Dilution Experiments: For an overview of the purpose and interpretation of dilution experiments see Landry (1993) and Worden & Binder (2003). Seawater samples were taken with Go-Flo bottles suspended on non-metallic cable. A portion of this water was gravity-filtered through 0.2 um pore-size capsule filters (Whatman Polycap 36 TC), and appropriate volumes of filtered and unfiltered seawater were added to 500 ml polycarbonate bottles to achieve the indicated dilutions. Time-0 (T0) samples were removed from each bottle and preserved for later flow cytometric analysis as described below. Incubation bottles were then distributed to nylon mesh bags and resuspended in the water column at the depths indicated. After 24 hours, the bottles were recovered and sampled for time-final (TF) counts. All sampling gear, filters, and incubation bottles were acid-washed per Fitzwater et al. (1982). The dates, times, and locations of the experiments were as follows:

 

Cruise      Exper    Date (UTC)    T0 (UTC)     E Long     N Lat

CH0409    X1         26-May-09     17:58:30     -71.998    30.171

CH0409    X2         29-May-09     13:28:09     -72.002    30.173

CH0510    X2         27-May-10     13:33:30     -72.683    30.699

 

Flow Cytometric Cell Counts: Samples were fixed with freshly titrated paraformaldehyde (pH 7.4–8.1, 0.1% final concentration), held in the dark for 10 min, frozen in liquid nitrogen, and stored in a ‑80 deg C freezer (CH0409 samples) or in liquid nitrogen (CH0510 samples) until analysis. Preserved samples were analyzed by flow cytometry on a modified Coulter-EPICS 753 flow cytometer (Binder et al. 1996). Samples were chosen in random order and defrosted in a 30 deg C water bath (just long enough to melt, ~5 min). Prior to analysis, polystyrene fluorescent beads (Flow Check® 0.494 um “BB”; Polysicences Inc., Washington, PA, USA), were added to each sample, and used to normalize cellular light scatter and fluorescence. Samples were typically run at an infusion rate of 20 uL min-1 for 1 to 20 min, depending on cell abundance within the sample. A minimum of 1,000 Prochlorococcus cells were analyzed, except for samples in which low cell concentrations made this impractical. Final cell concentrations tabulated here were calculated from 1-7 replicate counts (Count.n).