On sunken leaves in the waters of a Caribbean mangrove swamp, researchers discovered a bacterium that challenges the prevailing view of bacterial cell size; counter to the notion that microbes are only visible with a microscope, this one – named Thiomargarita magnifica – is larger than all other known giant bacteria by ~50-fold, and can be seen by the naked eye, the study’s authors say. It’s also quite complex in its structure, further challenging traditional concepts of bacterial cells. 

“This discovery adds to the group of large sulfur bacteria and helps to solve the puzzle of what factors limit cell size,” writes Petra Anne Levin in a related Perspective. Bacteria are commonly thought of as microscopic single cells with DNA free-floating in their cytoplasm. As a group, however, they often show a surprising range of diversity. In this study, Jean-Marie Volland et al. add to this diversity by reporting the discovery and characterization of a sulfur-oxidizing bacterium that can grow orders of magnitude over theoretical limits for bacterial cell size, with a complex membrane organization that likely allowed it to grow to such size, circumventing typical biophysical and bioenergetic limitations.

The organism was first discovered growing as thin white filaments on the surfaces of decaying mangrove leaves in shallow tropical marine mangrove swamps in Guadeloupe, Lesser Antilles. Using a range of techniques, Volland and colleagues aimed to characterize it. Though bacteria are typically visible only with a com­pound microscope capable of magni­fying 100 to 1,000 times, this one – reaching one centimeter in length – is visible without a microscope. 

And, instead of its DNA floating freely inside the cell as happens in other bacteria, the DNA is compartmentalized within membrane-bound structures, an innovation characteristic of more complex cells. These membrane-bound compartments are metabolically active, the authors’ analyses show, with activity occurring throughout the bacterium cell length, as opposed to just at its growing tip. It is possible that this unique spatial organization and bioenergetic membrane system, which indicate a gain of complexity in the Thiomargarita lineage, may have allowed T. magnifica to overcome size- and volume-related limitations typically associated with bacteria. 

Why these organisms need to be so large is an intriguing, open question, says Levin in the related Perspective, where she also suggests it unlikely that T. magnifica represents the upper limit of bacterial cell size. “… bacteria are endlessly adaptable and always surprising—and should never be underestimated,” writes Levin. The authors conclude: “The discovery…suggests that large and more complex bacteria may be hiding in plain sight.”