Retinalophototroph

Retinalophototrophs achieve adequate energy conversion via a proton-motive force.[3][4] In retinalophototrophs, proton-motive force is generated from rhodopsin-like proteins, primarily bacteriorhodopsin and proteorhodopsin, acting as proton pumps along a cellular membrane.[1][4]

To capture photons needed for activating a protein pump, retinalophototrophs employ organic pigments known as carotenoids, namely beta-carotenoids.[7][3][4] Beta-carotenoids present in retinalophototrophs are unusual candidates for energy conversion, but they possess high Vitamin-A activity necessary for retinaldehyde, or retinal, formation.[7][3][4] Retinal, a chromophore molecule configured from Vitamin A, is formed when bonds between carotenoids are disrupted in a process called cleavage.[7][3][4] Due to its acute light sensitivity, retinal is ideal for activation of proton-motive force and imparts a unique purple coloration to retinalophototrophs.[1][4] Once retinal absorbs enough light, it isomerizes, thereby forcing a conformational (i.e., structural) change among the covalent bonds of the rhodopsin-like proteins.[1][3][4] 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.[1][4] Ions diffusing outwards across the gradient through proton pumps are then bound to ATP synthase proteins on the cell’s surface.[1][4] 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.[1][4]

Many, if not all, retinalophototrophs are photoheterotrophs: 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 CO₂. This category includes retinalophototrophs that perform anaplerotic fixation, such as a flavobacterium that can use pyruvate and CO₂ to make malate. This ability does, however, help "stretch" limited supplies of carbon.[8]

Retinalophototrophs are found across all domains of life but predominantly in the Bacteria and Archaea taxonomic groups.[5][2][6] Scientists believe retinalophototroph’s general ecological abundance correlates to horizontal gene transfer 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).[3][4]

Despite their apparent simplicity, retinalophototrophs boast versatile ion usage that translates to their existence in relatively extreme environments.[3] 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.[3] 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.[4] Even though retinalophototrophs are widespread, research has shown they can be niche too.[1][6] Depending on their proximity to the oceans surface, retinalophototrophs have evolved to be better at absorbing light within specific wavelengths.[1][6] 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.[1][6]

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.[6]