There is an interesting feature in Nature which down scales the estimation of the number of microorganisms on the planet, or at least in the sea.
Geomicrobiologist Jens Kallmeyer and colleagues at the University of Potsdam in Germany have created the most accurate model so far of the geographical distribution of microbes in marine sediment. Their findings have been published by the Proceedings of the National Academy of Sciences.
The ocean floor is home to a staggering 2.9×1029 single-celled organisms — that's 10 million trillion microbes for every human on the planet — but vast though it is, this figure is only 8% of the previous estimate of 35.5×1029.
Kallmeyer and his colleagues have amassed new data by drilling holes in the sea floor to study microbes surviving at the extremes of nutrient starvation. They used these data to create global maps that could more accurately predict the biomass hidden in kilometers of ocean sediment. The new calculations decrease previous estimates by 92%, and reduce the estimated numbers of all microbes on Earth by around 50%, to between 9.2×1029 and 31.7×1029.
To read more, here is the research paper:
Kallmeyer, J., Pockalny, R., Adhikari, R., Smith, D. & D'Hondt, S. Proc. Natl Acad. Sci. USA http://dx.doi.org/10.1073/pnas.1203849109 (2012).