| Prochlorococcus spp. and Synechococcus spp. are abundant marine picocyanobacteria responsible for a large portion of the total primary production in oceanic systems. While a significant amount is known about the spatial and temporal distribution of these organisms, considerably less is known about their growth and grazing mortality rates. Consequently, the factors that regulate the dynamics of these organisms in nature remain poorly understood.; Most methods for estimating growth rate in natural picocyanobacterial populations involve bottle incubations, which may be subject to artifacts. One alternative, incubation-independent approach involves using cellular rRNA content to make inferences about the in situ growth rate of these populations. With this approach in mind, a method for whole-cell hybridization using fluorescently labeled rRNA-targeted peptide nucleic acid (PNA) probes was developed. In contrast to protocols using rRNA-targeted oligonucleotide probes, this method is capable of quantifying rRNA in Prochlorococcus and in oceanic (phycoerythrin-containing) Synechococcus, while preserving the natural fluorescence of these cells. The technique was applied to Prochlorococcus strains SS120, MED4 and MIT9202, and Synechococcus strains WH8101, WH8007 and WH7803 to measure rRNA cell–1 in cultures grown at different light-limited rates. Cellular RNA was related to growth rate in all these organisms in a tri-phasic manner. Among Synechococcus strains, normalization of these data to cellular forward angle light scatter lead to a more linear relationship with growth rate, and reduced the strain-specific differences.; In conjunction with these studies, experiments were conducted in the Sargasso Sea and California Current to assess the growth and grazing mortality of Prochlorococcus and Synechococcus in the natural environment. Using a combination of dilution experiments, cell-cycle analysis and diel abundance analysis it was determined that top-down controls play a significant role in population regulation. Rates of predation on these two organisms at the same station frequently differed. Grazing activity appeared to vary over the course of the day, in general being heaviest during or just after cell division. Stations with the lowest Prochlorococcus abundance had the highest growth and grazing mortality rates. Thus, the relative abundance of Prochlorococcus and Synechococcus does not necessarily reflect their relative importance with respect to growth, grazing, and production. |
