Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Prochlorococcus in situ Cell Cycle Phases Fractions.

Samples were taken using a rosette of Niskin bottles, 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 dual beam flow cytometry on a modified Coulter-EPICS 753 flow cytometer (Binder et al. 1996). Samples were chosen in random order, defrosted in a 30°C water bath (just long enough to melt, ~5 min), and stained with the DNA-specific stain Hoechst 33342 (0.5 ug mL-1 final concentration) (Invitrogen, Carlsbad, California) for a minimum of 20 min in the dark. Prior to analysis, polystyrene fluorescent beads (Flow Check® 1.0 um (YG) and 0.494 um (BB); Polysicences Inc., Washington, PA, USA), were added to each sample, and used to normalize cellular light scatter, red (chlorophyll-derived) fluorescence, and Hoechst fluorescence.

Samples were run at an infusion rate of 10 uL min-1 for 10 to 50 min, depending on cell abundance within the sample. A minimum of 10,000 Prochlorococcus cells were analyzed, except for samples in which low Prochlorococcus concentrations made this impractical.  

DNA frequency distributions for Prochlorococcus cells were obtained from Hoechst-derived blue fluorescence. These frequency distributions were deconvoluted into their component cell cycle stages (G1, S, G2) using Modfit software (Verity Software House, Topsham, ME, USA), and assuming a simple model comprised of two Gaussian populations (G1 and G2) and a broadened rectangle (S). 

BCO-DMO Data Processing Notes:

- separated Time.UTC column into two columns - date_UTC and time_UTC
- reformatted date to yyyy/mm/dd and time to 24hr time
- replaced #N/A to nd
- added ISO_DateTime_UTC column

The intellectual merit of the research is to extend our understanding of the biology and ecology of marine picophytoplankton, a group of microbes that are responsible for a large proportion of the total photosynthetic carbon fixation that occurs in the world's oceans. The importance of picophytoplankton as the dominant primary producers in open-ocean ecosystems is well-established. However, the factors that regulate the distribution and abundance of these populations remain poorly understood. The investigators will explore the dynamics of top-down (grazer-mediated) regulation of picophytoplankton populations in a specific context: the maintenance of summertime subsurface maxima in the pico-cyanobacterium Prochlorococcus (but not Synechococcus) in the Sargasso Sea. This phenomenon represents a relatively simple and predictable model system within which to test hypotheses about the regulation of oceanic picophytoplankton in general.
Recent results suggest that despite their abundance, Prochlorococcus in the subsurface maxi-mum are growing (and being grazed) rather slowly, as compared to the smaller population at the surface. In order to understand the factors responsible for this apparent paradox, this project will use a combination of field and laboratory studies to characterize and compare the interactions between Prochorococcus and its protozoan grazers at these two contrasting depths, and in relation to Synechococcus, which forms no such sub-surface maximum.
The broader impacts include training for graduate and undergraduate students. In addition, given the significance of picophytoplankton as primary producers at the base of oceanic microbial food webs, the results of this project should inform efforts to describe and model the broader oceanic ecosystem, and ultimately to understand its role in the global carbon cycle.