Complex sentence: There may be a population limit of around one billion cells per gram of soil, but estimates of the number of species vary widely from 50,000 per gram to over a million per gram of soil.
Split into simpler sentences: There may be a population limit of around one billion cells per gram of soil, but estimates of the number of species vary widely. Estimates range from 50,000 per gram to over a million species per gram of soil.