The charismatic megafauna may get all the attention, but it's amoebas, nematodes and microbes that make up the bulk of life on Earth. So how many microbes are in the sea floor? A new analysis published in Proceedings of the National Academies of Science suggests there are fewer cells than previously proposed.

Based on transects and sediment samples, scientists now estimate that there are 29,000 quadrillion cells in the subsurface of the ocean—their numbers constrained by a lack of available food in places where sedimentation rates are low. Of course, it is microbes living in just such extreme conditions that are challenging scientists' notion of what constitutes life by surviving for thousands of years.

We also don't know much about what's going on with the massive microbial biomes in the soil or deep beneath the Earth—microscopic worms that prey on bacteria have been found thriving more than a kilometer down. Estimates of microbial biomass, like estimates of the number of species on the planet, are only loose approximations.

The truth is: we have no idea how much life there is on Earth. And we've only just begun even attempting to find out.

—David Biello

[The above text is a transcript of this podcast.]