CELL-CYCLE DISTRIBUTIONS OF PROCHLOROPHYTES IN THE NORTH-WESTERN MEDITERRANEAN-SEA

Natural populations of oceanic prochlorophytes were sampled from the northwestern Mediterranean Sea in winter, stained with the DNA-specific fluorochrome DAPI, and analysed by flow cytometry. DNA histograms exhibited two peaks (G1 and G2 cells, containing one and two genome copies, respectively), separated by a trough of DNA-synthesizing cells (S cells). This suggested a cell cycle with a discrete S phase similar to that observed in eucaryotes or slow-growing procaryotes. Nitrogen and light were the two key environmental factors controlling the in situ cell cycle distributions of this procaryote. When nitrate levels were below 0.4-mu-M or light below 0.1 % of the surface intensity, most of the cells were found in G1, suggesting that they were not actively cycling. Cells arrested in G1 could be induced to cycle into S + G2 by incubating them with added nitrogen. Response was a function of initial nitrate concentration and decreased with depth, indicating that it was modulated by available light. These findings strongly suggest that prochlorophytes, which are one of the key components of the picoplankton community, may grow slowly in nitrogen-depleted waters, but still have the potential to respond quickly to nitrogen pulses.