Thiomargarita magnifica with a dime for scale. Image: Tomas Tyml
Scientists have discovered a bacteria with cells that measure a full centimeter in length, an astonishing size that makes it by far the largest bacterial species ever found and even “challenges our concept of a bacterial cell,” reports a new study. Bacteria are an extraordinarily diverse group of organisms that have inhabited Earth for billions of years and have evolved to occupy a dizzying variety of niches. Still, almost all of these microbes are composed of simple cells that measure about two microns in diameter, which is about 40 times smaller than a strand of human hair.
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Thiomargarita magnifica, a bacteria discovered on sunken red mangrove leaves in Guadeloupe, Lesser Antilles, has blown this standard scale out of the water. The species has evolved filamentary cells that are “larger than all other known giant bacteria by ~50-fold,” making them “visible to the naked eye,” according to a study published on Thursday in Science. Scientists led by Jean-Marie Volland, a marine biologist who holds joint appointments at the Laboratory for Research in Complex Systems and the Joint Genome Institute (JGI), a U.S. Department of Energy office at Lawrence Berkeley National Laboratory, suspect that this record-breaking adaptation is partly due to the astonishing number of duplicated genes wielded by T. magnifica, an ability that is known as polyploidy.“These cells grow orders of magnitude over theoretical limits for bacterial cell size” and “display unprecedented polyploidy of more than half a million copies of a very large genome,” Volland’s team note in the study. Such features “indicate gain of complexity in the Thiomargarita lineage and challenge traditional concepts of bacterial cells.”Scientists first stumbled across T. magnifica more than a decade ago, in the mangrove forests in the Caribbean island group of Guadeloupe, but the species has defied easy characterization for years. Though it belongs to a family of largely single-celled organisms called prokaryotes, which includes bacteria and archaea, its size and structure is more reminiscent of eukaryotes, which encompasses animals, plants, and other complex multicellular lifeforms.
“Bacteria and archaea are taxonomically and metabolically the most diverse and abundant organisms on Earth, but with only a small fraction of them isolated in culture, we remain grossly ignorant of their biology,” the team noted in the study. “Although most model bacteria and archaea are small, some remarkably large cells, referred to as giant bacteria, are evident in at least four phyla,” which is a term for a specific group of lifeforms, the researchers continued. “Such bacterial giants raise the question of whether other lineages of previously unidentified macrobacteria might exist.”This question, along with the mysterious combination of traits in T. magnifica, prompted an international effort to better understand the genetic makeup and cellular structure of these strange organisms. Volland and his colleagues used fluorescence, x-ray, and electron microscopy, along with genome sequencing, to unveil new details about the processes inside these huge cells. The results revealed that these bacteria contain DNA clusters in their cells, which are located in compartments bordered by membranes that the team called “pepins.” These organized pepins provide a stark contrast to the free-floating DNA seen in the cells of most bacteria. In addition, the team’s genetic sequencing revealed that T. magnifica contains hundreds of thousands of genome copies that are dispersed across the cell, adding up to about three times the number of genes in most bacteria, which is an extreme example of polyploidy.“These cellular features likely allow the organism to grow to an unusually large size and circumvent some of the biophysical and bioenergetic limitations on growth,” Volland and his colleagues said.The identification of T. magnifica represents a major milestone for microbiologists, but the study notes that there are still many open questions about the life cycle, cellular mechanisms, and evolutionary history of this undefeated bacterial giant. The researchers also suggest that the mere existence of this species hints at a secret world of huge microbes that may fundamentally reshape our understanding of life at these scales.“The discovery of [T. magnifica] suggests that large and more complex bacteria may be hiding in plain sight,” concluded Volland and his colleagues. “Investigating the biology, energy metabolism, and the formation, nature, and role of pepins will take us a step closer to understanding the evolution of biological complexity.”