In a Caribbean mangrove forest, scientists have discovered a type 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 the more well-known bacteria such as Escherichia coli. “It would be like meeting another person the size of Mount Everest,” said Jean-Marie Woland, a microbiologist at the United Genome Institute in Berkeley, California.
Dr. Woland and his colleagues published their study of a bacterium called Thiomargarita magnifica on Thursday in the journal Science.
Scientists once thought that bacteria were too simple to produce large cells. But Thiomargarita magnifica turns out to be remarkably complex. As most of the bacterial world has yet to be explored, it is entirely possible that even larger, even more complex bacteria will be waiting to be discovered.
It has been about 350 years since Dutch lens miller Anthony van Leeuwenhoek discovered bacteria by gnashing his teeth. When he placed the dental plaque under a primitive microscope, he was amazed to see single-celled organisms floating around. Over the next three centuries, scientists discovered many more species of bacteria, all of which were invisible to the naked eye. An E. coli cell, for example, is about two microns or less than ten thousandths of an inch in size.
Each bacterial cell is its own organism, which means that it can grow and divide into a pair of new bacteria. But bacterial cells often live together. Van Leeuwenhoek’s teeth were covered with a jelly-like film containing billions of bacteria. In lakes and rivers, some bacterial cells stick together to form small strands.
We humans are multicellular organisms, our bodies are made up of about 30 trillion cells. Although our cells are also invisible to the naked eye, they are usually much larger than those of bacteria. A human egg can reach about 120 microns in diameter or five thousandths of an inch.
The cells of other species can grow even larger: the green alga Caulerpa taxifolia produces blade-shaped cells that can grow up to a foot.
When the gap between small and large cells appeared, scientists turned to evolution to make sense of it. Animals, plants and fungi belong to the same evolutionary line called eukaryotes. Eukaryotes share many adaptations that help them build large cells. Scientists believe that without these adaptations, bacterial cells must remain small.
For starters, a large cell needs physical support to prevent it from collapsing or tearing. Eukaryotic cells contain solid molecular conductors that function as tent poles. However, bacteria do not have this cell skeleton.
A large cell also faces a chemical challenge: as its volume increases, it takes longer for molecules to move around and meet the right partners to perform precise chemical reactions.
Eukaryotes have developed a solution to this problem by filling cells with small compartments where various forms of biochemistry can take place. They keep their DNA wrapped in a sac called a nucleus, along with molecules that can read genes to produce proteins, or proteins produce new copies of DNA as the cell reproduces. Each cell generates fuel in sacs called mitochondria.
Bacteria have no compartments found in eukaryotic cells. Without a nucleus, each bacterium usually carries a loop of DNA floating freely around its interior. They also do not have mitochondria. Instead, they typically generate fuel with molecules embedded in their membranes. This arrangement works well for small cells. But as the volume of the cell increases, there is not enough space on the cell surface for enough fuel-generating molecules.
The simplicity of the bacteria seemed to explain why they were so small: they just didn’t have the complexity to grow.
However, this conclusion was made too hastily, according to Shailesh Date, founder of the Complex Systems Research Laboratory in Menlo Park, California, and co-author with Dr. Woland. Scientists have made extensive generalizations about bacteria after studying only a small part of the bacterial world.
“We just scratched the surface, but we were very dogmatic,” he said.
This dogma began to crack in the 1990s. Microbiologists have found that some bacteria have self-evolved compartments. They also found species that are visible to the naked eye. Epulopiscium fishelsoni, for example, came into the world in 1993. Living in a surgeon fish, the bacteria grow up to 600 microns in length – larger than a grain of salt.
Olivier Gro, a biologist at the University of the Antilles, discovered Thiomargarita magnifica in 2009 while exploring the mangrove forests of Guadeloupe, a group of Caribbean islands that are part of France. The microbe looked like tiny pieces of white spaghetti forming a coat on the dead leaves of trees floating in the water.
At first, Dr. Gross did not know what he had found. He thought the spaghetti might be a fungus, a small mushroom, or some other eukaryote. But when he and his colleagues took DNA from samples in the lab, it turned out to be bacteria.
Dr. Gross joined forces with Dr. Woland and other scientists to take a closer look at the strange organisms. They wondered if the bacteria were microscopic cells glued together in chains.
It turned out not to be so. When the researchers peeked inside the bacterial noodles with electron microscopes, they realized that each of them was its own giant cell. The middle cell was about 9,000 microns long, and the largest was 20,000 microns, long enough to cover a penny in diameter.
Research on Thiomargarita magnifica is slow because Dr. Valant and his colleagues have not yet figured out how to grow the bacteria in their lab. For now, Dr. Gross has to collect fresh supplies of bacteria every 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 located on sulfur-rich sediments in the mangrove forest. But bacteria seem to follow an unpredictable life cycle.
“I can’t find them in the last two months,” Dr. Gross said. “I don’t know where they are.”
In Thiomargarita magnifica cells, researchers found a strange, complex structure. Their membranes have many different types of compartments built into them. These compartments are different from those in our own cells, but can allow Thiomargarita magnifica to grow to enormous sizes.
Some of the compartments appear to be fuel factories, where the microbe can use the energy in nitrates and other chemicals it consumes in the mangrove forest.
Thiomargarita magnifica has other compartments that remarkably resemble human nuclei. Each of the compartments, which scientists call pepini after small seeds in fruits like kiwis, contains a loop of DNA. While the typical bacterial cell has only one loop of DNA, Thiomargarita magnifica has hundreds of thousands of them, each stored in its own pepin.
Even more remarkable is that each pepin contains factories to build proteins from its DNA. “They have essentially small cells in the cells,” said Petra Levin, a microbiologist at the University of Washington in St. Louis who was not involved in the study.
Thiomargarita magnifica’s vast supply of DNA can allow it to make the extra proteins it needs large. Each pepin can make special sets of proteins needed in its own region of the bacterium.
Dr. Woland and his colleagues hope that once they start growing the bacteria, they will be able to confirm these hypotheses. They will also deal with other mysteries, such as how the bacterium manages to be so healthy without a molecular skeleton.
“You can take a thread out of the water with tweezers and put it in another container,” Dr. Woland said. “How it sticks together and how it takes shape – these are questions we haven’t answered.”
Dr. Date said there may be more giant bacteria waiting to be found, perhaps even larger than Thiomargarita magnifica.
“We don’t really know how big they can get,” he said. “But now this bacterium has shown us the way.”
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