<p class="first" id="d10911534e89">Biofilms are a form of collective life with emergent
properties that confer many advantages
on their inhabitants, and they represent a much higher level of organization than
single cells do. However, to date, no global analysis on biofilm abundance exists.
We offer a critical discussion of the definition of biofilms and compile current estimates
of global cell numbers in major microbial habitats, mindful of the associated uncertainty.
Most bacteria and archaea on Earth (1.2 × 1030 cells) exist in the 'big five' habitats:
deep oceanic subsurface (4 × 1029), upper oceanic sediment (5 × 1028), deep continental
subsurface (3 × 1029), soil (3 × 1029) and oceans (1 × 1029). The remaining habitats,
including groundwater, the atmosphere, the ocean surface microlayer, humans, animals
and the phyllosphere, account for fewer cells by orders of magnitude. Biofilms dominate
in all habitats on the surface of the Earth, except in the oceans, accounting for
~80% of bacterial and archaeal cells. In the deep subsurface, however, they cannot
always be distinguished from single sessile cells; we estimate that 20-80% of cells
in the subsurface exist as biofilms. Hence, overall, 40-80% of cells on Earth reside
in biofilms. We conclude that biofilms drive all biogeochemical processes and represent
the main way of active bacterial and archaeal life.
</p>