Nucleic acid content of synechococcus spp. during growth in continuous light and light/dark cycles

Light-limited batch cultures of Synechococcus spp. strains PCC 6301 and WH 8103 exhibited a positive correlation between the specific growth rate and the cellular content of both RNA and DNA. The ratio of RNA to DNA increased with the growth rate in Synechococcus sp. strain WH 8103, as it does in heterotrophic bacteria, but remained constant in Synechococcus sp. strain PCC 6301. The cellular content of nucleic acids decreased during light periods and increased during dark periods in Synechococcus sp. strain PCC 6301 entrained by 12-h light/ 12-h dark (diurnal) cycles. This result was unexpected in light of experiments demonstrating a circadian increase in transcriptional activity during the subjective light periods and a decrease in transcriptional activity during subjective dark periods. The cyclical variation in cellular nucleic acid content during diurnal cycles appears to be in part a function of the timing of cell division and will influence estimates of in situ growth rates or relative abundances of cyanobacteria using oligonucleotide probes complementary to 16S rRNA.