IT is only a little over half a millimeter long. But to microbiologists, a newly identified bacterium is an awesome giant whose size challenges long-held notions about the nature and evolution of simple life forms.
Biologists already knew the microbe as an inhabitant of the gut of sturgeonfish - a symbiont that can't survive outside the fish. But they were misled by its size.
It has a million times the volume of the largest previously known bacteria. By that criterion, they considered it one of the eukaryotes - organisms (including multicelled organisms) whose cells contain a well defined nucleus. They thought it might be similar to an amoeba.
But Esther R. Angert and Kendall D. Clements of Indiana University at Bloomington and Norman R. Pace of Australia's James Cook University in Townsville, Queensland, report in the journal Nature that the microbe is definitely a bacterium. Instead of being an ordinary sized eukaryote, it is a giant among prokaryotes, the other basic class of organisms. Prokaryote cells lack the nucleus and certain other structures that eukaryote cells contain.
This distinction is biologically crucial. The emergence of eukaryote microbes opened the way for higher life forms to evolve. Scientists tracing that emergence in the fossil record can no longer use size alone to distinguish eukaryotes from bacteria. Also, biologists no longer can assume that giant microbes must have eukaryote-type machinery to function effectively.
Like most symbionts, the giant bacterium doesn't propagate outside its host. So biologists couldn't make the laboratory studies that would have revealed its bacterial nature. The research team was able to get around this problem by using a new technique. In effect, it read the code of some of the microbe's genetic material. That material's detailed molecular structure clearly identifies the microbe as closely related to what biologists call Gram-positive eubacteria. Thus the techniques of molecular biolo gy revealed what the microscope had failed to show.
Commenting on the Nature paper, Mitchell L. Sogin of the Marine Biological Laboratory at Woods Hole, Mass., says "this humbling discovery" makes the point that "many microorganisms that cannot be cultured in the laboratory may be stranger and more diverse than we realize."