Total counts of marine bacteria include a large fraction of non-nucleoid- containing bacteria (ghosts)

Abstract

Counts of heterotrophic bacteria in marine waters are usually in the order of 5 x 105 to 3 x 106 bacteria ml-1. These numbers are derived from unspecific fluorescent staining techniques (J. E. Hobble, R. J. Daley, and S. Jasper, Appl. Environ. Microbiol. 33:1225-1228, 1977; K. G. Porter and Y. S. Feig, Limnol. Oceanogr. 25:943-948, 1980) and are subsequently defined as total counts of bacteria. In samples from the Baltic Sea, the North Sea (Skagerrak), and the northeastern Mediterranean Sea, we found that only a minor fraction (2 to 32%) of total counts can be scored as bacteria with nucleoids. Lack of DNA no doubt means inactive cells; therefore, a much lower number of bacteria that grow at rates higher than those previously estimated must be responsible for the measured bacterial production in these seas. The remaining bacterium-sized and/or -shaped particles included in total counts may be cell residues of virus-lysed bacteria (ghosts) or remains of protozoan grazing.