What one counts is not necessarily what counts

The most abundant group of bacteria in the sea are SAR11 bacteria. On average, they make up about a third of all bacteria and are known to be slow-growing. We observed that during a spring algal bloom off Helgoland, they divided very rapidly, but their overall numbers decreased. This was unusual because cell numbers and growth rates are often closely linked. Presumably, SAR11 was heavily preyed upon and was unable to compensate for the losses, despite strong growth. Our results show that accurate microscopic observations are and will remain important for ecological studies.