How many bacteria out there?

At first sight, it seems a very difficult question to answer. How can we possibly estimate such a number? Well, William Whitman, David Coleman and William Wiebe - all from the University of Georgia, USA - have provided us with a very exciting proposition in a 1998 article published in the Proceedings of the National Academy of Sciences of the USA.

And the astonishing number is: ~5 x 10³⁰ bacteria!

~5,000,000,000,000,000,000,000,000,000,000

Our own 7,000,000,000 suddenly seem less impressive.
 

Whitman and his colleagues noted that the actual total number of bacterial cells had never been assessed, ‘because an estimation of the number of prokaryotes would seem to require endless cataloging of numerous habitats’. It certainly seemed to me that way, but they ended up with a convincing estimation after looking for representative habitats in both aqueous and terrestrial environments. What is striking is that many habitats that show very high densities of bacteria, such as, say, animals’ gut (up to 10¹¹ per g of human colon), account for a negligible fraction of the total. The main crowd is apparently to be found in subsurface sediments and terrestrial subsurface (probably >95%). Hence, what is directly accessible to us (plants, animals, soil, oceans, lakes, etc.) represents a mere 5% of the total bacterial environment. Talking about the tip of the iceberg…

 A couple of years ago, during a presentation of my PhD thesis to a non-professional audience, I used some data from Whitman et al. in a cartoon that was meant to illustrate the amazing ubiquity of bacteria on Earth:

References: Whitman, W. B., Coleman, D. C. and W. J. Wiebe (1998). Prokaryotes: The unseen majority. Proceedings of the National Academy of Sciences of the USA. Vol. 95, pp. 6578-6583.