Bacteria as Big as Tadpoles Discovered in Caribbean Swamp

A bacteria species so colossal that it changes everything we knew about bacteria has been found in a Caribbean mangrove swamp. As big as (small) tadpoles, these bacteria have been lurking in plain sight under mangrove leaves rotting in the slimy water. Sounds like fun.

How were they found? Being a centimeter (0.4 inches) long, this monster microbe is visible to the naked eye, researchers reported Thursday in Science. Every other bacterium known to humankind, including its cousins, requires microscopic assistance to be visible.

Named Thiomargarita magnifica, this monster is more than 50 times bigger than the biggest-known giant bacteria so far, writes Jean-Marie Volland of the Lawrence Berkeley National Laboratory/Laboratory for Research in Complex Systems with colleagues.

Why hadn’t magnifica been identified before? For one thing, one needs to root about beneath submerged vegetation in a scummy mangrove swamp in the Caribbean, notice the beast, and realize that it isn't a fungus and what one has really encountered.

Its discovery suggests there could be other titanic bacteria out there whose sheer immensity prevented their identification as such, the authors suggest.

“Confirmation bias related to viral size prevented the discovery of giant viruses for more than a century, and their ubiquity is only now being recognized,” the team points out.

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Is it unique, size-wise? So far, yes, but Volland and the team qualify that science has only managed to cultivate a tiny fraction of all known bacteria and archaea (formerly known as archaebacteria) in the lab. And we are far, far short of knowing them all. This means we are profoundly ignorant about our abundant planet-mates and possibly humongous bacteria are among us without our even noticing. Note that in 2021 a new species of whale was identified – and if that could evade our zoological radar, a germ could too.

How anomalous is Thiomargarita magnifica? There is no average size for bacteria because they are so diverse, but most range from 1 to 5 micrometers (i.e., 1 to 5 millionths of a meter). Until now, the biggest-known bacterium has been its cousin Thiomargarita namibiensis, which clocks in at a whopping 300 to 750 micrometers in length.

Magnifica is a centimeter long, which is 10,000 micrometers.

The lovely Thiomargarita namibiensis lives in ocean sediment of the Namibian continental shelf. There are reasons these super-giant bacteria evaded our inquiring eyes until now.

So how big is magnifica? Think of it this way: You have trillions of bacteria in your body. Now imagine they’re each a centimeter long and lined up like a string. One trillion centimeters is 10,000,000 kilometers. The distance between Earth and the Moon is 384,000 kilometers. If your bacteria were a centimeter long and lined up, they would stretch between Earth and the Moon more than 26 times. What does that indicate? Nothing. It’s just cool.

One fine day in a swamp

Magnifica is a sulfur bacterium. Sulfur bacteria can utilize molecules with sulfur (rather than oxygen) as an energy source. The famed bacterium – for instance, Desulfovibrio desulficans – “eats” sulfates in sewage and wet soil, producing hydrogen sulfide, which smells like rotten eggs. Thus sulfur bacteria contribute to the special aromatic ambiance we associate with swamps.

In general, sulfur bacteria tend to be big but magnifica is a whole other dimension. Why are sulfur bacteria big? Because they are like living storage containers for the inputs they need (soluble nitrogen and elemental sulfur), they are not dependent on finding immediately available sustenance, and can get about.

Thiomargarita bacteria, of which there are several types and to which our magnifica belongs, are among those flexible germs able to store lots of lovely substrate.

“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 an analysis accompanying the paper.

Half a million copies of its DNA

The titanic Thiomargerita magnifica has other anomalies too. Apparently its DNA isn’t sort of free-floating in its cellular space, like in the case of all other bacteria. Its DNA seems to be ensconced within organelles enveloped in membrane, which is akin to the state of affairs in advanced eukaryotic cells. (Leaving viroids and prions out of it, life can be split into prokaryotes – without nuclei – and eukaryotes, which have nuclei. You and your cat are eukaryotes.)

Organelles had been thought to be unique to eukaryotic cells. Recently it has become evident that some bacteria do have hitherto unappreciated structural complexities; cyanobacteria don’t have a nucleus, heaven forfend, but they do have membrane-bound organelles called carboxysomes that handle photosynthesis, for instance.

In magnifica’s case, the encapsulation of its genome in membrane may be key to its gigantism, which is far beyond what science had thought possible for bacteria, the writers say. The team notes there is another anomalous bacterium, Atribacteria, which may have membrane surrounding its DNA – it’s still under study.

Apropos DNA, magnifica displays a previously unknown level of polyploidy, the team writes, which means it has more than one copy of its genome.

Each of your cells has two copies of your genetic material, one from mom and one from dad. A single cell of magnifica can have more than half a million copies of genome, the team reports. Much more. Also, its genome is relatively large for a bacterium even before factoring in the extraordinary polyploidy.

One other weirdness: its manner of reproduction. Other bacteria reproduce by replicating their DNA into two copies and splitting into identical daughter cells, each with one copy of the DNA, ad infinitum. No such mundanity for magnifica: it buds asymmetrically in what is called a dimorphic life cycle. Copies of its genome are asymmetrically segregated into apparent dispersive daughter buds.

So what do we have here? A bacterium the size of a your fingernail with a strange life cycle and truckloads of DNA living under decaying leaves in the mangrove swamps of the Caribbean. Nice to meet. And we can expect it isn’t the biggest bacteria out there; we just haven’t found them yet, or realized what they are.