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[chem geek]

Bacteria have a generation rate, which is the time it takes to double in population, of from 15 minutes to an hour depending on species and conditions. If it's 15 minutes, then in 8 hours 1 bacterium may become over 4 billion bacteria. Even if it's the generation rate is 1 hour, then in 24 hours 1 bacterium may become over 16 million.

[EDIT] Algae have a generation rate of from 4 to 8 hours which is why it seems like it takes a while for algae to appear, but it grows (in ideal conditions) by a factor of 8 to 64 over a 24-hour period. So it seems like it starts out more slowly, but then appears to "bloom". [END-EDIT]

Richard