Large Sulfur Bacteria

In 2022, we described Thiomargarita magnifica, a bacterium of unprecedented size – up to two centimeters in length – making it easily visible to the naked eye. Each filamentous cell is a single bacterium, not a multicellular chain, and remarkably it contains novel membrane-bound compartments filled with DNA (akin to a nucleus in function). These organelles, which we named “pepins,” collectively house an extreme number of genome copies – on the order of hundreds of thousands (over half a million DNA copies per cell) – representing an unparalleled level of polyploidy in bacteria. T. magnifica lives in sulfidic mangrove swamps, sustaining itself through chemosynthesis (deriving energy by oxidizing hydrogen sulfide rather than sunlight in photosynthesis).

 

The discovery of this giant, sulfur-oxidizing microbe has profound implications: its enormous size, membrane-bound DNA organelles, and massive genome challenge traditional notions of bacterial simplicity, demonstrating that bacteria can evolve a higher level of cellular complexity than previously thought. This not only expands our understanding of microbial capabilities but also provides new insights into the evolutionary steps toward complex cell organization in living organisms.

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Bacteria and Archaea are taxonomically and metabolically the most diverse and abundant organisms on Earth, but with less than 0.1% of them isolated in culture, we remain grossly ignorant of their biology. To fill this gap we explore the evolution of bacterial complexity by combining environmental sampling, single-cell sorting, advanced microscopy, and genomics. We focus on large sulfur bacteria and other uncultivated species from extreme environments. While most model bacteria and archaea are small, our data show that bacterial gigantism has evolved independently at least 39 times—suggesting that large, structurally elaborate bacteria are not rare oddities, but recurring evolutionary innovations. 

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By revealing the hidden complexity of microbial life, we hope to reshape how we think about the origins of cellular organization—and the deep evolutionary roots of biological complexity.