingdoms of living organisms. Its distribution is episodic among the nonvertebrate groups in contrast to vertebrates. Nonvertebrate Hbs range from single-chain globins found in bacteria, algae, protozoa, and plants to large, multisubunit, multidomain Hbs found in nematodes, molluscs and crustaceans, and the giant annelid and vestimentiferan Hbs comprised of globin and nonglobin subunits. Chimeric hemoglobins have been found recently in bacteria and fungi. Hb occurs intracellularly in specific tissues and in circulating red blood cells (RBCs) and freely dissolved in various body fluids. In addition to transporting and storing O(2) and facilitating its diffusion, several novel Hb functions have emerged, including control of nitric oxide (NO) levels in microorganisms, use of NO to control the level of O(2) in nematodes, binding and transport of sulfide in endosymbiont-harboring species and protection against sulfide, scavenging of O(2 )in symbiotic leguminous plants, O(2 )sensing in bacteria and archaebacteria, and dehaloperoxidase activity useful in detoxification of chlorinated materials. This review focuses on the extensive variation in the functional properties of nonvertebrate Hbs, their O(2 )binding affinities, their homotropic interactions (cooperativity), and the sensitivities of these parameters to temperature and heterotropic effectors such as protons and cations. Whenever possible, it attempts to relate the ligand binding properties to the known molecular structures. The divergent and convergent evolutionary trends evident in the structures and functions of nonvertebrate Hbs appear to be adaptive in extending the inhabitable environment available to Hb-containing organisms.

Three-dimensional reconstruction of the hexagonal bilayer hemoglobin of the hydrothermal vent tube worm Riftia pachyptila by cryoelectron microscopy

A frozen-hydrated specimen of the V1 hemoglobin of the hydrothermal vent tube worm Riftia pachyptila was observed in the electron microscope and subjected to three-dimensional reconstruction by the method of random conical tilt series. The 3D volume possesses a D6 point-group symmetry. When viewed along its 6-fold axis the vertices of its upper hexagonal layer are 16 degrees clockwise rotated compared to those of the lower layer. A central linker complex is decorated by 12 hollow globular substructures. The linker complex comprises (i) a central hexagonal toroid, (ii) two internal bracelets onto which the hollow globular substructures are built, and (iii) six structures connecting the two hexagonal layers. The hollow globular substructures, related to the dodecamers of globin chains resulting from the dissociation of the hexagonal bilayer hemoglobin, have a local pseudo 3-fold symmetry and are composed each of three elongated structures visible when the volume is displayed at high threshold. At a resolution of 36 A, the 3D volumes of the hexagonal bilayer hemoglobins of Riftia pachyptyla and of the leech Macrobdella decora look almost perfectly identical.

The multi-hemoglobin system of the hydrothermal vent tube worm Riftia pachyptila. I. Reexamination of the number and masses of its constituents

The deep-sea tube worm Riftia pachyptila Jones possesses a well developed circulatory system and a large coelomic compartment, both containing extracellular hemoglobins. Fresh vascular blood is heterogeneous and contains two different hemoglobins (V1 and V2), whereas the coelomic fluid is homogeneous and comprises only one hemoglobin (C1). Their molecular weights have been determined by scanning transmission electron microscopy mass mapping (STEM) and by multi-angle laser light scattering (MALLS). Both methods yielded approximately the same molecular weights with masses significantly higher than the literature data for V1. V1, V2, and C1 had Mr of 3396 +/- 540 x 10(3), 393 +/- 71 x 10(3), and 410 +/- 51 x 10(3) by STEM, and 3503 +/- 13 x 10(3), 433 +/- 8 x 10(3), and 380 +/- 4 x 10(3) by MALLS, respectively. Transmission electron micrographs of V1 are typical of an hexagonal bilayer hemoglobin (HBL Hb). When submitted to dilution or osmotic shock, V1 dissociates into halves and one-twelfth subunits like annelid HBL Hbs. V1 is resistant to urea treatment, indicating that hydrophobic interactions play a small role in its quaternary structure. Conversely, V1 Hb is rather unstable in solution without denaturant, a property which seems to be characteristic of vestimentiferan HBL Hbs and could be explained by an important number of hydrogen bonds.

Adventitious variability? The amino acid sequences of nonvertebrate globins

1. The more than 140 amino acid sequences of non-vertebrate hemoglobins (Hbs) and myoglobins (Mbs) that are known at present, can be divided into several distinct groups: (1) single-chain globins, containing one heme-binding domain; (2) truncated, single-chain, one-domain globins; (3) chimeric, one-domain globins; (4) chimeric, two-domain globins; and (5) chimeric multi-domain globins. 2. The crystal structures of eight nonvertebrate Hbs and Mbs are known, all of them monomeric, one-domain globin chains. Although these molecules represent plants, prokaryotes and several metazoan groups, and although the inter-subunit interactions in the dimeric and tetrameric molecules differ from the ones observed in vertebrate Hbs, the secondary structures of all seven one-domain globins retain the characteristic vertebrate "myoglobin fold". No crystal structures of globins representing the other four groups have been determined. 3. Furthermore, a number of the one-, two- and multi-domain globin chains participate in a broad variety of quaternary structures, ranging from homo- and heterodimers to highly complex, multisubunit aggregates with M(r) > 3000 kDa (S. N. Vinogradov, Comp. Biochem. Physiol. 82B, 1-15, 1985). 4. (1) The single-chain, single-domain globins are comparable in size to the vertebrate globins and exhibit the widest distribution. (A) Intracellular Hbs include: (i) the monomeric and polymeric Hbs of the polychaete Glycera; (ii) the tetrameric Hb of the echiuran Urechis; (iii) the dimeric Hbs of echinoderms such as Paracaudina and Caudina; and (iv) the dimeric and tetrameric Hbs of molluscs, the bivalves Scapharca, Anadara, Barbatia and Calyptogena. (B) Extracellular Hbs include: (i) the multiple monomeric and dimeric Hbs of the larva of the insect Chironomus; (ii) the Hbs of nematodes such as Trichostrongylus and Caenorhabditis; (iii) the globin chains forming tetramers and dodecamers and comprising approximately 2/3 of the giant (approximately 3600 kDa), hexagonal bilayer (HBL) Hbs of annelids, e.g. the oligochaete Lumbricus and the polychaete Tylorrhynchus and of the vestimentiferan Lamellibrachia; and (iv) the globin chains comprising the ca 400 kDa Hbs of Lamellibrachia and the pogonophoran Oligobrachia. (C) Cytoplasmic Hbs include: (i) the Mbs of molluscs, the gastropods Aplysia, Bursatella, Cerithedea, Nassa and Dolabella and the chiton Liolophura; (ii) the three Hb of the symbiont-harboring bivalve Lucina; (iii) the dimeric Hb of the bacterium Vitreoscilla; and (iv) plant Hbs, including the Hbs of symbiont-containing legumes (Lgbs), the Hbs of symbiont-containing non-leguminous plants and the Hbs in the roots of symbiont-free plants.(ABSTRACT TRUNCATED AT 400 WORDS)