Scientists have discovered skeletons in the cyanobacterial closet. A never-before-seen species of cyanobacterium loads its cells with little bonelike lumps that may act as ballast, helping to anchor the beastie in its home waters of a Mexican lake. The discovery, described in the April 27 Science, is the first report of such a microbe creating calcified structures inside its cells, rather than externally.
Scientists aren’t sure what to make of the discovery. Related cyanobacteria play a major role in the planet’s geochemical cycles. “It’s interesting and opens up possibilities we hadn’t thought about before,” says Robert Riding of the University of Tennessee in Knoxville, who wrote a comment on the research in the same issue of Science. Because the microbe is the first of its kind, and so far, has been found in only one place, “it’s difficult to know where it will lead,” Riding says.
Geobiologist Karim Benzerara and his colleagues were investigating Lake Alchichica’s stromatolites, knobby pillars of sediment and microbes that can form in shallow waters. The researchers cultivated slimy films of the microbes in a lab aquarium. Looking at the slime under a microscope, the team saw that some cells looked like they were filled with little pearl-like granules. “That’s when we figured out that there was something special,” says Benzerara of the CNRS Institute of Mineralogy and Physics of Condensed Matter in Paris.
The granules are an unusual mixture of calcium, strontium, barium, magnesium and carbonate. Because the ratios of these ingredients aren’t the same in the granules as in the surrounding water, the researchers suspect that the cyanobacteria have some control over formation of the lumps and are actively transporting some of the ingredients into their cells. While the lumps occupy only about 6 percent of a cell, they change the microbe’s density, increasing it by 12 percent. This might help the microbes move from the water column to the surface of an underwater rock or stromatolite, the researchers speculate.
Even more intriguing is what the microbes might have been doing during Earth’s history, says geomicrobiologist Clara Chan of the University of Delaware in Newark. The new species is a type of cyanobacteria (also known as blue-green algae) known for making Earth’s atmosphere more breathable. “Cyanobacteria are really the movers and shakers of the Earth,” says Chan. “They were a major source of oxygen.”
Long before land plants were around, ocean-dwelling cyanobacteria were oxygenating the atmosphere, notes Chan. And when ocean chemistry favored the precipitation of minerals out of solution, calcified crystals formed on the outside of the cells of many of these cyanobacteria as a byproduct of photosynthesis. There’s good evidence of these calcified structures in the fossil record from about 1.2 billion years ago to 100 million years ago. Beyond 1.2 billion years ago, the record is much spottier, even though cyanobacteria are thought to have existed for the past 2.7 billion years.
The new species, called Candidatus Gloeomargarita lithophora, may help explain the gaps in the record. A genetic analysis by Benzerara and his colleagues suggests that the species is part of an ancient lineage of cyanobacteria, the Gloeobacterales. If back in the day, related species created internal granules, rather than external shell-like structures, perhaps the granules wouldn’t show up in the fossil record. The researchers don’t yet know whether the granules dissolve or leave a trace after the cyanobacterium dies.
