Ultradian Growth in Prochlorococcus spp - PubMed (original) (raw)

Ultradian Growth in Prochlorococcus spp

A Shalapyonok et al. Appl Environ Microbiol. 1998 Mar.

Abstract

Species of the widespread marine prokaryote Prochlorococcus exhibited ultradian growth (faster than 1 division per day) both in situ and in culture, even though cell division is strictly phased to the light-dark cycle. Under optimal conditions a second DNA replication and cell division closely followed, but did not overlap with, the first division. The timing of cell cycle events was not affected by light intensity or duration, suggesting control by a light-triggered timer or circadian clock rather than by completion of a light-dependent assimilation phase. This mode of ultradian growth has not been observed previously and poses new questions about the regulation of cellular rhythms in prokaryotes. In addition, it implies that conclusions regarding the lack of nutrient limitation of Prochlorococcus in the open ocean, which were based on the appearance that cells were growing at their maximal rate, need to be reconsidered.

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Figures

FIG. 1

FIG. 1

Prochlorococcus division pattern in situ (22 to 23 July 1995; 19°10′N, 67°10′E). (A) Cell concentrations and percentage of cells in G1 and G2 phases; _t_0, ta, and tb are the timing points used for growth rate calculations (equations 2 and 3). (B) DNA frequency distributions for selected samples (indicated by dashed lines) during the period of DNA replication and cell division. At other time points only G1 cells were present. Ticks on the x axis mark the origin for each distribution.

FIG. 2

FIG. 2

Prochlorococcus strain MIT 9302 division pattern in laboratory culture on a light-dark cycle (light intensity, 120 microeinsteins m−2 s−1). (A) Cell concentrations and percentage of cells in G1 and G2 phases. Symbols are as explained for Fig. 1. (B) DNA frequency distributions for selected samples (indicated by dashed lines).

FIG. 3

FIG. 3

DNA frequency distributions of Prochlorococcus strain MIT 9302 at 1800 to 2100 at division rates of 0.69 div d−1 (30 microeinsteins m−2 s−1) (A) and 1.34 div d−1 (120 microeinsteins m−2 s−1) (B). Note that when the division rate is less than 1 div d−1 (panel A), there is no sign of new G1 cells entering S phase, in contrast to the clearly visible cohort of cells reentering S phase during ultradian growth (panel B).

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