On receipt
Almost all life on earth depends directly or indirectly on the sun. So it is not surprising that bacteria also know how to use sunlight effectively. In a recently discovered germ, two molecules work hand in hand. In every crisis lies an opportunity. This wisdom, which is often quoted by life, business or political consultants, also applies to evolution. Because when the funds in the primordial soup slowly became scarce at the beginning of life, this probably meant the end for most of the life forms that had emerged up to that point. However, a few were able to save themselves with an innovation: They "invented" photosynthesis.
By using the energy of sunlight to build up organic matter themselves, the resourceful organisms made themselves independent of the food supply from the environment. Others, in turn, were able to benefit from this innovative achievement: they ate their photosynthetic fellow creatures.
Almost all life on our planet is based on photosynthesis, which is not just limited to green plants. In addition to the cyanobacteria, it is above all the primeval archaea that understand the art of using light. Here a protein called bacteriorhodopsin works as a light trap. It is similar, as the name suggests, to the light-sensitive pigment rhodopsin that helps our eyes see.
But the "real" bacteria also have derivatives of visual purple for energy use. The germ Salinibacter ruber, which Spanish researchers led by Josefa Antón from the University of Alicante only described in 2002, has emerged as a new candidate. The species is particularly notable for its preferences – it thrives in brine on the Mediterranean coast with s alt concentrations between 20 and 30 percent – and its deep red color.
The microbiologists identified a molecule in the cell membrane as the cause of the coloration, which they named "salinixanthin". So far, the researchers assumed that the substance, which is one of the carotenoids, only stabilizes the membrane of the bacterium and protects it from excessive solar radiation.
So just an umbrella? Antón traveled to sunny California to explore the germ here – in Janos Lanyi's laboratory at the University of California at Irvine – with Sergei Balashov and other scientists.
As it turned out, Salinibacter ruber is not at all averse to sunlight. On the contrary: With sufficient irradiation, it thrives splendidly - and in doing so it acidifies its surrounding medium vigorously by releasing protons to the outside.
This looked familiar to the researchers. Because other microorganisms also have a light-driven proton pump: bacteriorhodopsin uses the energy of light to transport protons out of the cell against the concentration gradient. With the resulting proton gradient, the cells can produce the universal energy carrier ATP.
And Balashov and Co actually discovered such a proton pump in the cell membrane of Salinibacter ruber, which runs on solar energy. This is also a rhodopsin relative, which the researchers dubbed "xanthorhodopsin".
But what is the role of salinixanthin? Since the ratio between xanthorhodopsin and salinixanthin is exactly one to one, a participation in the use of light is obvious. The suspicion was confirmed: Salinixanthin works as an antenna. It absorbs sunlight and transmits the energy to xanthorhodopsin, greatly increasing its effectiveness.
Such antenna pigments are not new to the scientists. Finally, green plants also use carotenoids as light scavengers to increase the efficiency of their photosynthesis. But their energy converter is called chlorophyll – the researchers had not yet come across antenna pigments that work together with rhodopsin.
So Salinibacter ruber has the simplest light-driven proton pump equipped with an auxiliary antenna. The researchers suspect that this effective use of energy must have appeared very early in evolution. Maybe even when the primordial soup was fished out in a critical way at the beginning of life. Because: In every crisis there is an opportunity.
