The World's Largest Bacteria
Without even looking at it under a microscope, Andreas Teske could tell immediately that this bacterium was unusual.
That’s because he didn’t need a microscope.
Unlike other bacteria, this one could be seen with the naked eye. The newly discovered bacterium is about as large as the period at the end of this sentence, making it gargantuan by bacterial standards. It was found buried in smelly, sulfur-rich, sedimentary, seafloor muck off the coast of Namibia. Growing in long lines of single cells, each stuffed with reflective white globules of sulfur, the bacterium resembled a string of pearls to its discoverers, who named it Thiomargarita namibiensis—“Sulfur pearl of Namibia.”
With a volume about 3 million times greater than average bacteria, Thiomargarita namibienus is by far the world’s largest bacterium ever found. It shattered the conventional wisdom that bacteria’s inherent physiology prevented them from ever getting so big.
But Teske, who has studied bacteria for many years, is well aware of their capabilities. They don’t have nuclei, let alone brains, yet bacteria have an impressive capacity for innovation and enterprise. For billions of years, they have tenaciously survived, no matter what life throws at them. They find their niche in the world, often by devising ways to thrive in conditions that would kill other living things (even other bacteria). Whatever environmental situation exists out there, chances are good that bacteria have been there and done that. They have found ways to exploit any chemical reaction that exists to create energy. In their genes, they still have the original blueprints for a wide range of possible biological structures and processes—some retained and others discarded by life forms on Earth and elsewhere in the universe perhaps.
“When you follow the evolution of bacteria, you can see the history of biochemical invention,” Teske said.
Thus, the bacterial world, whose breadth humans have barely begun to explore, can show us life’s full range of capabilities. Bacteria can provide clues to the origins and evolution of life on Earth, and the search for extraterrestrial life. They offer insights into biochemical reactions that affect cells within our bodies and the ecological balance of our world.
Take Thiomargarita namibiensis, for example, which was found serendipitously during a hunt for other recently discovered large sulfide-eating marine bacteria called Thioploca and Beggiatoa. These two were found in sediments off the coast of Chile and Peru. In these areas, upwelling currents bring to the surface deep, nutrient-rich waters that promote blooms of microscopic marine plants and explosions of microscopic animals that eat the plants. All this life eventually dies, sinks to the seafloor, and decays in the bottom sediments. This process produces sediments that are depleted of oxygen but rich in chemicals such as nitrate and the foul-smelling gas hydrogen sulfide. These chemicals provide a banquet for Thioploca, Beggiatoa, and Thiomargarita. Consuming sulfide the way we eat food and taking in nitrate the way we breathe in oxygen, they produce the energy and organic compounds they need to live and grow. These sulfide-oxidizing bacteria play an important ecological role. They act as detoxifying agents, removing poisonous sulfide that would otherwise seep into and easily accumulate in ocean waters, especially if oxygen levels are already low.
“Without these bacteria,” Teske said, “the waters would stink and the fish would die.”
Since bacteria readily settle in every niche that offers them the “right” chemical menu, microbiologists theorized that sulfide-oxidizing bacteria should be found in many marine habitats where the chemical properties suited them. To test this theory, an international team of scientists from the Max Planck Institute for Marine Microbiology and the University of Oldenburg in Germany, the University of Barcelona in Spain, and from Woods Hole began to search for Thioploca and Beggiatoa in other places where oceanic conditions resembled those off South America. One such place is the western coast of southern Africa, and the team examined sediments off Namibia collected aboard the Russian research vessel Petr Kottsov. The samples contained smatterings of Thioploca, but they were teeming with something completely new: the bacteria that they soon named Thiomargarita.
Teske examined key genes of the new bacterium that define its position in the evolutionary tree of life and determined that Thiomargarita was closely related to its sulfide-eating, nitrate-respiring cousins Thioploca and Beggiatoa. But they looked very different. Thioploca and Beggiatoa cells are smaller and grow tightly stacked on each other in long, motile filaments, which make them look “like overcooked angel hair pasta,” Teske said. Their filamentous shape allows them to move as necessary. Thioploca shuttle down into the sediments to find more sulfide, for example, or up near the seafloor to bathe in seawater filled with dissolved nitrate.
Thiomargarita, on the other hand, grow in rows of individual balloon-shaped cells that do not form motile filaments; therefore Thiomargarita have no means of locomotion. Stuck in the mud, they have perfected “the strategy to stay put,” Teske said, by evolving very large nitrate-storing bubbles, or vacuoles, that allow them to survive long periods of nitrate starvation.
“It is like a scuba tank,” he said. “Without nitrate, Thiomargarita would suffocate, the way we would without oxygen. In times when nitrate is plentiful in surrounding seawater, they accumulate nitrate in their vacuoles, which they can subsequently draw on when nitrate supplies diminish. These vacuoles are unique in the microbial world. Only Thiomargarita and their close relatives, Thioploca and some marine Beggiatoa, have them.”
The mammoth vacuoles give Thiomargarita the ability to sit tight until ocean currents sweep nitrate-containing waters past them again. If humans had scuba tanks with equivalent capacity, they could breathe under water for weeks.
The vacuoles also give Thiomargarita a size that scientists thought bacteria could not achieve. One of the fundamental differences between bacteria and eukaryotes, the domain of life to which fungi, plants, animals, and protists belong, is that the latter have cells with internal structures for transporting food and wastes within the cells. Bacteria rely on diffusion to move chemicals around, a process that works only over tiny distances. So to transact business with the outside world, their cytoplasms—the liquid, metabolically active cell interiors—have to be close to their cell walls. That intrinsically limits how big they can get.
But Thiomargarita got around that size constraint in a novel way. Their cytoplasm forms a thin layer along the peripheral cell membrane, while the nitrate-storing vacuole occupies almost the complete interior of the cell. The large vacuoles fill Thiomargarita cells like a balloon and give these bacteria their record-breaking bulk.
“The discovery of Thiomargarita shows the oceans’ barely tapped potential for new and exciting findings in the microbial world,” Teske said.
The genes of microbes, he said, are often like “palimpsests—old medieval parchments that have been erased and written over many times.” To him, discovering Thiomargarita is like finding a long-lost and unique page in our evolutionary history, one that carries crucial information about the microbial chemistry of today’s oceans.