For centuries, humanity remained oblivious to the fact that we share our planet with a population so vast it defies standard comprehension. Eubacteria (or simply “Bacteria”) represent the single most prolific lineage of life on Earth. Unlike the “extremophile” Archaea, which often dominate niche, energy-poor environments, Bacteria have successfully colonized every accessible surface and fluid on the planet.¹ From the upper atmosphere to the deepest ocean trenches, they form a continuous, living skin over the globe. Modern estimates place the total number of bacterial cells on Earth between $2 \times 10^{30}$ and $5 \times 10^{30}$. To visualize this nonillion-scale figure, if you were to stack all the bacteria on Earth, the column would stretch well beyond our galaxy.

In terms of physical mass, Bacteria are the heavyweights of the microbial world. A landmark census of Earth’s biomass reveals that Bacteria contribute approximately 70 gigatons of carbon (Gt C) to the global total.² This makes them the second-largest biomass group on the planet, surpassed only by plants (which hold about 450 Gt C).³ Crucially, bacterial biomass dwarfs that of all humans and livestock combined by a factor of more than 350. While plants dominate the visible landscape, Bacteria dominate the invisible metabolic processes of the planet, driving the essential cycles of nitrogen, carbon, and sulfur that allow plants and animals to exist.

The distribution of this massive population is not uniform. Paradoxically, the vast majority of bacteria do not live in places we typically associate with “life,” such as lush forests or coral reefs. Instead, they reside in the Deep Biosphere.⁴ Roughly 70% of all bacterial and archaeal biomass is locked deep underground—buried in the terrestrial subsurface or beneath the ocean floor.⁵ In these dark, nutrient-starved environments, bacteria live in “zombie-like” states, reproducing perhaps once every few hundred or thousand years. Despite their slow metabolic rates, their sheer abundance in these deep layers makes them the largest reservoir of biological diversity on Earth.

Surface populations, while smaller in total biomass, are incredibly dense and active. In the open ocean, bacteria are the engines of the marine food web, numbering approximately ⁶$1.2 \times 10^{29}$ cells.⁷ In soil, the numbers are even more concentrated; a single gram of healthy soil can contain up to 10 billion bacterial cells belonging to thousands of different species. This immense soil population is responsible for decomposing organic matter and “fixing” atmospheric nitrogen into a form that plants can use, effectively acting as the world’s natural fertilizer factory.

Finally, the relationship between bacteria and humans has recently been subject to a major scientific correction. For decades, it was widely cited that bacterial cells in the human body outnumbered human cells by 10 to 1.⁸ However, revised calculations from 2016 have debunked this myth. We now know that the ratio is roughly 1:1. The average “reference man” is composed of approximately 30 trillion human cells and 38 trillion bacterial cells.⁹ While the numbers are roughly equal, the bacterial cells are significantly smaller, contributing only about 0.2 kg to our total body weight.¹⁰ Nevertheless, this “microbiome” is essential for our survival, training our immune systems and synthesizing vitamins we cannot produce ourselves.

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References

1. Bar-On, Y. M., Phillips, R., & Milo, R. (2018).¹¹ The biomass distribution on Earth. Proceedings of the National Academy of Sciences (PNAS).
2. Whitman, W. B., Coleman, D. C., & Wiebe, W. J. (1998). Prokaryotes: The unseen majority. Proceedings of the National Academy of Sciences.
3. Sender, R., Fuchs, S., & Milo, R. (2016). Revised Estimates for the Number of Human and Bacteria Cells in the Body. PLOS Biology.
4. Kallmeyer, J., et al. (2012). Global distribution of microbial abundance and biomass in subseafloor sediment. Proceedings of the National Academy of Sciences.