“Let there be light”

The Dead Sea, contrary to its name, is brimming with life of perplexing microbial forms that can survive the harsh conditions of hyper salinity, low dissolved oxygen and intense UV radiations. In fact some of these forms use the intense radiation to their advantage!

Halobacterium salinarum, popularly misconceived to be a bacterium, is actually an ancient archaeal form that has a mechanism of obtaining ATP using light in a simpler way than the method employed by plants and cyanobacteria. They have an amazing trans-membrane protein called bacteriorhodopsin that chemically simulates the action of what rhodopsin pigment in the human eye does except, of course, there is no circuit to an “archaeal brain” otherwise even archaeal cells would have vision. Instead they make ATP molecules using light but without consuming CO2 and skipping all the steps, jumping right into creating a proton gradient needed to synthesize the ATP! It really redefines the phrase, “let’s cut to the chase”. How do they achieve this without a conventional electron transport chain?

Bacteriorhodopsin has a molecule of Vitamin A that is highly sensitive to light energy. The double bond in the retinal molecule switches from the Trans to the cis form in the presence of light. The Trans form is straight in orientation while the cis form is bent. To put it in crude words the protein is facilitating the “kicking” out of the protons by rapidly changing its conformation from a straight to a bent form. The proton gradient created enables the gamma sub-unit of ATP synthase to rotate and make ATP. However this mechanism only serves as an energy source for the organism. The carbon source has to be obtained elsewhere since they do not use CO2.

Here is one more reason why archaeal forms are one of the most wondrous forms to exist. You do not witness ATP synthesis without the preceding reactions of glycolysis, Krebs’s cycle, photosynthesis or the electron transport chain elsewhere now, do you?

About Netra Kadambi