A small volume of a few milliliters containing phytoplankton particles between about 1 µm and 1000 µm in diameter and suspended in water, are pumped up by a flexible tube and brought in the CytoSense flowcytometer (CytoSense by CytoBuoy bv). The particles are transferred by a gentle constant stream one by one in a cuvette, which is illuminated by one or more lasers. Each particle is illuminated-excited by these focused laser beams one by one. Pigments and photo absoroption systems inside autotrophic cells (such as chlorophyl and phyco-erithrin -cyanin) will be induced by the laser light energy, and emit specific colors of light dependent of the pigments inside. The detectors of the flow cytometer are made color sensitive and will capture each individual species signal while it is passing the laser beam. Any particle (autotrophic as well as hetrotrophic or inorganic) particle will induce light scatter as well. If laser light meets the particle, this light will reflect, bow, transmit, diffract, absorp, a kind of collection of physical processes which are dependent of the properties of the particle such as the size, volume, surface properties, inside properties, color etc. These light scatter results in forward and sideward direction of the particle towards the laser beam are also detected. They also give important particle information. Calcite at the surface (Coccolithofores will induce relative hight scatter signals compared to other similar size species). This chlorophyl-pigments and light scatter information as a function of length of a particle form an kind of ‘optical fingerprint’ of species. Species with almost similar fingerprints are grouped together as a phytoplankton similar optical group. Specific fingerprints can characterize species to identify specific them. The number of particles which can be measured in a short time is a big advantage of flow cytometry compared to microscopy. Typical numbers are 10000 particles within a few minutes. This objective and fast way of counting makes the technique very suitable and effective for the automation of phytoplankton monitoring in natural waters.
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