Carotenoids that capture light and transform it into energy in the oceans

Carotenoids that capture light and transform it into energy in the oceans

Three researchers from the University of Huelva (Rosa León, Patricia Gómez-Villegas and Ana Molina-Márquez) are part of the international team that unveils the role of carotenoids as antennae in the capture of light and its transformation into energy in the oceans and continental waters.

The study, published in the prestigious journal Nature, confirms that retinal proteins, type I rhodopsins (related to type II rhodopsins, responsible for vision in animals) are present in more than half of the non-photosynthetic bacteria and archaea in aquatic environments and that almost half of them may have a second chromophore group, which captures light and transmits it to the retinal. These rhodopsins act as proton or ion pumps, converting the sun’s energy into chemical energy and contributing to the bacteria’s metabolism. Researchers estimate that, in aquatic environments, this energy uptake by rhodopsin pumps may exceed that carried out in photosynthesis. The participation of hydroxylated carotenoids, such as lutein and zeaxanthin, which are very abundant in nature, increases the range of radiation that rhodopsin pumps can capture and provides evidence of the importance that these pigment-antennas may play in rhodopsin-based phototropy and energy fluxes in oceanic and inland waters.

Until recently, the main pigments involved in the uptake and transformation of solar energy were considered to be chlorophylls, through photosynthesis, but the contribution of retinal, assisted in many cases by carotenoids, in aquatic heterotrophic organisms may be even greater than that of photosynthesis, revealing the importance of photoheterotrophy (metabolism based on the uptake of light and assimilation of organic carbon) and the need to further study this type of metabolism, as yet little explored, which may change the paradigms of matter and energy flow in nature and be of great importance in marine ecology.

The essential chromophore group of rhodopsins is retinal. To date, only two or three cases have been found of bacteria in which a second chromophore of an isoprenoid nature contributes to the uptake of light and its transfer to the retinal. Specifically, the ketocaroteoids salinixanthin and echinenone in Salinibacter and the cyanobacterium Gloeobacter violaceus, respectively. Salinixanthin, which acts as an antenna or secondary chromophore in xanthorhodopsin, is the most studied; it was discovered in the Marismas de Santa Pola, thanks to researchers from the University of Alicante (Josefa Antón, Science 2005). It is now revealed that the presence of this second chromophore group is not something exceptional or exclusive to extreme environments, but could be a very generalised fact, involving very abundant hydroxylated carotenoids.

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