Clinging to sunken debris in the shallow marine mangrove forests of the French Caribbean, tiny threadlike organisms – perfectly visible to the naked eye – have earned the title of the largest bacteria ever known.
Measuring around a centimeter in length, they are about the size and shape of a human eyelash, beating the competition at 5,000 times the size of garden bacteria and 50 times the size of bacteria previously considered giant. In human terms, it is like meeting someone as tall as Mount Everest.
Olivier Gros, a biologist at the University of the West Indies, discovered prokaryotes in 2009 by seeing them swaying gently in the sulphurous waters of the mangroves of the Guadeloupe archipelago. The bacteria cling to leaves, branches, oyster shells and bottles that have sunk into the tropical swamp, Gros said during a press briefing.
He and his colleagues first thought they might be complex eukaryotic organisms or perhaps a chain of related organisms. But years of genetic and molecular research have revealed that each chain is, in fact, a towering bacterial cell, genetically related to other sulfur-oxidizing bacteria. “Of course, it was quite a surprise,” said Jean-Marie Volland, a microbiologist at the Joint Genome Institute in Berkeley, Calif., during the briefing.
This week, Gros and his colleagues published an article in Science laying out everything they’ve learned about the huge new bacterium, which they’ve dubbed Candidatus (Ca.) Thiomargarita magnifica.
Their findings expand our understanding of microbial diversity in ways microbiologists never thought possible. Scientists had previously hypothesized that the size of bacteria would be limited by several factors, including the lack of intracellular transport systems, reliance on inefficient chemical diffusion, and a surface-to-volume ratio needed to meet energy requirements. Yet the volume of a single California. T. magnifica cell is at least two orders of magnitude higher than the predicted maximum a bacterium can theoretically reach, Volland said.
Volland, Gros and their colleagues are still learning how – and why exactly –California. T. magnifica handles its massive size. But so far it is clear that California. T. magnifica oxidizes hydrogen sulfide from its sulfur-rich environment and reduces nitrate. About 75 percent of its cell volume is a sac of stored nitrate. The bag smashes against the cell envelope, limiting the depth that nutrients and other molecules need to diffuse.
While bacteria tend to have free-floating DNA, California. T. magnifica appears to have more than half a million copies of its genome clumped together in numerous membrane-bound compartments that the researchers named pips, after the small seeds in the fruit. The distribution of seeds on the outer edges of the bacterium could allow localized production of proteins, thus eliminating the need to transport proteins over long distances.
The next step in studying these gargantuan bacteria is for scientists to figure out how to grow them in the lab. So far, researchers have collected new specimens from the mangrove forests each time they run out. But that was tricky because they seem to have a mysterious life cycle or seasonality. For two months, Gros hasn’t been able to find any. “I don’t know where they are,” he said.