Riftia

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Riftia pachyptila is a marine invertebrate in the phylum of segmented worms, Annelida,<ref>Template:Cite book</ref> which include the other "polychaete" tube worms commonly found in shallow water marine environments and coral reefs. R. pachyptila lives in the deep sea, growing on geologically active regions of the Pacific Ocean's seafloor, such as near hydrothermal vents. These vents provide a natural ambient temperature ranging from Template:Convert,Template:Contradictory inline Template:Verify source<ref name=Bright2008>Template:Cite book</ref> and emit large amounts of chemicals such as hydrogen sulfide, which this species can tolerate at extremely high levels. These worms can reach a length of Template:Convert,<ref name=McClain>Template:Cite journal</ref> and their tubular bodies have a diameter of Template:Convert.

Historically, the genus Riftia (which only contains this species) was placed within the phyla Pogonophora and Vestimentifera. It has been informally known as the giant tube worm or the giant beardworm; however, the former name is also used for the largest living species of shipworm, Kuphus polythalamius, which is a type of bivalve (a group of molluscs which includes clams, mussels, and scallops).

Discovery

R. pachyptila was discovered in 1977 during an expedition led by geologist Jack Corliss to the Galápagos Rift; the survey was sponsored by the Woods Hole Oceanographic Institution and the National Science Foundation, and the dive was carried out by the American bathyscaphe DSV Alvin. The presence of thermal springs near mid-oceanic ridges was presumed, and the expedition aimed to confirm this.<ref name="SmithArch"/>

The discovery of life around hydrothermal vents was unexpected, as the team was originally planning to study hydrothermal vents, which were assumed to be sterile environments due to high heat emitted from them (around Template:Cvt);<ref>Template:Cite web</ref><ref>Template:Cite journal</ref> no biologists were included in the expedition. Many of the species discovered during this expedition had never been seen before, as they are found exclusively near hydrothermal vents. Observed species included "foot-long" bivalves (the mussel Bathymodiolus thermophilus and clam Calyptogena magnifica), white crabs, and polychaetes such as R. pachyptila.<ref>Template:Cite journal</ref> The discovery was compared to that of Columbus' discovery of the Americas by one of the geologists.<ref name="SmithArch"/><ref>Template:Cite journal</ref> Hydrothermal activity at this site was determined to have begun during the early 1970s, with the tubeworms ecologically dominating the site prior to 1979.<ref name="BioChem"/>

Though initially thought to be an isolated phenomenon, this vent ecosystem (located at Template:Coord;<ref name="OG"/> dubbed the "Rose Garden" site due to the prevalence of the red Riftia worms)<ref name="SmithArch"/> proved to be the first of many such ecosystems that would later be discovered on other geologically active sections of the sea floor. An expedition in 1985 returned to the Rose Garden site and found that the Riftia were fewer in number, having been displaced by clams and mussels despite having "essentially the same" water chemistry as the initial discovery in 1979.<ref name="BioChem">Template:Cite journal</ref> Another returning expedition in 2002 found that it had been destroyed by a lava flow sometime in the prior decade, though a second vent ecosystem was found near the original site, which it was dubbed "Rosebud".<ref name="SmithArch"/>

Description

The generic name Riftia alludes to the rift that formed the geothermal vents where the species inhabits, while pachyptila (pachy; thick + ptilon; feather) refers to the anterior plume of the worm.<ref name="OG">Template:Cite journal</ref> The original specimen or holotype, USNM 59951, is held by the National Museum of Natural History (USNM).<ref name="SmithArch">Template:Cite web</ref>

Anatomy

Template:Copy edit Isolating the vermiform body from white chitinous tube, a small difference exists from the classic three subdivisions typical of phylum Pogonophora:<ref>Template:Cite journal</ref> the prosoma, the mesosoma, and the metasoma.

The first body region is the vascularized branchial plume, which is bright red due to the presence of hemoglobin that contains up to 144 globin chains (each presumably including associated heme structures). These tube worm hemoglobins are remarkable for carrying oxygen in the presence of sulfide, without being inhibited by this molecule, as hemoglobins in most other species are.<ref>Template:Cite journal</ref><ref name="Hahlbeck_2005">Template:Cite journal</ref> The soluble hemoglobins, present in the tentacles, are able to bind O₂ and H₂S, which are necessary for chemosynthetic bacteria. Due to the capillaries, these compounds are absorbed by bacteria.<ref>Template:Cite journal</ref> The plume provides essential nutrients to bacteria living inside the trophosome.Template:Clarify After Paralvinella grasslei, Riftia has the second highest branchial surface area among aquatic animals, having Template:Convert of branchial area per gram of wet mass (with the sampled worms ranging from Template:Convert). Larger, typically mature worms have proportionally smaller branchial areas compared to smaller immature ones.<ref name="Biometry">Template:Cite journal</ref> If the tubeworm perceives a threat or is touched, it retracts the plume and the tube is closed due to the obturaculum, a particular operculum that protects and isolates the animal from the external environment.<ref>Template:Cite book</ref> The collagenous obturaculum supports the respiratory lamellae.<ref>Template:Cite journal</ref>

The second body region is the vestimentum, formed by muscle bands, having a winged shape, and it presents the two genital openings at the end.<ref>Template:Cite book</ref><ref>Template:Cite book</ref> The heart, extended portion of dorsal vessel, enclose the vestimentum.<ref>Template:Cite book</ref>

In the middle part, the trunk or third body region, is full of vascularized solid tissue, and includes body wall, gonads, and the coelomic cavity. Here is also located the trophosome, a spongy tissue where a billion symbiotic, thioautotrophic bacteria and sulfur granules are found.<ref>Template:Cite web</ref><ref name="Stewart_2006">Template:Cite journal</ref> Since the mouth, digestive system, and anus are missing, the survival of R. pachyptila is dependent on this mutualistic symbiosis.<ref name="Cavanaugh_1981">Template:Cite journal</ref> This process, known as chemosynthesis, was recognized within the trophosome by Colleen Cavanaugh.<ref name="Cavanaugh_1981"/>

In the posterior part, the fourth body region, is the opisthosome, which anchors the animal to the tube and is used for the storage of waste from bacterial reactions.<ref>Template:Cite web</ref>

Tubes

RiftiaTemplate:'s tubes possess very thick walls compared to cold-seep tubeworms and pogonophorans, especially at the base.<ref name="Biometry"/> They are composed of chitin associated with proteins,<ref>Template:Cite journal</ref> which are secreted out from cup-shaped microvilli-like structures within glands which form crystallite chitin layers over time.<ref name="SynthGrow"/> These tubes are resistant to enzymatic attack by bacteria, lasting 2.5 years in comparison to degrading in less than 36 days for the exoskeleton of the crab Bythograea thermydron.<ref>Template:Cite journal</ref>

The worms are able to remodel both the top and the base of their tubes, which allows for some adaptability in the highly competitive and crowded spaces they grow in.<ref>Template:Cite journal</ref> Chitinolytic activity has been detected on RiftiaTemplate:'s opisthosome; the lysis of chitin is one mechanism which allows tube remodelling.<ref name="SynthGrow"/> Growth rate of the tubes range from Template:Cvt per year.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The amount of chitin produced approaches 100 times that of pelagic and benthic ecosystems.<ref name="SynthGrow">Template:Cite journal</ref>

Physiology

Template:Technical Because of the peculiar environment in which R. pachyptila thrives, this species differs greatly from other deep-sea species that do not inhabit hydrothermal vent sites; the activity of diagnostic enzymes for glycolysis, the citric acid cycle and electron transport in the tissues of R. pachyptila is very similar to the activity of these enzymes in the tissues of shallow-living animals. This contrasts with the fact that deep-sea species usually show very low metabolic rates, which in turn suggests that low water temperature and high pressure in the deep sea do not necessarily limit the metabolic rate of animals and that hydrothermal vent sites display characteristics that are completely different from the surrounding environment, thereby shaping the physiology and biological interactions of the organisms living in these sites.<ref name="Hand_1983" />

Hydrothermal vent tubeworms get organic compounds from bacteria that live in their trophosome

The discovery of bacterial invertebrate chemoautotrophic symbiosis, particularly in vestimentiferan tubeworms R. pachyptila<ref name = "Cavanaugh_1981" /> and then in vesicomyid clams and mytilid mussels revealed the chemoautotrophic potential of the hydrothermal vent tube worm.<ref name="Felbeck_1981">Template:Cite journal</ref> Scientists discovered a remarkable source of nutrition that helps to sustain the conspicuous biomass of invertebrates at vents.<ref name="Felbeck_1981" /> Many studies focusing on this type of symbiosis revealed the presence of chemoautotrophic, endosymbiotic, sulfur-oxidizing bacteria mainly in R. pachyptila,<ref>Template:Cite journal</ref> which inhabits extreme environments and is adapted to the particular composition of the mixed volcanic and sea waters.<ref name="zor">Template:Cite journal</ref> This special environment is filled with inorganic metabolites, essentially carbon, nitrogen, oxygen, and sulfur. In its adult phase, R. pachyptila lacks a digestive system. To provide its energetic needs, it retains those dissolved inorganic nutrients (sulfide, carbon dioxide, oxygen, nitrogen) into its plume and transports them through a vascular system to the trophosome, which is suspended in paired coelomic cavities and is where the intracellular symbiotic bacteria are found.<ref name = "Stewart_2006" /><ref name="Childress_1993">Template:Cite journal</ref><ref name="Childress_1984">Template:Cite journal</ref> The trophosome<ref name="Robidart_2008">Template:Cite journal</ref> is a soft tissue that runs through almost the whole length of the tube's coelom. It retains a large number of bacteria on the order of 10⁹ bacteria per gram of fresh weight.<ref name="Hand_1983">Template:Cite journal</ref> Bacteria in the trophosome are retained inside bacteriocytes, thereby having no contact with the external environment. Thus, they rely on R. pachyptila for the assimilation of nutrients needed for the array of metabolic reactions they employ and for the excretion of waste products of carbon fixation pathways. At the same time, the tube worm depends completely on the microorganisms for the byproducts of their carbon fixation cycles that are needed for its growth.

Initial evidence for a chemoautotrophic symbiosis in R. pachyptila came from microscopic and biochemical analyses showing Gram-negative bacteria packed within a highly vascularized organ in the tubeworm trunk called the trophosome.<ref name="Cavanaugh_1981" /> Additional analyses involving stable isotope,<ref>Template:Cite journal</ref> enzymatic,<ref>Template:Cite journal</ref><ref name="Felbeck_1981" /> and physiological<ref>Template:Cite book</ref> characterizations confirmed that the end symbionts of R. pachyptila oxidize reduced-sulfur compounds to synthesize ATP for use in autotrophic carbon fixation through the Calvin cycle. The host tubeworm enables the uptake and transport of the substrates required for thioautotrophy, which are HS⁻, O₂, and CO₂, receiving back a portion of the organic matter synthesized by the symbiont population. The adult tubeworm, given its inability to feed on particulate matter and its entire dependency on its symbionts for nutrition, the bacterial population is then the primary source of carbon acquisition for the symbiosis. Discovery of bacterial–invertebrate chemoautotrophic symbioses, initially in vestimentiferan tubeworms<ref name="Cavanaugh_1981" /><ref name="Felbeck_1981" /> and then in vesicomyid clams and mytilid mussels,<ref name="Felbeck_1981" /> pointed to an even more remarkable source of nutrition sustaining the invertebrates at vents.

A wide range of bacterial diversity is associated with symbiotic relationships with R. pachyptila. Many bacteria belong to the phylum Campylobacterota (formerly class Epsilonproteobacteria)<ref name="López-García_2002">Template:Cite journal</ref> as supported by the recent discovery in 2016 of the new species Sulfurovum riftiae belonging to the phylum Campylobacterota, family Helicobacteraceae isolated from R. pachyptila collected from the East Pacific Rise.<ref name="Sulfurovum riftiae sp. nov., a meso">Template:Cite journal</ref> Other symbionts belong to the class Delta-, Alpha- and Gammaproteobacteria.<ref name="López-García_2002" /> The Candidatus Endoriftia persephone (Gammaproteobacteria) is a facultative R. pachyptila symbiont and has been shown to be a mixotroph, thereby exploiting both Calvin Benson cycle and reverse TCA cycle (with an unusual ATP citrate lyase) according to availability of carbon resources and whether it is free living in the environment or inside a eukaryotic host. The bacteria apparently prefer a heterotrophic lifestyle when carbon sources are available.<ref name="Robidart_2008" />

Evidence based on 16S rRNA analysis affirms that R. pachyptila chemoautotrophic bacteria belong to two different clades: Gammaproteobacteria<ref>Template:Cite journal</ref><ref name = "Stewart_2006" /> and Campylobacterota (e.g. Sulfurovum riftiae)<ref name="Sulfurovum riftiae sp. nov., a meso"/> that get energy from the oxidation of inorganic sulfur compounds such as hydrogen sulfide (H₂S, HS⁻, S^2-) to synthesize ATP for carbon fixation via the Calvin cycle.<ref name = "Stewart_2006" /> Unfortunately, most of these bacteria are still uncultivable. Symbiosis works so that R. pachyptila provides nutrients such as HS⁻, O₂, CO₂ to bacteria, and in turn it receives organic matter from them. Thus, because of a lack of a digestive system, R. pachyptila depends entirely on its bacterial symbiont to survive.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

In the first step of sulfide-oxidation, reduced sulfur (HS⁻) passes from the external environment into R. pachyptila blood, where, together with O₂, it is bound by hemoglobin, forming the complex Hb-O₂-HS⁻ and then it is transported to the trophosome, where bacterial symbionts reside. Here, HS⁻ is oxidized to elemental sulfur (S⁰) or to sulfite (SO₃^2-).<ref name = "Stewart_2006" />

In the second step, the symbionts make sulfite-oxidation by the "APS pathway", to get ATP. In this biochemical pathway, AMP reacts with sulfite in the presence of the enzyme APS reductase, giving APS (adenosine 5'-phosphosulfate). Then, APS reacts with the enzyme ATP sulfurylase in presence of pyrophosphate (PPi) giving ATP (substrate-level phosphorylation) and sulfate (SO₄^2-) as end products.<ref name = "Stewart_2006" /> In formulas:

<chem>AMP + SO3^2- ->[APSreductase] APS</chem>
<chem>APS + PPi ->[ATP sulfurylase] ATP + SO4^2-</chem>

The electrons released during the entire sulfide-oxidation process enter in an electron transport chain, yielding a proton gradient that produces ATP (oxidative phosphorylation). Thus, ATP generated from oxidative phosphorylation and ATP produced by substrate-level phosphorylation become available for CO₂ fixation in Calvin cycle, whose presence has been demonstrated by the presence of two key enzymes of this pathway: phosphoribulokinase and RubisCO.<ref name="Felbeck_1981" /><ref>Template:Cite journal</ref>

To support this unusual metabolism, R. pachyptila has to absorb all the substances necessary for both sulfide-oxidation and carbon fixation, that is: HS⁻, O₂ and CO₂ and other fundamental bacterial nutrients such as N and P. This means that the tubeworm must be able to access both oxic and anoxic areas.

Oxidation of reduced sulfur compounds requires the presence of oxidized reagents such as oxygen and nitrate. Hydrothermal vents are characterized by conditions of high hypoxia. In hypoxic conditions, sulfur-storing organisms start producing hydrogen sulfide. Therefore, the production of in H₂S in anaerobic conditions is common among thiotrophic symbiosis. H₂S can be damaging for some physiological processes as it inhibits the activity of cytochrome c oxidase, consequentially impairing oxidative phosphorylation. In R. pachyptila the production of hydrogen sulfide starts after 24h of hypoxia. In order to avoid physiological damage some animals, including Riftia pachyptila are able to bind H₂S to haemoglobin in the blood to eventually expel it in the surrounding environment.

Nitrate and nitrite are toxic, but are required for biosynthetic processes. The chemosynthetic bacteria within the trophosome convert nitrate to ammonium ions, which then are available for production of amino acids in the bacteria, which are in turn released to the tube worm. To transport nitrate to the bacteria, R. pachyptila concentrates nitrate in its blood, to a concentration 100 times more concentrated than the surrounding water. The exact mechanism of R. pachyptila's ability to withstand and concentrate nitrate is still unknown.<ref name="Hahlbeck_2005" />

Carbon fixation and organic carbon assimilation

Unlike metazoans, which respire carbon dioxide as a waste product, R. pachyptila-symbiont association has a demand for a net uptake of CO₂ instead, as a cnidarian-symbiont associations.<ref name="Van_Dover_2004">Template:Cite journal</ref> Ambient deep-sea water contains an abundant amount of inorganic carbon in the form of bicarbonate HCO₃⁻, but it is actually the chargeless form of inorganic carbon, CO₂, that is easily diffusible across membranes. The low partial pressures of CO₂ in the deep-sea environment is due to the seawater alkaline pH and the high solubility of CO₂, yet the pCO₂ of the blood of R. pachyptila may be as much as two orders of magnitude greater than the pCO₂ of deep-sea water.<ref name="Van_Dover_2004" />

CO₂ partial pressures are transferred to the vicinity of vent fluids due to the enriched inorganic carbon content of vent fluids and their lower pH.<ref name = "Stewart_2006" /> CO₂ uptake in the worm is enhanced by the higher pH of its blood (7.3–7.4), which favors the bicarbonate ion and thus promotes a steep gradient across which CO₂ diffuses into the vascular blood of the plume.<ref>Template:Cite journal</ref><ref name="Stewart_2006" /> The facilitation of CO₂ uptake by high environmental pCO₂ was first inferred based on measures of elevated blood and coelomic fluid pCO₂ in tubeworms, and was subsequently demonstrated through incubations of intact animals under various pCO₂ conditions.<ref name="Childress_1984" />

Once CO₂ is fixed by the symbionts, it must be assimilated by the host tissues. The supply of fixed carbon to the host is transported via organic molecules from the trophosome in the hemolymph, but the relative importance of translocation and symbiont digestion is not yet known.<ref name="Childress_1984" /><ref name="Bright_2000">Template:Cite journal</ref> Studies proved that within 15 min, the label first appears in symbiont-free host tissues, and that indicates a significant amount of release of organic carbon immediately after fixation. After 24 h, labeled carbon is clearly evident in the epidermal tissues of the body wall. Results of the pulse-chase autoradiographic experiments were also evident with ultrastructural evidence for digestion of symbionts in the peripheral regions of the trophosome lobules.<ref name="Bright_2000" /><ref>Template:Cite journal</ref>

Sulfide acquisition

In deep-sea hydrothermal vents, sulfide and oxygen are present in different areas. Indeed, the reducing fluid of hydrothermal vents is rich in sulfide, but poor in oxygen, whereas seawater is richer in dissolved oxygen. Moreover, sulfide is immediately oxidized by dissolved oxygen to form partly, or totally, oxidized sulfur compounds like thiosulfate (S₂O₃^2-) and ultimately sulfate (SO₄^2-), respectively less, or no longer, usable for microbial oxidation metabolism.<ref>Template:Cite journal</ref> This causes the substrates to be less available for microbial activity, thus bacteria are constricted to compete with oxygen to get their nutrients. In order to avoid this issue, several microbes have evolved to make symbiosis with eukaryotic hosts.<ref>Template:Cite journal</ref><ref name="Stewart_2006"/> In fact, R. pachyptila is able to cover the oxic and anoxic areas to get both sulfide and oxygen<ref>Template:Cite journal</ref><ref>Template:Cite book</ref><ref>Template:Cite journal</ref> thanks to its hemoglobin that can bind sulfide reversibly and apart from oxygen by functional binding sites determined to be zinc ions embedded in the A2 chains of the hemoglobins.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> and then transport it to the trophosome, where bacterial metabolism can occur. It has also been suggested that cysteine residues are involved in this process.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

During the chemosynthesis, the mitochondrial enzyme rhodanase catalyzes the disproportionation reaction of the thiosulfate anion S₂O₃^2- to sulfur S and sulfite SO₃^2- .<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The R. pachyptila's bloodstream is responsible for absorption of the O₂ and nutrients such as carbohydrates.

Biology

Template:Multiple image Riftia pachyptila are sessile and are found clustered together around deep-sea hydrothermal vents of the East Pacific Rise and the Galapagos Rift.<ref name="Coykendall_2011"/> The size of a patch of individuals surrounding a vent is can span tens of metres.<ref>Template:Cite journal</ref> R. pachyptila has the fastest growth rate of any known marine invertebrate. These organisms have been known to colonize a new site, grow to sexual maturity, and increase in length to Template:Cvt in less than two years.<ref name="Lutz_1994">Template:Cite journal</ref>

Study of the Rose Garden site determined that the three species of major sessile animals present have different microhabitat requirements;<ref name="BioChem"/> that is to say, they are able to coexist at the same sites through niche partitioning. Riftia apparently requires areas of high water flow, temperature, and sulfide concentrations, in contrast to Calyptogena magnificaTemplate:'s need of low flow and temperature, and Bathymodiolus thermophilusTemplate:' flexibility; B. thermophilus mussels may grow in places ranging from the periphery of the vent field, to directly on RiftiaTemplate:'s tubes.<ref name="BioChem"/> The Rose Garden was estimated to have become hydrothermically active in the early 70s, and while Riftia and the two bivalve species probably colonized the site shortly after, RiftiaTemplate:'s greater "autotrophic potential" allowed them to grow faster and dominate the site before the 1979 observation. Between 1979 and 1985, the more opportunistic mussels reached their maximum sizes, and their dense growth on Riftia tubes altered the local microhabitat; vent flows were diverted away from the tubeworm's plumes and mussel growth may have interfered with the growth of smaller Riftia worms. As a result, the tubeworms dominance over the site subsided, with the bivalves displacing the tubeworms as the ecosystem matured with a stable hydrothermal flow, and the two species are thought to persist for "much longer" than the tubeworms if the flow gradually declined.<ref name="BioChem"/>

It is suggested that the smaller-bodied tubeworm Tevnia may facilitate later vent colonization by Riftia.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Reproduction

R. pachyptila is dioecious, having two sexes.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The male's spermatozoa are thread-shaped and are composed of three distinct regions: the acrosome (6 μm), the nucleus (26 μm) and the tail (98 μm). Thus, the single spermatozoon is about 130 μm long overall, with a diameter of 0.7 μm, which becomes narrower near the tail area, reaching 0.2 μm. The sperm is arranged into an agglomeration of around 340–350 individual spermatozoa that form a torch-like shape. The cup part is made up of acrosomes and nucleus, while the handle is made up by the tails. The spermatozoa in the package are held together by fibrils. Fibrils also coat the package itself to ensure cohesion.Template:Citation needed

The large ovaries of females run within the gonocoel along the entire length of the trunk and are ventral to the trophosome. Eggs at different maturation stages can be found in the middle area of the ovaries, and depending on their developmental stage, are referred to as: oogonia, oocytes, and follicular cells. When the oocytes mature, they acquire protein and lipid yolk granules.Template:Citation needed

Males release their sperm into seawater. While the released agglomerations of spermatozoa, referred to as spermatozeugmata, do not remain intact for more than 30 seconds in laboratory conditions, they may maintain integrity for longer periods of time in specific hydrothermal vent conditions. Usually, the spermatozeugmata swim into the female's tube. Movement of the cluster is conferred by the collective action of each spermatozoon moving independently. Reproduction has also been observed involving only a single spermatozoon reaching the female's tube. Generally, fertilization in R. pachyptila is considered internal. However, some argue that, as the sperm is released into seawater and only afterwards reaches the eggs in the oviducts, it should be defined as internal-external.Template:Citation needed

R. pachyptila develops from a free-swimming, pelagic, non-symbiotic trochophore larva, which enters juvenile (metatrochophore) development, becoming sessile, and subsequently acquiring symbiotic bacteria.<ref name=Bright>Template:Cite web</ref><ref>Template:Cite web</ref> The symbiotic bacteria, on which adult worms depend for sustenance, are not present in the gametes, but are acquired from the environment through the skin in a process similar to an infection. The digestive tract is transiently connected from a mouth at the tip of the ventral medial process to a foregut, midgut, hindgut, and anus and was previously thought to have been the method by which the bacteria are introduced into adults. After symbionts are established in the midgut, they undergo substantial remodeling and enlargement to become the trophosome, while the remainder of the digestive tract has not been detected in adult specimens.<ref name="Jones 1989">Template:Cite journal</ref>

R. pachyptila is completely dependent on the production of volcanic gases and the presence of sulfide-oxidizing bacteria. Therefore, its metapopulation distribution is profoundly linked to volcanic and tectonic activity that creates active hydrothermal vent sites with a patchy and ephemeral distribution. The distance between active sites along a rift or adjacent segments can be very high, reaching hundreds of km.<ref name="Coykendall_2011">Template:Cite journal</ref> This raises the question regarding larval dispersal. R. pachytpila is capable of larval dispersal across distances of 100 to 200 km<ref name="Coykendall_2011" /> and cultured larvae have been shown to be viable for 38 days.<ref>Template:Cite journal</ref><ref>Template:Cite web</ref> Though dispersal is considered to be effective, the genetic variability observed in R. pachyptila metapopulation is low compared to other vent species. This may be due to high extinction events and colonization events, as R. pachyptila is one of the first species to colonize a new active site.<ref name="Coykendall_2011" />

Some sources list Riftia pachyptila as one of the world's longest living organisms,<ref>Template:Cite web</ref> but the unstable vent environment and high ecological turnover makes this claim unlikely.<ref>Template:Cite web</ref> Tube worms inhabiting stable cold seep environments such as Lamellibranchia and Escarpia have better claims to longevity, with age measurements of both species exceeding a century.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite web</ref>

Symbiont acquisition

The endosymbionts of R. pachyptila are not passed to the fertilized eggs during spawning, but are acquired later during the larval stage of the vestimentiferan worm. R. pachyptila planktonic larvae are transported through sea-bottom currents until they reach active hydrothermal vents sites, are referred to as trophocores. The trophocore stage lacks endosymbionts, which are acquired once larvae settle in a suitable environment and substrate. Free-living bacteria found in the water column are ingested randomly and enter the worm through a ciliated opening of the branchial plume. This opening is connected to the trophosome through a duct that passes through the brain. Once the bacteria are in the gut, the ones that are beneficial to the individual, namely sulfide-oxidizing strains are phaghocytized by epithelial cells found in the midgut are then retained. Bacteria that do not represent possible endosymbionts are digested. This raises questions as to how R. pachyptila manages to discern between essential and nonessential bacterial strains. The worm's ability to recognise a beneficial strain, as well as preferential host-specific infection by bacteria have been both suggested as being the drivers of this phenomenon.<ref name="Jones 1989"/>

The acquisition of a symbiont by a host can occur in these ways:

Environmental transfer (symbiont acquired from a free-living population in the environment)
Vertical transfer (parents transfer symbiont to offspring via eggs)
Horizontal transfer (hosts that share the same environment)

Evidence suggests that R. pachyptila acquires its symbionts through its environment. In fact, 16S rRNA gene analysis showed that vestimentiferan tubeworms belonging to three different genera: Riftia, Oasisia, and Tevnia, share the same bacterial symbiont phylotype.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

This proves that R. pachyptila takes its symbionts from a free-living bacterial population in the environment. Other studies also support this thesis, because analyzing R. pachyptila eggs, 16S rRNA belonging to the symbiont was not found, showing that the bacterial symbiont is not transmitted by vertical transfer.<ref>Template:Cite journal</ref>

Another proof to support the environmental transfer comes from several studies conducted in the late 1990s.<ref>Template:Cite journal</ref> PCR was used to detect and identify a R. pachyptila symbiont gene whose sequence was very similar to the fliC gene that encodes some primary protein subunits (flagellin) required for flagellum synthesis. Analysis showed that R. pachyptila symbiont has at least one gene needed for flagellum synthesis. Hence, the question arose as to the purpose of the flagellum. Flagellar motility would be useless for a bacterial symbiont transmitted vertically, but if the symbiont came from the external environment, then a flagellum would be essential to reach the host organism and to colonize it. Indeed, several symbionts use this method to colonize eukaryotic hosts.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Thus, these results confirm the environmental transfer of R. pachyptila symbiont.

References

External links

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Riftia

Contents

Discovery