How genetic plunder transformed a microbe into a pink, salt-loving scavenger
Most cells would shrivel to death in a salt lake. But not the Halobacteria. These microbes thrive in brine, painting waters a gentle pink or crimson red wherever they bloom. The Halobacteria live in every salt lake on this planet, from the Dead Sea of Israel to the vast salt flats at the feet of the Sierra Nevada. But these hardy microbes haven’t always called salty depths their home. Their genomes reveal a tale of a dramatic transformation through genetic plunder.
Organisms that can survive in waters of extreme salinity are called ‘halophiles’ – or salt lovers. There exist salt-tolerant algae, fungi and even shrimp. But of all the salt lovers in the world, the pink Halobacteria are the most passionate. They don’t just cope with brine. They embrace it.
Most halophiles do their best to keep their cells clear of salt. But the Halobacteria just don’t care. The insides of their cells are as salty as the lakes they live in. This strategy, the Halobacteria have come to utterly dependent on salt, up to the point were fresh water is as deadly for them as salt water is for others. Placed in a freshwater lake, their cells would swell and pop like bloated water balloons.
Confusing enough, Halobacteria are not bacteria, but archaea, which have a completely different biochemistry. As a general rule, archaea are more hardy and robust than their bacterial counterparts, living in a wider range of extreme environments.
Microbiologists have long noted something odd about the Halobacteria. In all their evolutionary analyses, they found that Halobacteria are part of a branch of archaea called the ‘methanogens’. What bothers microbiologists is that as microbes, methanogens and Halobacteria couldn’t be more different. In every scheme ever devised to differentiate among micro-organisms, methanogens and Halobacteria end up on opposing sides of the divide. If microbes were spices, methanogens would be the pepper to the halobacterial salt.
Methanogens are the self-reliant survivalists, able to liberate energy from the most basic of molecules. A pinch of hydrogen (H2), a dash of carbon dioxide (CO2) and a spoonful of minerals is all a methanogen needs to carve out a living. This sober lifestyle has earned them the moniker of ‘rock eaters’ (lithotrophs).
Halobacteria, on the other hand, fancy their molecules ready-to-eat. They are scavengers, scrounging the salty waters for carbon compounds that they burn using oxygen (methanogens loathe oxygen). As an alternative energy supply, halobacteria are also able to harvest energy from sunlight.
Two types of microbes with radically different life strategies, yet one evolved from the other. So how did the Halobacteria cross the line?
Shijulal Nelson-Sathi thinks he has found the answer. In their latest paper, he and his colleagues show that the ancestor of all Halobacteria acquired as much as a thousand genes from another microbe, a bacterium. And through this act of plunder, the microbiologists write, the Halobacteria left their methanogenic ways behind, becoming salt-loving scavengers in the process.