The effects of global warming, pollution in river effluents, and changing ocean currents can be studied by characterizing variations in phytoplankton populations. We demonstrate the design and fabrication of a microflow cytometer for characterization of phytoplankton. Guided by chevron-shaped grooves on the top and bottom of a microfluidic channel, two symmetric sheath streams wrap around a central sample stream and hydrodynamically focus it in the center of the channel. The lasers are carefully chosen to provide excitation light close to the maximum absorbance wavelengths for the intrinsic fluorophores chlorophyll and phycoerythrin, and the excitation light is coupled to the flow cytometer through the use of an optical fiber. Fluorescence and light scatter are collected using two multimode optical fibers placed at 90-degree angles with respect to the excitation fiber. Light emerging from these collection fibers is directed through optical bandpass filters into photomultiplier tubes. The cytometer measured the optical and side scatter properties of Karenia b., Synechococcus sp., Pseudo-Nitzchia, Alexandrium, Nitzschia, and Thallassiosira pseudonana. The microflow cytometer proved sensitive enough to detect and characterize picoplankton with diameter approximately 1 μm and larger phytoplankton of up to 80 μm in length. The wide range in size discrimination coupled with detection of intrinsic fluorescent pigments suggests that this microflow cytometer will be able to distinguish different populations of phytoplankton on unmanned underwater vehicles. We also studied the effect of the sheath-to-sample flow-rate ratio on the light scatter and fluorescence of these marine microorganisms.

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