ABSTRACT
The grazing rate of the bacterioplankton community may be estimated by determining the rate of disappearance of genetically labelled minicells, added in low concentrations to natural water samples. The turnover rate of minicells is subsequently calculated to grazing rate of natural bacteria via the bacteria to minicell ratio in the sample. "Minicells" are small cells with a size distribution similar to marine bacteria that bud off from bacterial strains with an aberrant cell division pattern. Minicells cannot divide due to the lack of chromosome, but may harbor plasmids. Choosing a plasmid coding for a highly expressed protein enables quantification of minicells via the radioactively labeled marker protein. High specific activity of the minicells is crucial for obtaining a sensitive enough detection limit in the field experiments. The method assumes that all grazing bactivores will accept minicells as normal food particles subsequently ingest and digest the minicells.
