Riftia pachyptila symbiont

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Characteristics of the symbiont/pathogen

What kind of microbe is it (eg Cell morphology, shape, phylogenetic classification)? Is its genome sequenced, and if so, how big is the genome?

Characteristics of Riftia pachyptila

Riftia pachyptila is a giant tubeworm that inhabits the volcanic deep sea vents of the Pacific Ocean. A plume protrudes from the R. pachyptila protective tube and contacts the surrounding water. The plume has a large, highly vascularized surface which allows for the exchange of metabolites between R. pachyptila and the environment. Other tissues within the R. pachyptila tube include the vestimentum, which allows R. pachyptila to position itself in the tube, and the richly vascularized trophosome.[1] R. pachyptila does not have a digestive tract and must live in an obligate symbiosis with a sulfur-oxidizing chemoautotrophic bacterium. This mutualistic symbiosis is localized in the R. pachyptila trophosome cells, which are densely colonized by the bacterium. [2]. The bacterium is estimated to represent as much as 35% of the total volume of the trophosome [4]. R. pachyptila larvae have a digestive tract which disappears during development, so likely the trophosome must be colonized with the bacterium for each generation [3].

Riftia pachyptila with visible red plume due to hemoglobin NOAA

The circulatory system includes a pump located in the vestimentum region that promotes blood circulation in the entire body, including to the trophosome cells which bring nutrients to the bacterium. The plume is rich with blood, which can be visualized by the red color of the plume. The circulatory system mediates all metabolite exchanges between R. pachyptila and the surrounding water [4][5].R. pachyptila are adapted to their volcanic deep sea environment and use its composition, which include carbon, nitrogen, oxygen, and sulfur, in metabolic pathways that rely on the symbiotic relationship with the bacterium. The R. pachyptila hemoglobin is the transporter of both oxygen and sulfide to the bacterium which produce metabolic energy for both itself and R. pachyptila [6].

Host-Symbiont Interaction

Assimilation of Carbon

R.pachyptila absorb carbon dioxide produced by the surrounding hydrothermal vents using its brachial plume []. Once absorbed, carbon dioxide can be used in many ways. Carbon dioxide can be transported by the circulatory system to the trophosome where bacteria are located []. In addition, carboxylation in the plume results in malate [], which can be transported immediately to the trophosome by blood circulation []. In the bacteria, the carbon dioxide from the plume provided either by the environment or as a result from the decarboxylation of the transported malate enters the Calvin-Benson cycle and serves as a precursor for different small organic metabolites []. These metabolites, such as as ribulose-1,5-biphosphate and ribulose-5-phosphate, can be delivered to the different tissues of R. pachyptila for its own metabolism and ATP production [].

Assimilation of Nitrogen

The bacterial symbiont has a high demand for nitrogen due to its large biomass [] and high growth rate []. This is consistent with the high level of nitrate in the surrounding environment []. Ammonia resulting from the reduction of nitrate by the bacterial symbiont can be utilized by R. pachyptila as well as produce metabolites, such as amino acids and nucleotides, for the bacterial symbiont []. Ammonia along with carbon dioxide can also be used in the biosynthetic pyrimidine and arginine pathways.

R. pachyptila lack enzymes required for the de novo pyrimidine pathway as well as those required for the biosynthesis of polyamines, while the bacterial symbiont lacks enzymes required for the pyrimidine salvage pathway.

Pyrimidine metabolism

The de novo pathway, which utilizes carbon and nitrogen, and the salvage pathway, which utilizes nucleic acids, are the two metabolic pathways responsible for the production of pyrimidine nucleotides. Enzymes required in the pyrimidine de novo pathway are only present in the bacterial symbiont []. R. pachyptila is unable to synthesize pyrimdine nucleotides through the de novo pathway and must rely on the salvage pathway. R. pachyptila contain all of the enzymes required for this pathway []. R. pachyptila is completely dependent on the bacterial symbiont for the de novo biosynthesis of the pyrimidine nucleotides.

Arginine metabolism

Arginine carboxylase and ornithine decarboxylase have key roles in the synthesis of polyamines for the R. pachyptila cell tissue. Polyamines are involved in membrane stability and growth []. R. pachyptila cannot utilize arginine metabolism because it lacks key enzymes and therefore must rely on the bacterial symbiont. Putrescine, the product of polyamine degradation, can serve as an alternative source of inorganic carbon and nitrogen for R. pachyptila [].

Molecular Insights into the Symbiosis

Sulfide Acquisition

Sulfide and oxygen are both required for the bacterial symbiont to produce ATP. However, sulfide spontaneously reacts with oxygen to form sulfur compounds []. This decreases the available free sulfide for the bacterial symbiont to oxidize. The bacterial symbiont must compete with oxygen for free sulfide and reside at the interface between oxic and anoxic zones so it can acquire oxygen but without prematurely oxidizing the free sulfide. The bacterial symbiont has adapted to this by residing with R. pachyptila [].

Simultaneous acquisition of sulfide and oxygen occurs in R. pachyptila because of a biochemical adaptation. Sulfide typically interacts to inhibit oxygen-binding on hemoglobin, [], however, the multi-hemoglobin complex synthesized by R. pachyptila reversibly binds sulfide independent of oxygen[]. Sulfide, primarily as hydrogen sulfide, and oxygen can then be transported by the R. pachyptila circulatory system to the trophosome for use by the bacterial symbiont.

Carbon Acquisition

The bacterial symbiont uses the Calvin-Benson cycle for carbon fixation and therefore requires carbon dioxide, which diffuses readily through biological membranes. While the majority of dissolved carbon in the sea is bicarbonate due to the higher pH of the sea (pH 8.0)[], the lower pH around hydrothermal vents (pH 6.0) generates higher concentrations of carbon dioxide and gives organisms that utilize the Calvin-Benson cycle an advantage. R. pachyptila must offset the proton-generating reaction of sulfur oxidation by bacterial symbionts to create the gradient required to intake carbon dioxide. R. pachyptila rely on H^+^-ATPases to export proton ions []. This maintains R. pachyptila blood at an alkaline pH of 7.5 []. While the low pH of the surrounding hydrothermal vent water results in a greater carbon dioxide concentration, the alkaline pH of the R. pachyptila blood favors the conversion of carbon dioxide to bicarbonate, which establishes a carbon dioxide gradient across the R. pachyptila plume []. This gradient, from higher external concentration of carbon dioxide to lower internal concentration of carbon dioxide, drives the diffusion of carbon dioxide into the R. pachyptila blood. Carbon dioxide is then transported by the blood to the trophosome for use by the bacterial symbiont in carbon fixation via the Calvin-Benson cycle [].

Ecological and Evolutionary Aspects

What is the evolutionary history of the interaction? Do particular environmental factors play a role in regulating the symbiosis?

Recent Discoveries

Describe two findings on the symbiosis published within the last two years.

References

[Sample reference] [[1] Seemanapalli SV, Xu Q, McShan K, Liang FT. 2010. Outer surface protein C is a dissemination-facilitating factor of Borrelia burgdorferi during mammalian infection. PLoS One 5:e15830.]

1) Gaill, F. (1993) Aspects of life development at deep sea hydrothermal vents. FASEB J. 7, 558–565.

2) Hand, S.C. (1987) Trophosome ultrastructure and the characterization of isolated bacteriocytes from invertebrate-sulfur bacteria symbioses. Biol. Bull. 173, 260–276.

3) Edwards, D.B., Nelson, D.C. (1991) DNA-DNA Solution Hybridization Studies of the Bacterial Symbionts of Hydrothermal Vent Tube Worms (Riftia pachyptila and Tevnia jerichonana). Appl Environ Microbiol. 5:1082–1088

4) Zal, F., Lallier, F.H., Green, B.N., Vinogradov, S.N. & Toulmond, A. (1996) The multi-hemoglobin system of the hydrothermal vent tube worm Riftia pachyptila. II. Complete polypeptide chain composition investigated by maximum entropy analysis of mass spectra. J. Biol. Chem. 271, 8875–8881.

5) Zal, F., Lallier, F.H., Wall, J.S., Vinogradov, S.N. & Toulmond, A. (1996) The multi-hemoglobin system of the hydrothermal vent tube worm Riftia pachyptila. I. Reexamination of the number and masses of its constituents. J. Biol. Chem. 271, 8869–8874.

6) Goffredi, S.K., Childress, J.J., Desaulniers, N.T. & Lallier, F.J.(1997) Sulfide acquisition by the vent worm Riftia pachyptila appears to be via uptake of HS–, rather than H2S. J. Exp. Biol. 200, 2609–2616.

Edited by [Crystal Leibrand], students of Grace Lim-Fong

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