Size dependence of metabolism within marine picoplankton populations

Cell size is broadly applied as a convenient parameterization of ecosystem models and is widely applicable to constrain the activities of organisms spanning large size ranges. However, the size structure of the majority of the marine picoplankton assemblage is narrow and beneath the lower size limit of the empirical allometric relationships established so far (typically >1 mu m). We applied a fine-resolution (0.05 mu m increments) size fractionation method to estimate the size dependence of metabolic activities of picoplankton populations in the 0.10-1.00 mu m size interval within the surface North Pacific Subtropical Gyre microbial assemblage. Group-specific carbon retained on each filter was quantified by flow cytometric conversion of light scatter to cellular carbon quotas. Median carbon quotas were 25.7, 22.6, and 5.9 fg C cell(-1) for populations of the picocyanobacterium Prochlorococcus, high-scatter heterotrophs, and low-scatter heterotrophs, respectively. Carbon-specific rates of primary production as a function of cell size, using the C-14 method, and phosphate transport, using P-33 radiotracers, resulted in negative power scalings (b) within populations of Prochlorococcus and heterotrophs of b = -1.3 and b = -1.1, respectively. These findings are in contrast to the positive empirical power scaling comprising the broader and larger prokaryote category (b = 0.7) and point to within-population variability in cell physiology and metabolism for these important microbial groups.