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Aquatic Microbial Ecology

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AME 58:31-44 (2009)  -  DOI:

Phytoplankton taxon-specific orthophosphate (Pi) and ATP utilization in the western subtropical North Atlantic

John R. Casey1, Michael W. Lomas1,*, Vanessa K. Michelou2, Sonya T. Dyhrman3, Elizabeth D. Orchard3, James W. Ammerman4, Jason B. Sylvan5

1Bermuda Institute of Ocean Sciences, 17 Biological Lane, St. George’s GE01, Bermuda
2College of Marine and Earth Studies, University of Delaware, 700 Pilottown Road, Lewes, Delaware 19958, USA
3Biology Department, Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, Massachusetts 02543, USA
4School of Marine and Atmospheric Sciences, 121 Discovery Hall, Stony Brook University, Stony Brook, New York 11794-5001, USA
5Department of Biological Sciences, Marine Environmental Biology Section, University of Southern California, 3616 Trousdale Parkway, Los Angeles, California 90089-0371, USA
*Corresponding author. Email:

ABSTRACT: Utilization rates of inorganic and organic phosphorus by different picophytoplankton in the oligotrophic ocean are not well quantified. We used radioisotope tracers of orthophosphate (Pi) and the nucleic acid adenosine 5’triphosphate (ATP) to quantify P utilization into flow cytometrically sorted groups of picophytoplankton during the summer and fall of 2007 in the western Sargasso Sea. Dissolved organic phosphorus (DOP) dominated the dissolved P pool (mean ± SD 71 ± 56%), while soluble reactive phosphorus (SRP) concentrations were consistently <5 nmol l–1. All of the groups studied assimilated PiPi) and ATP (ρ*ATP) at significant rates. In addition, ρPi increased with ambient SRP concentrations, while ambient DOP concentrations had no apparent effect on either ρPi or ρ*ATP. Consistent with community composition and contributions to autotrophic biovolume, prokaryotes were primarily responsible for Pi and ATP turnover. In regions where SRP was depleted to <3 nmol l–1, ATP accounted for >70% of the total P utilized. Among the individual taxa, ρPi and ρ*ATP increased in the order Prochlorococcus, Synechococcus, pico-, and nanoeukaryotes, when uptake was normalized to cell number, but the opposite relationship was observed when normalized to cell volume. This suggests that cyanobacteria are physiologically superior to the larger eukaryotes with respect to scavenging both Pi and ATP in the oligotrophic Sargasso Sea. A comparison of estimated C:P utilization rates with particulate C:P ratios suggests that different phytoplankton groups may be experiencing different degrees of P stress in the same ambient nutrient environment. Collectively, these data suggest that the labile DOP pool, assuming that ATP is a reasonable proxy for the labile DOP pool, in the Sargasso Sea may constrain primary productivity in the absence of sufficient SRP, and that cyanobacteria have a physiological advantage for P utilization under these conditions.

KEY WORDS: Flow cytometry · Phosphate utilization · DOP utilization · Sargasso Sea · Picophytoplankton

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Cite this article as: Casey JR, Lomas MW, Michelou VK, Dyhrman ST, Orchard ED, Ammerman JW, Sylvan JB (2009) Phytoplankton taxon-specific orthophosphate (Pi) and ATP utilization in the western subtropical North Atlantic. Aquat Microb Ecol 58:31-44.

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