User:Kbseah/Chemosynthetic symbiosis

Chemosynthetic symbiosis is a type of symbiotic relationship between two or more species, where at least one partner is able to grow and reproduce using inorganic chemicals as an energy source (chemosynthesis). The other partner may rely on its chemosynthetic symbiont for food. Examples include the tubeworm Riftia pachyptila from deep-sea hydrothermal vents, and the bivalve family Lucinidae from seagrass ecosystems in coastal environments.

Chemosynthesis
Chemosynthesis is the ability to use energy from chemical oxidation to synthesize new biomass from one-carbon compounds, such as carbon dioxide or methane. Those organisms that use carbon dioxide obtain the energy for doing so by oxidizing other inorganic chemicals, such as hydrogen sulfide, a type of metabolism known as chemolithoautotrophy. Methane, on the other hand, can serve as a source of both carbon and energy, and organisms that can use it are known as methanotrophs. All known chemosynthetic organisms are prokaryotes. The term "chemosynthesis" was first used by Wilhelm Pfeffer in 1897, as a parallel to "photosynthesis".

Discovery

 * First discovered in 1977 at hydrothermal vents off the Galapagos
 * The abundance of animal life at these vents was unexpected
 * Several theories were proposed for how mouthless, gutless animals could feed themselves to grow and reproduce
 * Shown that they have sulfide-oxidizing bacteria in their bodies, which can gain energy from chemicals in the hydrothermal vent fluids, and fix carbon dioxide for growth.
 * Animals in other habitats, such as shallow-water seagrass beds, were also found to harbor such symbionts.

Diversity
Chemosynthetic symbiosis has evolved independently several times in different groups of eukaryotic hosts and bacterial symbionts. They are associated with many different host species, most of which are animals. They are found in at least six different phyla within the animal kingdom. One group of protists, the ciliates, also have such symbionts. The vestimentiferan tube worms were originally classified in the phylum Pogonophora (beard worms), but subsequent studies showed that pogonophorans are actually annelid worms (phylum Annelida), and not a distinct phylum of their own.

The symbiotic microbes also come from different groups within the Bacteria. Most of the sulfur-oxidizing symbionts are members of the class Gammaproteobacteria, but some also belong to the Epsilonproteobacteria (e.g. symbionts of Rimicaris), or Alphaproteobacteria (symbionts of Paracatenula).

The known examples of methanotrophic symbionts are most closely related to the Type I subgroup of free-living aerobic methanotrophs within the class Gammaproteobacteria.

Several species of chemosynthetic symbiotic bacteria have been given provisional scientific names, indicated by the prefix Candidatus. These taxa are only "putative", because they have not been cultivated in the laboratory, which is a requirement for the formal description of a microbial species. Some host species have more than one kind of chemosynthetic bacterium living as a symbiont. Several species of deep-sea mytilid mussels have both sulfur-oxidizing and methanotrophic bacteria living inside the cells of its gills. The annelid worm Olavius algarvensis contains two different kinds of sulfur-oxidizing bacteria, in addition to three other types of bacteria that are heterotrophs.

Digestive system
Many host species have lost or reduced their digestive tract (mouth, gut, anus) in the course of evolution. They are therefore thought to be completely reliant on their symbionts for food. Examples include Paracatenula, Olavius, and Astomonema, whose name is a direct reference to its lack of a mouth. The vestimentiferan tubeworms have a digestive tract during the larval stage of their life cycle, but this is lost during metamorphosis to the adult stage.

In contrast, some hosts still have a digestive tract, e.g. the stilbonematine nematodes and the Rimicaris shrimp. They may feed like non-symbiotic animals, or could be grazing on the symbionts.

Location of symbionts in host body
Symbionts can be located at different parts of the host organism's body, depending on the species. The vestimentiferan tubeworms, such as Riftia pachyptila, have a specialized symbiont-bearing tissue called the trophosome that is located within the coelomic cavity in the trunk of the adult animal. The trophosome is well-supplied with blood vessels, and the bacteria are located inside cells called bacteriocytes. In bivalves, the symbionts are often located in the gills. For example, the clam Solemya velum has unusually large gills (compared to other filter-feeding bivalves) that make up 38% of the animal's total weight. The bacteria are also contained within bacteriocyte cells, which alternate with symbiont-free cells in the gill tissue. Symbionts can also be located extracellularly, such as the layer of symbionts attached to the cuticle of roundworms in the subfamily Stilbonematinae, or attached to the carapace within the gill chamber of the deep-sea shrimp Rimicaris exoculata.

Energy sources
Many chemosynthetic symbionts use hydrogen sulfide, or other chemically-reduced species of sulfur, as a source of energy and reducing equivalents for growth, with the following net reaction, where [CH2O] represents biomass:


 * CO2 + H2S + 2 H2O -> [CH2O] + H2SO4.

Methanotrophs, which oxidize methane (CH4), can use it as both a source of energy and carbon for growth. Methanotrophic symbionts belong to the Type I subgroup of methanotrophs, which use the ribulose monophosphate (RuMP) pathway for assimilation of carbon.

Symbionts that use hydrogen (H2) and carbon monoxide (CO) have also been discovered.

Carbon-fixation pathways
A variety of metabolic pathways for autotrophic carbon fixation are used by chemosynthetic symbionts to produce new biomass. The majority that fix carbon dioxide (CO2) use the Calvin-Benson-Bassham cycle, also known as the reductive pentose phosphate cycle. Methanotrophs, which use methane rather than carbon dioxide, use the RuMP pathway to fix carbon, as described above. The symbionts of the vent tubeworm Riftia pachyptila use two different autotrophic pathway: the Calvin-Benson-Bassham cycle and the reductive tricarboxylic acid cycle (Arnon-Buchanan cycle), which is an alternative autotrophic pathway found in many anaerobic bacteria.

Ecology