Red and salty

Halobacterium salinarum, Microbe of the Year 2017, belongs to the archaea - primordial forms of life that resemble bacteria but are actually more closely related to plants and animals. They are often adapted to unusual habitats, for example hot springs, extremely acidic waters, places without oxygen or - like H. salinarum - to high salt concentrations. The microorganism grows in salines and brines. Thanks to special channel proteins in the cell envelope, H. salinarum can adapt its salt content to the external conditions. It even survives in salt crystals for hundreds of years. Since the microbes contain red pigments, they make salt lakes and sea salt extraction plants appear reddish-purple. The pigments (carotenoids) of Halobacterium salinarum accumulate in the food chain: Small brine shrimp feed on the microorganisms, and they are again eaten by flamingos, turning their feathers pink.

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Precursor of our visual system

The protein Bacteriorhodopsin from Halobacterium absorbs light and converts it into energy for the metabolism of the cell: The colour of the bacteriorhodopsin changes from violet to yellow in the process. The dyes in the cell membrane of Halobacterium salinarum are not only responsible for the red colour, but also for a special type of photosynthesis that converts light into usable energy for the cell. The discovery of the import role of bacteriorhodopsin from H. salinarum was made by the biochemist Dieter Oesterhelt in 1971. A fascinating link: a comparable rhodopsin is responsible for the visual process in our eyes. The evolution of the molecular basis of our sense of sight probably has its roots in ancient microbial forms.

 

Light switch for new treatments

After the discovery of the bacteriorhodopsin from Halobacterium salinarum, a new field of research developed: optogenetics. Today, rhodopsins are used as molecular "light switches" to specifically investigate and control the behaviour of nerve cells. Initial successes indicate that neuronal defects could be treatable in the future, for example retinal diseases, Parkinson's or epilepsy.

But Halobacterium salinarum offers even more special features: It regulates its cell density with the help of gas vesicles that are filled with air and enclosed in a waterproof protein shell. Like a diver, it can thus float in certain water depths and seek out suitable oxygen and light conditions. It can also swim around in these water layers: Thanks to pushing of long projections (filaments), it screws itself through the viscous salt solution according to the principle of a propeller. For this purpose, the archaea have "invented" their own molecular rotary motor, which can randomly change the direction of rotation and thus the orientation of the cell in response to a cell's own signal.

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