A magnificent megabacterium discovered in the mangroves of a Caribbean island
“There are about one trillion species of microbes on Earth, and 99.999 percent of them have yet to be discovered” – Jay T. Lennon, Biologist, Indiana University
Ask any bacteriologist, and they will tell you that bacteria are microscopic, free-living, single-celled organisms with circular DNA floating freely in the cell. As biologists, we may have to revisit that description. In a new paper published in Science, researchers report discovering a centimeter-long, filamentous bacterium with its genetic material enclosed in membrane-bound compartments. Scientists Jean-Marie Volland, Olivier Gros, and Tanja Woyke take us through their remarkable findings.
These gigantic bacterial cells, now classified as Candidatus (Ca.) Thiomargarita magnifica, were found on the sulfur-rich mangrove sediments of the Guadeloupe archipelago by Oliver Gros, a professor of marine biology at Université des Antilles. “I had a hunch they were prokaryotic because they lacked organelles such as nuclei and mitochondria,” says. Prof. Gros. Later along with Dr. Silvina Gonzalez-Rizzo, who performed 16S rRNA gene sequencing, and Dr. Chantal Guidi-Rontani, they realized that the white filaments he collected in mangrove waters were, in fact, gigantic single bacterial cells.
The Guadeloupe team then approached Jean-Marie Volland, a scientist at the Laboratory for Research in Complex Systems and Lawrence Berkeley National Laboratory, to study these creatures in detail. Looking at their electron microscopy images, Jean-Marie observed some fascinating structures present in the filaments. To his surprise, these structures contained the genetic material of the cell. “My first reaction, when I looked at their electron microscopy pictures was not so much related to the extreme cell size but rather about interesting structures that were present within these giant cells. Those structures would turn out to be the membrane-bound organelles that we now call—pepins.”, explains Jean-Marie. Another bacterium belonging to the same genus Thiomargarita (T. namibiensis) found in the ocean sediments in 1997, is now the third-largest, however, structures such as, pepins have not been reported in them.
This exciting finding opens doors for many more questions— what led to these elongated filamentous structures? how does one culture these bacteria in a laboratory? or how abundant are these around us?
“How and why they became such gigantic filaments is still an open question”, says Jean-Marie adding, “Because we have sequenced and analyzed their genome, we know that they have lost some of the genes that are considered essential to divide the way most bacteria do. And on the other hand, some of the genes that are involved in cell elongation are present in multiple copies. The loss of “division genes” and the duplication of “elongation genes” may be responsible for producing the unusually long filaments of T. magnifica but how this evolved and why remain to be studied.”
Until now these giant sulfur-oxidizing bacteria remain unculturable and the team’s next goal will be to establish the cultivation conditions. T. magnifica requires a very subtle mixture of oxygen and hydrogen sulfide to grow, and setting this unstable chemical environment can be a challenging task. Sulfur oxidizing bacteria (organisms that oxidize inorganic sulfur such as hydrogen sulfide) are found in many extreme environments (volcanoes, lakes, hydrothermal vents, caves, and beyond), and research teams have thus far been able to propagate many of them in labs. “Even some large sulfur bacteria, such as Beggiatoa (~200 microns), have been enriched and isolated by researchers”, points out biologist Tanja Woyke further adding that they are working on a custom cultivation device to mimic the unique natural conditions that T. magnifica is used to.
Although this finding shatters all records about how gigantic bacteria can be, this will hopefully facilitate more explorative studies and a lookout for biologically complex organisms. As Tanja Woyke mentions, “I think we’ll see even more “unusual” morphological features in bacteria in the future, as scientists develop even better techniques to grow them in the lab and to characterize their morphological traits.”
This is a Behind-the-discovery article featuring the researchers’ vision behind the experiments and future directions.