Table of Contents

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

This dataset contains abundances of cyanobacteria (Synechococcus, and Prochlorococcus), picoalgae, non-pigmented picoplankton (bacteria), and heterotrophic protists acquired through flow cytometry.  Latitude, longitude, timestamp, and sample depth are also included in this dataset.  

Water samples were acquired during the Oligotrophy to UlTra-oligotrophy PACific Experiment (OUTPACE) cruise between New Caledonia and Tahiti from February to April 2015.

Analytical procedure:

Microbial cell abundances were determined using an Influx flow cytometer. Briefly, pigmented groups (Prochlorococcus, Synechococcus, and picoalgae) were enumerated in unstained samples by their chlorophyll and forward scatter signatures (Marie et al. 1999). The high phycoerythrin signal in Synechococcus was used to distinguish this group from Prochlorococcus and picoalgae.

To visualize non-pigmented picoplankton (bacteria) and heterotrophic protists, samples were stained with SYBR Green I DNA dye (SG, 1:10000 final concentration, for 10 min at 4C in the dark) (Zubkov et al. 2007, Christaki et al. 2011). Because bacteria and Prochlorococcus groups exhibit overlapping signal characteristics in surface samples after SG staining, the abundances of bacteria in surface samples were calculated by subtracting the Prochlorococcus abundance, determined in the unstained aliquot, from the total SG-stained group abundance. An internal standard of 1-um diameter fluorescent microspheres (Fluoresbrite, Polysciences) was added to each sample.

Sampling and Analytical Methodology: 

Seawater samples were acquired from bottle samples collected by a CTD rosette during the OUTPACE cruise. 1.8 ml water samples were collected in duplicate, fixed with 0.25% (w/v) paraformaldehyde, flash frozen, and preserved at -80C. One of the duplicate samples was used to count phytoplankton (Prochlorococcus, Synechococcus, picoalgae), and the other was used to count bacteria and protists.

For pigmented groups (Prochlorococcus, Synechococcus, and picoalgae), cells were excited using a combination of two lasers (488+456 nm) focused into one pinhole; while for non-pigmented groups (bacteria and heterotrophic protists), cells were excited using the 488 nm laser only. For calibration, a solution of 1-um diameter fluorescent microspheres (Fluoresbrite, Polysciences) was used.

References: 

Christaki U, Courties C, Massana R, Catala P, Lebaron P, Gasol JM, Zubkov MV (2011) Optimized routine flow cytometric enumeration of heterotrophic flagellates using SYBR Green I. Limnology and Oceanography-Methods 9:329-339. https://doi.org/10.4319/lom.2011.9.329

Marie D, Partensky F, Vaulot D, Brussaard C (1999) Enumeration of Phytoplankton, Bacteria, and Viruses in Marine Samples. Current Protocols in Cytometry:11.11.11-11.11.15. https://doi.org/10.1002/0471142956.cy1111s10

Zubkov M, Burkill PH, Topping JN (2007) Flow cytometric enumeration of DNA-stained oceanic planktonic protists. Journal of Plankton Research 29:79-86.  https://doi.org/10.1093/plankt/fbl059

Flow cytometry data were analyzed using FCS Express Pro (De Novo Software). 

BCO-DMO Processing Notes:

* added ISO timestamp from OUTPACE cruise CTD logs 
* stripped out_c_ from station column for matching with bottle data 
* column station renamed cast to be consistent with CTD log
* added a conventional header with dataset name, PI name, version date
* modified parameter names to conform with BCO-DMO naming conventions

Description from NSF award abstract:
Unicellular cyanobacteria are major contributors to primary production and carbon export in the open ocean. They also play an important role in the control of nutrient availability. The ability of these microbes to harvest light energy benefits a range of physiological functions, but the effect of light on their metabolism (other than for photosynthesis) is poorly known and controversial. This project will investigate the role of light in uptake of organic substrates (carbon and nutrients) by unicellular cyanobacteria and elucidate the importance of photoheterotrophy. The ability of these organisms to assimilate organic compounds and its modulation by light and nutrients will provide additional hints about the ecological success of unicellular cyanobacteria in the ocean. The proposed work will involve field experiments in the southwest Pacific Ocean, complemented by laboratory experiments in controlled cultures of ecologically relevant cyanobacteria. The study will employ innovative methods, including single cell assays and molecular tools that target individual cyanobacteria and evaluate their response to light for the assimilation of organic substrates. This research project will lead to an increased understanding of the microbial adaptations to light and nutrient gradients and the role these adaptations play in elemental cycling in oceanic habitats.

Unicellular cyanobacteria inhabit the surface ocean (generally <150 m deep), and use solar energy to compete for a limited supply of available nutrients. Therefore, they are expected to utilize light energy not only for photosynthesis but also to enhance their metabolism of dissolved organic compounds. Yet, the role of light in the uptake of organic compounds (both carbon and nutrients) and the importance of photoheterotrophy are still poorly understood. This proposal seeks to investigate the ecological drivers and significance of photoheterotrophy in the unicellular cyanobacteria Prochlorococcus and Synechococcus, the most abundant groups of primary producers in the ocean, and Crocosphaera an important nitrogen fixing organism. This proposal argues that adaptations to specific light regimes must shape spatiotemporal partitioning of resources among microbial groups in the ocean. Field experiments along a west-east transect in the southwest Pacific Ocean will cover a range of nutrient conditions and cyanobacterial abundances. Radioactive substrate incubations combined with flow cytometry cell sorting and microautoradiography paired to catalyzed reporter deposition fluorescence in situ hybridization (MICRO-CARD-FISH) will allow the separation of unicellular cyanobacteria from non-pigmented bacterioplankton and evaluation of their response to light for the uptake of different organic substrates. These experiments will be complemented by laboratory tests in controlled cultures of axenic strains representative of different ecologically relevant functional groups of cyanobacteria. Lastly, the capacity of the important nitrogen fixer Crocosphaera watsonii to feed on glucose will be tested, taking advantage of the sequenced genome of the representative strain WH8501 in targeting the expression of genes involved in glucose metabolism in situ.

Project investigators will participate in a Southwest Pacific cruise, the OUTPACE (Oligotrophy to UlTraoligotrophy PACific Experiment) expedition. The cruise will sample stations along a West-East transect between New Caledonia and Tahiti.

More information:
* OUTPACE cruise (doi: http://dx.doi.org/10.17600/15000900)
* OUTPACE website: https://outpace.mio.univ-amu.fr/?lang=en