We usually think of bacteria as organisms so small they can be seen only through a microscope. But scientists discovered a giant white bacterium lurking on rotting leaves in the brackish waters of a red mangrove swamp in Guadeloupe in the Lesser Antilles.

It’s so large it can be seen easily with the naked eye. But size isn’t the only astonishing trait of this long, filamentous microbe; it has a more complex structure than any other bacterium previously discovered, and, unlike most, it stores its DNA in tidy little packets.

Previously discovered giant bacteria, some of which can also form centimeters-long filaments, are composed of hundreds to thousands of cells. But the newfound bacteria, which is roughly the shape and