Oligotrophic bacterioplankton with a novel single-cell life strategy.

A large fraction of the marine bacterioplankton community is unable to form colonies on agar surfaces, which so far no experimental evidence can explain. Here we describe a previously undescribed growth behavior of three non-colony-forming oligotrophic bacterioplankton, including a SAR11 cluster representative, the world's most abundant organism. We found that these bacteria exhibit a behavior that promotes growth and dispersal instead of colony formation. Although these bacteria do not form colonies on agar, it was possible to monitor growth on the surface of seawater agar slides containing a fluorescent stain, 4',6'-diamidino-2-phenylindole (DAPI). Agar slides were prepared by pouring a solution containing 0.7% agar and 0.5 micro g of DAPI per ml in seawater onto glass slides. Prompt dispersal of newly divided cells explained the inability to form colonies since immobilized cells (cells immersed in agar) formed microcolonies. The behavior observed suggests a life strategy intended to optimize access of individual cells to substrates. Thus, the inability to form colonies or biofilms appears to be part of a K-selected population strategy in which oligotrophic bacteria explore dissolved organic matter in seawater as single cells.