TABLE 1.
Estimation of the number of prokaryotic cells and of the number of virus-like particles in various habitats on Earth
| Sample source | No. of prokaryotes, based ona: |
No. of viruses, based on Bar-On et al.a | VTM, based on Bar-On et al. | Habitat volb | Concn of microbesc | |
|---|---|---|---|---|---|---|
| Whitman et al. | Bar-On et al. | |||||
| Ocean | 0.12 | 0.12 | 2 | 17 | 103 | 1 |
| Ocean subsurface | 3.56 | 0.4 | 0.4 | 1 | 1 | 4 × 103 |
| Ocean crust | Not assessed | 0.2 | 6d (2–20) | 30d (3–100) | 103 | ∼1 |
| Soil | 0.26 | 0.3 | 0.6 | 2 | 1 | 3.5 × 103 |
| Terrestrial subsurface | 0.25 | 2 | 2 | 1 | 15 | 1.5 × 103 |
| Microbiomes | Insignificant | 0.01e | 0.01 | 1 | ||
| Land water | Insignificant | Insignificant | Insignificant | |||
| Air | Insignificant | Insignificant | Insignificant | |||
| Total | 4.19 | 3 | 11 | |||
The numbers are for cell or particle counts, divided by 1030. Whitman et al. is reference 15; Bar-On et al. is reference 18.
Volumes are in cubic meters and divided by 1015. The volumes were estimated by taking the area of the ocean and area of land on Earth (3.6 × 1014 and 1.5 × 1014, respectively) and multiplying by the effective thickness of each habitat from Bar-On et al. (18).
Concentrations are cells or virions per cubic meter, divided by 1011. The values are obtained by dividing the virus counts by the habitat volume.
This measure has a large uncertainty (18).
The corrected number of bacteria plus archaea in the adult human body is 3.8 × 1013 (22), which, for the entire Earth population of 7.5 × 109 humans (assuming adult sizes for all instead of the actual proportion of adults [70%], to obtain an upper bound), is 3 × 1023 prokaryotes. Animal biomass is 300 times larger than human biomass; assuming that the animal microbiome size scales linearly with biomass (which is realistic given that the majority of the animal microbiome inhabits the digestive systems), there are at most 1026 bacteria in the entire animal microbiome. Plant biomass (mostly trees) is 250 times greater than animal biomass; if plant microbiomes live mostly on plant surfaces, not inside largely metabolically inactive xylem, the plant microbiome size would scale as the power of 2/3 the mass. All told, just under 1028 microbes may live in and on all macroorganisms on Earth. Considering the high density of microbes in these habitats, a low VTM applies, giving ∼1028 virus particles in the organismal virome on Earth.