Intercalibration = instruments presentation

The Cytosub flow cytometers (Cytobuoy b.v.) are able to analyse large phytoplanktonic cells (1 to 1000 µm and a few mm in length, but a specific optical unit (picoplankton option) can look at < 1 µm ) and relatively large water volumes (up to 4 ml per sample).

The fluidic, optics and electronic are modified in order to resolve the low concentrated and highly diversified phytoplankton cells in natural waters. They are capable for operating on line or as a submersible instrument. The Cytosubs can be deployed down to 200 m depth helpful to a cylindrical aluminium covering. The seawater was pumped to fill a sample loop of 4 ml before entering the flow cell in order to avoid external turbulences and operate at atmospheric pressure. Minimal sampling frequency is approx. one sample every 5 minute corresponding to the flushing of the sample loop and the data transfer, depending on the concentration and on the size of the cells.

LMGEM’s Cytosub had an external power source (24V AC) coming from an on land desk through a waterproof data-power cable. This cable allowed the sending of the analysed sample after each run. The sample flow was controlled by a peristaltic pump working at 7.8 µl.s-1 during the temporal study in the Bay of Marseille. It flowed at 8.3 µl.s-1 ¬during the intercalibration test with the NOCS’s Cytosub because the sample tubing was replaced. The sheath fluid flowed at a rate of 80 ml.min-1.

Each particle was intercepted by the solid-state laser beam (Coherent Saphyre, 488 nm, 15 mW). The side scatters and fluorescence signals were dispersed by a concave holographic grating and collected via a HPMT (Hybrid Photomultiplier). The forward scatter signal was collected via a PIN photodiode. The red (FLR), orange (FLO) and yellow (FLY) fluorescences were collected in the wavelength ranges 734-668, 601-668, 536-601 respectively. Data recording was triggered by the forward scatter signal. The shape of the signals was encoded with a frequency of 4 MHz and data were saved in distinct 64 kbits grabbers. Particles flew at a rate of 2 m.s-1 through the 5 µm laser beam so that for instance the signal shapes of 1 µm beads would be defined by 12 points. The sheath fluid and the seawater sample were mixed together at the exit of the flow cell and filtered onto 0.2 µm nucleopore filters in order to be recycled.

NOCS’s Cytosub was similar to the LMGEM one except that power source came from lithium battery (24V DC) encapsulated in an external waterproof and pressurised cylinder. Further more, the analysed samples were saved in a data logger and communication (data transfer and sampling programming) to a computer used a BlueTooth connection. Sample flow reached 11 µl.s-1.