Retinalophototroph
   HOME

TheInfoList



OR:

A retinalophototroph is one of two different types of
phototroph Phototrophs () are organisms that carry out photon capture to produce complex organic compounds (e.g. carbohydrates) and acquire energy. They use the energy from light to carry out various cellular metabolic processes. It is a list of common m ...
s, and are named for
retinal Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). Some microorganisms use ret ...
-binding proteins (
microbial rhodopsin Microbial rhodopsins, also known as bacterial rhodopsins, are retinal-binding proteins that provide light-dependent ion transport and sensory functions in halophilic and other bacteria. They are integral membrane proteins with seven transmembr ...
s) they utilize for cell signaling and converting light into energy. Like all phototrophs, retinalophototrophs absorb photons to initiate their cellular processes. In contrast with chlorophototrophs, retinalophototrophs do not use
chlorophyll Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...
or an
electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules which transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples th ...
to power their chemical reactions. This means retinalophototrophs are incapable of traditional
carbon fixation Biological carbon fixation, or сarbon assimilation, is the Biological process, process by which living organisms convert Total inorganic carbon, inorganic carbon (particularly carbon dioxide, ) to Organic compound, organic compounds. These o ...
, a fundamental photosynthetic process that transforms inorganic carbon (carbon contained in molecular compounds like
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
) into organic compounds. For this reason, experts consider them to be less efficient than their chlorophyll-using counterparts, chlorophototrophs.


Energy conversion

Retinalophototrophs achieve adequate energy conversion via a
proton-motive force Chemiosmosis is the movement of ions across a semipermeable membrane bound structure, down their electrochemical gradient. An important example is the formation of adenosine triphosphate (ATP) by the movement of hydrogen ions (H+) across a membra ...
. In retinalophototrophs, proton-motive force is generated from rhodopsin-like proteins, primarily
bacteriorhodopsin Bacteriorhodopsin (Bop) is a protein used by Archaea, most notably by Haloarchaea, a class of the Euryarchaeota. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell. The res ...
and
proteorhodopsin Proteorhodopsin (PR or pRhodopsin) belongs to the Protein family, family of Bacteria, bacterial Transmembrane protein, transmembrane Rhodopsin, rhodopsins (Retinylidene protein, retinylidene proteins). In 1971, the first Microorganism, microbial ...
, acting as proton pumps along a cellular membrane. To capture photons needed for activating a protein pump, retinalophototrophs employ organic pigments known as carotenoids, namely beta-carotenoids. Beta-carotenoids present in retinalophototrophs are unusual candidates for energy conversion, but they possess high Vitamin-A activity necessary for retinaldehyde, or retinal, formation. Retinal, a chromophore molecule configured from Vitamin A, is formed when bonds between carotenoids are disrupted in a process called cleavage. Due to its acute light sensitivity, retinal is ideal for activation of proton-motive force and imparts a unique purple coloration to retinalophototrophs. Once retinal absorbs enough light, it isomerizes, thereby forcing a conformational (i.e., structural) change among the covalent bonds of the rhodopsin-like proteins. Upon activation, these proteins mimic a gateway, allowing passage of ions to create an electrochemical gradient between the interior and exterior of the cellular membrane. Ions diffusing outwards across the gradient through proton pumps are then bound to ATP synthase proteins on the cell’s surface. As they diffuse back into the cell, their protons catalyze the creation of ATP (from ADP and a phosphorus ion), providing energy for retinalophototrophic self-sustenance and proliferation.


Interaction with carbon

Many, if not all, retinalophototrophs are
photoheterotroph Photoheterotrophs (''Greek language, Gk'': ''photo'' = light, ''hetero'' = (an)other, ''troph'' = nourishment) are heterotrophic phototrophs—that is, they are organisms that use light for energy, but cannot use carbon dioxide as their sole carbon ...
s: although sufficient ATP is produced by light, they cannot subsist on light and inorganic substances alone because they cannot produce needed organic materials from only . This category includes retinalophototrophs that perform anaplerotic fixation, such as a flavobacterium that can use pyruvate and CO2 to make
malate Malic acid is an organic compound with the molecular formula . It is a dicarboxylic acid that is made by all living organisms, contributes to the sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms ( ...
. This ability does, however, help "stretch" limited supplies of carbon.{{Cite journal , doi = 10.1073/pnas.0712027105 , issn = 0027-8424 , volume = 105 , issue = 25 , pages = 8724–8729 , author1=González, José M. , author2=Fernández-Gómez, Beatriz , author3=Fernàndez-Guerra, Antoni , author4=Gómez-Consarnau, Laura , author5=Sánchez, Olga , author6=Coll-Lladó, Montserrat , author7=del Campo, Javier , author8=Escudero, Lorena , author9=Rodríguez-Martínez, Raquel , author10=Alonso-Sáez, Laura , author11=Latasa, Mikel , author12=Paulsen, Ian , author13=Nedashkovskaya, Olga , author14=Lekunberri, Itziar , author15=Pinhassi, Jarone , author16=Pedrós-Alió, Carlos , display-authors=6 , title=Genome Analysis of the Proteorhodopsin-Containing Marine Bacterium Polaribacter Sp. MED152 (Flavobacteria) , journal=Proceedings of the National Academy of Sciences , date=2008-06-24 , df=dmy-all , pmid=18552178 , pmc=2438413, doi-access = free


Taxonomy

Retinalophototrophs are found across all domains of life but predominantly in the ''Bacteria'' and ''Archaea'' taxonomic groups. Scientists believe retinalophototroph’s general ecological abundance correlates to
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...
since only two genes are required for retinalophototrophy to occur: essentially, one gene for retinal-binding protein synthesis (bop) and one for retinal chromophore synthesis (blh).


Interactions with environment

Despite their apparent simplicity, retinalophototrophs boast versatile ion usage that translates to their existence in relatively extreme environments. For instance, retinalophototrophs can thrive at depths over 200 meters where, despite a lack of inorganic carbon, sufficient light as well as sodium, hydrogen, or chloride concentrations harbor conditions capable of supporting their vital metabolic processes. Studies have also shown sodium and hydrogen ions correlate directly with retinalophototroph’s nutrient uptake and ATP synthesis, while chloride drives processes responsible for osmotic equilibrium. Even though retinalophototrophs are widespread, research has shown they can be niche too. Depending on their proximity to the oceans surface, retinalophototrophs have evolved to be better at absorbing light within specific wavelengths. Most importantly, retinalophototrophs prevalence as a primary producer contributes substantially to the bottom-up mechanics of marine environments and, consequently, success of fauna and flora worldwide. Although retinalophototrophs are less efficient at converting light than chlorophototrophs, the simplicity makes it the preferred system in a large number of environments. For example, because retinalophototrophs requires no iron in the reaction center, they are well-adapted to the iron-poor ocean environment. At high light level, they are more efficient in terms of protein investment to energy output due to the small size.


References

Photosynthesis Trophic ecology Microbial growth and nutrition Biology terminology