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

Van Mooy, Benjamin A. S., Allan H. Devol

Limnol. Oceanogr., 53(1), 2008, 78-88 | DOI: 10.4319/lo.2008.53.1.0078

ABSTRACT: 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 33PO43- 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.

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