Hans and his band of ragged rare bacteria waited patiently for the currents of change. Once the conditions were right, they would ... eat and multiply!
There are abundant and rare groups of bacteria in seawater. A recent paper from our lab found that some rare groups in a community act as "seeds"; when the conditions are favorable to them, they become more abundant. Here is the link to the not-super-sciencey version http://www.udel.edu/udaily/2012/jul/bacteria-ocean-072011.html **, ***
* Until conditions change again, heh.
** No, mystery is not solved. There are few absolute truths in biology, sorry.
*** There are not thousands of bacteria in a teaspoon of seawater, it's more like 3 million.