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Essentials invisible to the naked eye

Wilhelm Kühne, known for coining the term “enzyme”, was the biochemist who extracted something he called “visual purple” from a bovine retina. In the midst of the 19th century, the scientist proposed that this molecule was a key part of the vision process. He wasn’t wrong. This light-capturing “antenna” is the main protein component of the membrane surrounding the outer segment disc of the retina’s rod cells. It is a pigment that is still giving technological cores much to work on, nearly two centuries after Kühne’s initial discoveries.

Light decomposes the rhodopsin molecule in the opsin protein and the retinal (vitamin A aldehyde), a mechanism that triggers a cascade of reactions to stimulate the optic nerve, which then transmits the nerve impulses to the brain. Kühne also discovered that rhodopsin photobleaches when exposed to visible light. For that reason, the retina has often been compared to photographic film, albeit with one key difference:  after its “exposure” it regenerates to receive a new image.

Crystal structure of rhodopsin

Source: Dasalam42 (Wikimedia)

In the 1970’s, Stoeckenius and Oesterhelt made another landmark discovery. After analyzing colorful microorganisms found in a pond with a high salt concentration, they found that Halobacterium salinarum contained something similar to the rhodopsin of our retinas. This way, they determined the existence of bacteriorhodopsin, a protein that acts like a proton pump to turn light into energy, and in some cases, it can occupy 50% of cells’ surface area. In addition to the basic studies conducted to understand its structure and functions, a number of researchers have explored the biotechnological applications of this molecule in recent years. They include, among others, the generation of electricity or the production of hydrogen.

A study led by the National Center for Biotechnology (CNB-CSIC) has reviewed current knowledge on the genetic diversity, physiology and ecology of rhodopsins. The study, published in Microbiology and Molecular Biology Reviews, reviews the latest discoveries around this molecule, which allows more than half of non-photosynthetic marine microorganisms to harness energy from sunlight. This finding was possible thanks to the application of metagenomics, a technology that revealed the existence of a molecule similar to the one discovered by Kühne two centuries ago. The analysis of the genetic sequences of microorganisms that live in the Arctic, for example, in which the CSIC took part, determined the pivotal role played by proteorhodopsin. Researchers found that the expression of this photoactive protein is surprisingly high in months of more darkness than it is in June and July. Metagenomics and transcriptomics made it possible to confirm that this molecule plays a fundamental role in arctic bacteria’s growth and orientation. It gives the microorganisms a competitive advantage that helps them survive in the darkness.

“With most research techniques and approaches, you find in nature what you’re looking for. Everything else stays invisible,” state the CNB-CSIC researchers. In their opinion, metagenomics has been a fundamental technique to reveal the variety and diversity of molecules in the environment. But other technological cores have also made contributions to this research. Sequencing, a central element in the discovery of rhodopsins in the Californian bay of Monterrey, the Red Sea or the Mediterranean, among other regions, has been coupled to flow cytometry to know more about these proteins. What’s more, cellular sorting has helped confirm the presence of rhodopsins in Bacteroidetes in the Gulf of Maine and in proteobacteria and archea of the Atlantic, Mediterranean or Pacific.

Polariobacter MED152 is one of the bacteria using rhodopsin to capture sunlight energy

Source: Carlos Pedrós-Alió, CNB-CSIC

From the initial discovery of a genetic fragment that coded rhodopsin on the coasts of California, a finding reported in Science in 2000, the exploration and knowledge of this photoactive protein’s genetic diversity has varied greatly. Beyond this variability, the CNB-CSIC scientists are optimistic about the results that could be offered by missions like that of Malaspina, as they will help better understand the geographic distribution of the organisms that contain this molecule. The review recently published shows that the protein is highly conserved throughout evolution, both in archea and bacteria as well as eukaryotes. Thanks to this molecule, marine organisms can use sunlight to grow, move and survive even facing a complete lack of nutrients.

“Up to now, unconventional entry of solar energy had been ignored in all biology studies”, states Carlos Pedrós-Alió, of the CNB-CSIC. His article, in which he reviews the impact that technological cores have had on development of rhodopsin knowledge, also shows that the origin of this molecule dates back to distant times, prior to the separation of bacteria and archea. But these photoactive proteins not only play an important role in marine ecology, they also have important biotechnological applications. In addition to those already mentioned, scientists point to the potential of optogenetics, in which rhodopsins are introduced into neuronal cells for them to become photosensitive. This technique has already been shown to stop epilepsy attacks in mice. These results show the possibilities of molecules that still have a great role to play in basic and applied research, where the work of technological cores has been fundamental, precisely to explore that potential.