A not very micro microorganism

world's largest bacterium

Microorganisms are often defined as any organism too small to be visible to the unaided eye, such as bacteria, protists and some fungi. 2022 will go down in the annals of microbiology as a big year, a year when a super-sized bacterium was discovered.  A bacterium fifty times larger than others.  A microorganism that fails the microscopic criterion in the name.  But that’s not the oxymoronic giant microbe’s only claim to fame.

Collected underwater, from decaying leaves in a Caribbean mangrove, the bacterium is filamentous and averages about 10 mm in length.  Were that not impressive enough, the longest among these individual cells are up to 20 mm!  The new species, Thiomargarita magnifica is a sulfur oxidizing bacterium whose discovery took place in slow motion.  First encountered several years ago, biologists initially assumed it was something else, a fungus perhaps.  Afterall microbes are not usually visible to the naked eye.

Other bacteria in the genus Thiomargarita are considered large, but the longest among them is only about 750 microns.  The new one is 20,000 microns long! Other filamentous bacteria have been collected from mangroves, but their threadlike structure is comprised of a large number of cells strung together.  The new species is just one very, very long cell.

Thiomargarita is one of about 250 genera of gram negative Gammaproteobacteria, representing the most genus-rich and biologically diverse taxon among prokaryotes.  Gammaproteobacteria are important in nutrient cycling in coastal and marine ecosystems, and the group includes species that have uses in bioremediation of oil spills and biosynthesis, including promising biodegradable plastics.  They are common in soil as well as both fresh and salt water and, on average, make up about 14% of oceanic bacterioplankton.  And among them are important pathogens, including Pseudomonas aeruginosa, Escherichia coli, and some Salmonella. Others are pathogens of plants and marine life, from corals and shrimp to fish.

Prokaryotes, we were all taught in biology class, have their DNA floating free in the cytoplasm of the cell.  It is the containment of DNA within a nuclear membrane, after all, that differentiates the eukaryotes.  The new bacterium has many thousands of membrane-encased structures called pepins each of which hold DNA, ribosomes, and proteins.  All told, one cell’s pepins may contain more than half a million copies of the bacterium’s genome.  Like its size, this seems like overkill.  Although gargantuan among bacteria, these impressive giants aren’t even close to being the largest cells.  Myxomycetes, plasmodial slime-molds, can have plasmodia that contain thousands of nuclei within a single cell membrane that holds the content of one gargantuan cell measuring up to four feet in diameter.

World records are always fun, including this, the largest bacterium ever found by far.  But the containment of DNA in pepins enticingly blurs what had seemed a clear line between the comparatively complex intracellular organization of eukaryote cells and the supposedly simpler, at least by comparison, prokaryotic cell.  It also reminds us just how little we know of earth’s species and the incredible innovations possible on a hospitable planet, given a few billion years of evolution.

References

Volland, J.-M. et al. (2022) A centimeter-long bacterium with DNA contained in metabolically active, membrane-bound organelles. Science 376: 1453-1458.

Munn, C.B. (2020) Marine Microbiology: Ecology and Applications. Third Edition. London: CRC Press.