Estimates of the order of magnitude for the total number of microbial species on Earth range from 10(3) to 10(9). Despite global dispersal of microorganisms, this number is probably rather large. The total biodiversity of an ecosystem is composed of two elements: first, a set of abundant taxa that carry out most ecosystem functions, grow actively and suffer intense losses through predation and viral lysis. These taxa are retrievable with molecular techniques but are difficult to grow in culture. Second, there is a seed bank of many rare taxa that are not growing or grow extremely slowly, do not experience viral lysis and predation is reduced. Such taxa are seldom retrieved by molecular techniques but many can be grown in culture, which explains the dictum 'everything is everywhere'.