Species composition shift of confined bacterioplankton studied at the level of community DNA

Abstract

Using a new DNA hybridization technique that does not require culturing, we compared the species composition of natural planktonic bacterial assemblages before and after confinement in 20 l containers for ca 2 d. Although confinement is known to cause species shifts, possibly by stimulating growth of certain types of cells near the container wall, we found that such shifts were minor; 5 to 15% of the communities had changed during confinement. The greatest shift occurred in the samples that had the fastest bacterial growth rate measured by ( super(3)H)thymidine incorporation. Despite the minor changes in species composition, the fraction of the cultivable cells (colony forming units; CFU) increased 4- to 23-fold, but amounted to < 2% of total counts even after confinement. Interestingly, CFU on unsupplemented media increased more rapidly than CFU on supplemented media. Comparisons to previous studies suggest that our use of large (20 l) containers and our efforts to minimize contamination with organic material may have decreased the "wall effect". We conclude that confined microbial communities did not undergo drastic changes of species composition in 2 d in 20 l containers.