I read a review article about phage therapy today (citation below*) with the following opening sentence:
The human gut contains approximately 1015 bacteriophages (the ‘phageome’), probably the richest concentration of biological entities on earth.
Is that claim actually true? They cite this Lepage et al. Gut paper; those folks estimate that 1014 microorganisms (that is, distinct cells) live in any single human gut. We usually guess that an environment contains at least 10 times as many individual bacteriophage as potential host cells, so 1015 bacteriophages doesn't seem like a bad estimate. That being said, could there be a more densely-populated reservoir out there? I've seen population counts for chickens as high as 19 billion but I wasn't able to find any estimates of their gut microbiome diversity. We know they're a potential reservoir of pathogens and their population exceeds that of humanity.

Update: I've been thinking about this and realized that the phrase "richest concentration of biological entities" likely refers to a single human gut rather than the sum of all human gut microbiomes and viriomes. I like to think about ecological niches on a grand scale; the total number of different variations in phage genomes is higher when we include every similar environment in the total rather than the contents of just one human gut. My qualms about the superlative remain. I'd suspect that some sewer systems may contain richer, more diverse arrays of phages, and that's without employing much creativity. Could other species on this planet maintain more diverse microbiomes and/or viriomes?

*Dalmasso M, Hill C, Ross RP (2014) Exploiting gut bacteriophages for human health. Trends in microbiology 22: 399–405.