Assessing nutrient limitation of Prochlorococcus in the North Pacific subtropical gyre by using an RNA capture method

It has been hypothesized that the planktonic community of the North Pacific subtropical gyre (NPSG) underwent a domain shift'' in the early 1980s in which phytoplankton of the domain Eukarya were supplanted by phytoplankton of the domain Bacteria, primarily Prochlorococcus. P limitation of eukaryotic phytoplankton was implicated as the causative chemical factor in the domain shift, and we sought to investigate the current nutrient limitation status of Prochlorococcus, now 2 decades since this event. We measured ribonucleic acid (RNA) synthesis rates by NPSG plankton at Station ALOHA in (PO43-)-P-33 tracer incubations and found that RNA synthesis was the single largest biochemical sink for dissolved P, accounting for about half of the total PO43- uptake. We also found that NH4+ stimulated RNA synthesis but that PO43- did not, which suggested N limitation of plankton growth. We developed a new RNA capture procedure, termed radioisotope-based tracking of RNA synthesis by hybridization and capture (RIBOTRACE), to measure RNA synthesis rates by Prochlorococcus exclusively. Data from this procedure showed that NH4+ stimulated RNA synthesis by Prochlorococcus and confirmed that Prochlorococcus was N limited and not P limited. Our RIBOTRACE data do not necessarily refute the domain shift hypothesis, but suggest that any critical period of P limitation required for the domain shift must have subsided and given way to the N-limiting conditions that existed previously.