In a Caribbean mangrove forest, scientists have discovered a species of bacteria that grows to the size and shape of a human eyelash.
These cells are the largest bacteria ever observed, thousands of times larger than more familiar bacteria like Escherichia coli. “It would be like meeting another human the size of Mount Everest,” said Jean-Marie Volland, a microbiologist at the Joint Genome Institute in Berkeley, California.
Dr. Volland and his colleagues published their study of the bacteria, called Thiomargarita magnifica, in the journal Science on Thursday.
Scientists once thought bacteria were too simple to produce large cells. But Thiomargarita magnifica turns out to be remarkably complex. With most of the bacterial world yet to be explored, it is entirely possible that even larger and more complex bacteria are waiting to be discovered.
About 350 years ago, Dutch lentil grinder Antonie van Leeuwenhoek discovered bacteria while scraping his teeth. When he placed dental plaque under a primitive microscope, he was amazed to see single-celled organisms swimming around. Over the next three centuries, scientists discovered many other types of bacteria, all invisible to the naked eye. A cell of E. coli, for example, measures about two microns, or less than one ten thousandth of an inch.
Each bacterial cell is its own organism, which means it can grow and divide into a pair of new bacteria. But bacterial cells often live together. Van Leeuwenhoek’s teeth were covered in a gelatinous film containing billions of bacteria. In lakes and rivers, some bacterial cells clump together to form tiny filaments.
We humans are multicellular organisms, our body made up of about 30 trillion cells. Although our cells are also invisible to the naked eye, they are generally much larger than those of bacteria. A human egg can reach about 120 microns in diameter, or five thousandths of an inch.
Cells of other species can grow even larger: The green alga Caulerpa taxifolia produces blade-like cells that can reach 30 cm in length.
As the chasm between small and large cells emerged, scientists looked to evolution to make sense of it. Animals, plants and fungi all belong to the same evolutionary line, called eukaryotes. Eukaryotes share many adaptations that help them build large cells. Scientists reasoned that without these adaptations, bacterial cells must remain small.
To begin with, a large cell needs physical support so that it does not collapse or tear. Eukaryotic cells contain rigid molecular threads that function like poles in a tent. Bacteria, however, lack this cell skeleton.
A large cell also faces a chemical challenge: as its volume increases, molecules take longer to drift and meet the right partners to perform precise chemical reactions.
Eukaryotes have developed a solution to this problem by filling cells with tiny compartments where distinct forms of biochemistry can take place. They keep their DNA coiled up in a bag called the nucleus, along with molecules that can read genes to make proteins, or proteins produce new copies of DNA when a cell reproduces. Each cell generates fuel inside pockets called mitochondria.
Bacteria lack the compartments found in eukaryotic cells. Without a nucleus, each bacterium usually carries a free-floating loop of DNA around its interior. They also lack mitochondria. Instead, they typically generate fuel with molecules embedded in their membranes. This arrangement works well for small cells. But as the volume of a cell increases, there is not enough room on the surface of the cell for enough fuel-generating molecules.
The simplicity of bacteria seemed to explain why they were so small: they simply lacked the complexity essential to grow large.
However, that conclusion was drawn too hastily, according to Shailesh Date, the founder of the Complex Systems Research Laboratory in Menlo Park, Calif., and co-author with Dr. Volland. Scientists have made sweeping generalizations about bacteria after studying a tiny fraction of the bacterial world.
“We only scratched the surface, but we were very dogmatic,” he said.
This dogma began to crack in the 1990s. Microbiologists discovered that some bacteria developed their own compartments independently. They also discovered species visible to the naked eye. Epulopiscium fishelsoni, for example, was discovered in 1993. Living inside the surgeonfish, the bacterium reaches 600 microns in length, more than a grain of salt.
Olivier Gros, a biologist at the University of the West Indies, discovered Thiomargarita magnifica in 2009 while surveying the mangrove forests of Guadeloupe, a group of Caribbean islands that are part of France. The microbe looked like miniature pieces of white spaghetti, forming a coat on dead tree leaves floating in the water.
At first, Dr. Gros didn’t know what he had found. He thought spaghetti could be mushrooms, tiny sponges, or some other eukaryote. But when he and his colleagues extracted DNA from samples in the lab, it revealed they were bacteria.
Dr. Gros teamed up with Dr. Volland and other scientists to take a closer look at the strange organisms. They wondered if bacteria were microscopic cells stuck together in chains.
This turned out not to be the case. When the researchers looked inside the bacterial noodles with electron microscopes, they realized each was its own gigantic cell. The average cell was about 9,000 microns long, and the largest was 20,000 microns – long enough to cover the diameter of a penny.
Studies on Thiomargarita magnifica have progressed slowly because Dr. Vallant and his colleagues have yet to figure out how to grow the bacteria in their lab. For now, Dr. Gros must collect a new stock of bacteria each time the team wants to conduct a new experiment. He can find them not only on leaves, but also on oyster shells and plastic bottles lying on the sulfur-rich sediments of the mangrove forest. But bacteria seem to follow an unpredictable life cycle.
“For the past two months, I can’t find them,” Dr. Gros said. “I don’t know where they are.”
Inside the cells of Thiomargarita magnifica, the researchers discovered a bizarre and complicated structure. Their membranes contain many different types of compartments. These compartments are different from those in our own cells, but they can allow Thiomargarita magnifica to grow to enormous sizes.
Some of the compartments appear to be fuel-generating factories, where the microbe can draw energy from the nitrates and other chemicals it consumes in the mangrove.
Thiomargarita magnifica also has other compartments that remarkably resemble human nuclei. Each of the compartments, which scientists call pips after the small seeds of fruits like kiwis, contains a loop of DNA. While a typical bacterial cell has just one loop of DNA, Thiomargarita magnifica has hundreds of thousands, each nestled in its own backbone.
More remarkably, each seed contains factories to build proteins from its DNA. “They basically have little cells within cells,” said Petra Levin, a microbiologist at Washington University in St. Louis, who was not involved in the study.
Thiomargarita magnifica’s huge store of DNA can allow it to create the extra proteins it needs. Each seed can make a special set of proteins needed in its own region of the bacteria.
Dr. Volland and his colleagues hope that after they start culturing the bacteria, they can confirm these hypotheses. They will also tackle other mysteries, such as how the bacterium manages to be so resistant without a molecular skeleton.
“You can take a single filament out of the water with tweezers and put it in another container,” Dr. Volland said. “How it holds together and how it acquires its shape – those are questions we haven’t answered.”
Dr Date said there could be more giant bacteria waiting to be discovered, perhaps even bigger than Thiomargarita magnifica.
“How big they can get, we don’t really know,” he said. “But now this bacterium has shown us the way.”