User:Pierangelo Ferrara/sandbox

Endosymbiosis with thioautotrophic bacteria
Evidences based on 16S rRNA analysis affirm that Riftia chemoautotrophic bacteria belong to two different phyla of Proteobacteria superphylum: Gammaproteobacteria phylum and Epsilonproteobacteria phylum (e.g. Sulfurovum riftiae) that get energy from the oxidation of inorganic sulfur compounds such as hydrogen sulfide (H2S, HS-, S2-) in order to synthetize ATP for carbon fixation via Calvin cycle. Unfortunately, most of these bacteria are still uncultivable. Symbiosis works so that Riftia provides nutrients such as HS-, O2, CO2 to bacteria, and in turn it receives a lot of organic matter from them. Thus, because of lack of digestive system, Riftia depends entirely on its bacterial symbiont in order to survive.

In the first step of sulfide-oxidation, reduced sulfur (HS-) passes from the external environment into Riftia blood, where, together with O2, it's bound by hemoglobin forming the complex Hb-O2-HS- and then it's transported to the trophosome, where bacterial symbiont resides. Here, HS- is oxidized to elemental sulfur (S0) or to sulfite (SO32-).

In the second step, Riftia symbiont makes sulfite-oxidation thanks to "APS pathway", in order 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 (SO42-) as end products. In formulas:





The electrons released during the entire sulfide-oxidation process enter in an electron transport chain, yielding a proton gradient that produces ATP (oxydative 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 (PRK) and RubisCO.

To support this unsual metabolism, Riftia has to assume all the substances necessary for both sulfide-oxidation and carbon fixation that is: HS-, O2 and CO2 and other fundamental bacterial nutrients like N and P. This means that Riftia must be able to access both oxic and anoxic areas.

Sulfide acquisition
In deep-sea hydrothermal vents, sulfide and oxygen are present in different areas. Indeed vent fluid of hydrothermal vents is rich in sulfide, but poor in oxygen, whereas sea water is rich in oxygen. Moreover, sulfide reacts immediately with oxygen to form sulfur compounds like S2O32- or  SO42-, unusable for microbial metabolism. This causes the substrates are 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 eucariotic hosts. In fact Riftia pachyptila is able to cover the oxic and anoxic areas in order to get both sulfide and oxygen  thanks to its hemoglobin that can bind sulfide reversibly and apart from oxygen by means of two cysteine residues  , and then transport it to the trophosome where bacteria metabolism can occur.

Symbiont acquisition
The acquisition of a symbiont by an host can occur in three different ways:


 * by environmental transfer (symbiont acquered from a free-living population in the environment);
 * by vertical transfer (parents transfer symbiont to offspring via eggs);
 * by horizontal transfer (hosts that share the same environment).

Evidences suggest that Riftia pachyptila acquires its symbiont via 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. This proves that Riftia takes its symbiont from a free-living bacterial population in the environment. Other studies also support this thesis, beacause analyzing R.pachyptila eggs there were not found 16S rRNA belonging to symbiont, showing that bacterial symbiont is not transmitted via vertical transfer.

Another proof to support the environmenal transfer comes from several studies conducted in the late 90's. PCR was used to detect and identify a Riftia symbiont gene whose sequence was very similar to fliC gene which encodes some primary protein subunities (flagellin) required for flagellum synthesis. Analysis showed that Riftia symbiont has at least one gene needed for flagellum synthesis. Hence the question was: what the flagellum was for. 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 expedient to colonize eukaryotic hosts.

Thus, these results led to confirm the thesis of environmental transfer of R.Pachyptila symbiont.

Curiosity

 * 1) The association between Riftia pachyptila and its symbiont was the first symbiosis described for a marine invertebrate and chemoautotrophic bacteria.
 * 2) Riftia pachyptila has very high growth rates, in fact it is estimated an increase of up 1.4% of its organic carbon per day.