Bacterioplankton growth in seawater: I. Growth kinetics and cellular characteristics in seawater cultures

Abstract

The major objective of this study was to examine the ability of marine bacteria to grow in paricle-free unenriched seawater and to characterize this growth. A natural assemblage of marine bacteria was inoculated into particle-free (0.22 pm filtered) seawater. The inoculum, mainly free-living bacteria, was prepared by filtering seawater through a 0.6 pm Nuclepore filter. Growth on dissolved organic matter was unambiguously demonstrated by parallel increases in cell number, ATP, and biovolume. Doubling times of these 'seawater cultures' were 9 to 18 h, comparable to those of similar bacterial assemblages in the natural environment. These similarities in growth kinetics imply that for at least some of the bacteria, the growth conditions were similar to those in the sea, and growth was not an artifact of containment The average cell volume more than doubled during the log phase of growth and then returned to near initial levels. Preferential predation on such large cells is suggested as an explanation for their rare occurrence in the sea. Seawater cultures should be useful for studies of the nutrition, chemical composition, and other aspects of the physiological ecology of marine bacteria.