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.

Temperature-dependent stress response in oysters, Crassostrea virginica: pollution reduces temperature tolerance in oysters.

Combined effects of temperature and a toxic metal, cadmium (Cd), on energy metabolism were studied in a model marine bivalve, the eastern oyster Crassostrea virginica, acclimated at 20, 24 and 28 degrees C and exposed to 50microgl(-1) of Cd. Both increasing temperature and Cd exposure led to a rise in standard metabolic rates, and combined stressors appeared to override the capability for aerobic energy production resulting in impaired stress tolerance. Oysters exposed to elevated temperature but not Cd showed no significant change in condition, survival rate and lipid peroxidation, whereas those exposed to both Cd and temperature stress suffered high mortality accompanied by low condition index and elevated lipid peroxidation. Furthermore, RNA/DNA ratios indicative of protein synthesis rate, and levels of glutathione, which is involved in metal detoxification, increased in Cd-exposed oysters at 20 degrees C but not at 28 degrees C. Implications of the synergism between elevated temperatures and cadmium stress on energy metabolism of oysters are discussed in the light of the potential effects of climate change on oyster populations in polluted areas.

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.