R.pachyptila lives in complete darkness and therefore is not able to gain energy from photoautotrophic primary producers. Lacking a mouth and digestive system, tubeworms are also incapable of ingesting organic material and breaking it down for energy (heterotrophy). Instead, similar to a number of other hydrothermal vent associated animals, R.pachyptila has developed the ability to harness the primary productivity of chemosynthetic prokaryotes as an energy source. The high concentrations of proteobacterium housed in the worm’s trophosome produce organic compounds as a result of chemosynthesis (Cavanaugh et al., 1981).
Bacterial chemosynthesis is a two stage process. The first stage is the oxidisation of hydrogen sulphide. This provides chemical energy in the form of ATP, allowing inorganic carbon to be fixed from carbon dioxide via the Calvin-Benson cycle. The second stage of this process produces the amino acid glutamine which is used by R.pachyptila as its food source (Van Dover, 2000).
In order for symbionts to produce food for the worm via chemosynthesis they require a number of metabolites (inorganic carbon, oxygen, sulphide and nitrogen) to be provided to them by the host.
Unlike other heterotrophic animals, R.pachyptila has a net intake of carbon dioxide. This is because the chemosynthetic need for carbon dioxide is double the amount of carbon dioxide produced by the respiration in the tubeworm’s cells (Childress et al., 1991). Because of this, the tubeworms need to uptake carbon dioxide from the surrounding seawater.
The environmental Carbon dioxide required is absorbed by the plume (figure 4) into the blood by diffusion (figure 5). The low pH of venting fluid aids with the diffusion of carbon dioxide because acidic seawater has a higher partial pressure (the pressure of gas that would be observed if it filled the entire volume) of carbon dioxide. Partial pressures of carbon dioxide in vent fluids are around three orders of magnitude higher than in the surrounding deep ocean water (Childress et al., 1993).
Another aid to the diffusion of carbon dioxide into the tubeworm’s blood is the presence of the enzyme carbonic anhydrase. This enzyme, most active in the plume and trophosome, catalyses the reaction of carbon dioxide into bicarbonate, therefore maintaining a carbon dioxide gradient between the environment and the body of the tubeworm (Kochevar and Childress, 1996).
Oxygen is diffused into the blood from the external environment via the plume (figure 5). Unlike carbon dioxide, which is free in the blood, oxygen binds to haemoglobin. Both the cells of the host and the symbionts require oxygen for respiration. Large amounts of haemoglobin are present in the blood as the symbionts require twice as much respiratory oxygen as the host does (Childress et al., 1991). Also the dilute mix of venting fluid has a lower oxygen concentration than that of normal deep water, therefore tubeworms require efficient uptake of oxygen (Van Dover, 2000).
The high affinity of haemoglobin for oxygen helps uptake and transport and also crucially prevents a build up of internal, dissolved oxygen. Without the maintenance of internal oxygen levels, the gradient of oxygen important for diffusion into the blood, would be reduced. Sulphides in the blood would also react with the oxygen, oxidising spontaneously (Childress and Fisher, 1992).
Due to the environment that tubeworms grow in, they experience a temperature gradient where seawater temperature is warmest at the base of the tube. At warmer temperatures the affinity of haemoglobin for oxygen is reduced (Arp et al. 1985) and the rate of dissociation of oxygen is increased (Wittenberg et al., 1981). The features help pass on oxygen from the blood to bacterial cells in the trophosome, which is located in the lower portion of the tubeworm (Van Dover, 2000).
Hydrogen sulphide provides the energy for the first stage of chemosynthesis (see above). Similarly to oxygen and carbon dioxide, sulphide in the form HS‾, diffuses into the blood by diffusion via the vascular plume of the tubeworm (figure 5). Hydrogen sulphide can be highly toxic to animals as it is able to bind to and block cytochrome-c oxidase, an enzyme important in oxidative phosphorylation (the final stage of respiration in which energy, in the form of ATP, is synthesised) (Van Dover, 2000). Free sulphide also reacts with oxygen resulting in the formation of compounds that are less reduced. These compounds can’t be used by the symbionts in chemosynthesis (Belkin et al., 1986) and so would interrupt the tubeworm’s source of nourishment if such a reaction occurred in the blood.
Tubeworms manage to avoid both of these detrimental effects as they have a larger form of haemoglobin in the blood that specifically binds to and has a high affinity for sulphide. This means that sulphide and oxygen do not compete for haemoglobin binding sites (Somero et al., 1989). While bound to haemoglobin, sulphide is more stable and does not bind to cytocrome-c oxidase (Van Dover, 2000). The high affinity for sulphide and large numbers of haemoglobin in the vascular system also allow the uptake of large amounts of sulphide without the accumulation of free sulphides in the blood that could react with oxygen (Childress et al., 1991).
Nitrogen is important for both the tubeworm host and its symbionts. Nitrate can be found in all amino acids, which build proteins. Nitrate can also be used by the symbionts during respiration as a terminal electron acceptor, if oxygen is absent, which is important for producing energy in the form of ATP (Cole and Ferguson, 1988). The lack of a digestive system indicates that R.pachyptila uptakes nitrogen from the environment rather than ingesting it in organic material (Jones, 1981). Nitrogen occurs at hydrothermal vent ecosystems in the form of dissolved nitrate (Johnson et al., 1988) and ammonia, carried in venting water (Lilley et al., 1993). Little is known however, about the mechanisms involved in the uptake and transportation of nitrogen in R.pachyptila (Van Dover, 2000; De Cian et al., 2000; Girguis et al., 2000).