I have always taken (and used) for granted the 10³¹ number of phages in the planet. Normally, this is calculated from the estimation that there are 10 phages per prokaryotic cells, and the latter are estimated to be 10³⁰. Usually the references to these numbers are: Jiang & Paul 1998, PMID 9687430 and Whitman 1998, PMID 9618454

Today I found what might be an older reference: Bergh et al. 1989, PMID 2755508, High abundance of viruses found in aquatic environments

Once I get access to the full-text paper (“thanks to” Nature’s unwillingness to open even older articles), I can confirm the exact phage number as claimed in 1989.

If you know of a better (aka older) reference, feel free to share it.

This number (10³¹), by the way, can be read as: ten nonillions (by the US numbering system)