Unbalanced growth in natural assemblages of marine bacterioplankton

Abstract

We tested whether natural assemblages of marine bacterioplankton undergo periods when rates of macromolecular synthesis are uncoupled (unbalanced growth). In seawater cultures of bacteria, rates of DNA and protein syntheses (thymidine and leucine incorporation) and changes in the DNA amount and cell size were compared to fluctuations in growth rate. Rates of DNA and protein syntheses became uncoupled when the bacterial assemblage shifted between growth rates. During these periods of unbalanced growth, rates of bacterial protein synthesis changed faster than rates of DNA synthesis. Variations in the DNA concentration and size of cells paralleled changes in rates of DNA and protein syntheses. The C:DNA ratio of bacteria varied 2-fold depending on the growth state with an average value of 4.6 .+-. 0.48. The delay between unbalanced growth and shifts in the growth rate was on the order of hours and was always shorter than the generation time. Information about unbalanced growth may reveal the delay between fluctuations in the environment and the corresponding bacterial response. In addition, the unbalanced growth model may explain why rates of thymidine and leucine incorporation occasionally do not covary in pelagic ecosystems.