Wespecifically used 57Co-labeled vitamin B12, as well as 14Cbicarbonate(all from MP-Biomedicals) to assess productivityrates as described above. Triplicate 10 mL culture tubes at eightvitamin concentrations (0, 7, 15, 23, 31, 82, 184 and 368 pmol L1 ofB12) were inoculated with vitamin B12 replete and depleted culturesin early exponential growth phase, prepared as described above, to afinal cell concentration of 1 103 cells mL1, a density typical of C.polykrikoides blooms (Gobler et al., 2008), and spiked with 57Colabeledvitamin B12 while another set was spiked with 14Cbicarbonateas described above. The radio-labeled B12 was added at trace levels (<10%) to avoid any concentration effects on uptakerates. The tubes were incubated for 2 h at 21 8C and exposed to lightat 100 mmol quanta m2 s1. Experiments were terminated byfiltering each tube onto 2 mm filters, rinsing filters with 0.2 mmfiltered seawater, and analyzing them for tracer incorporation.
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