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Green biflagellated microalgae have proven to be of interest in biotechnology and biomedicine due to the production of lipids, carotenoids, and other components that have an environment dependent yield. In this work, we use back focal plane interferometry to obtain information about the behavior of microalgae held by an optical trap under different conditions. It has been observed that the elongated body of a microalga entering an optical trap will align along the beam axis and rotate counter-clockwise. The rotation is produced by the beating flagella, as we conclude from our observation of non-rotation of deflagellated or photodamaged cells. The dependence of rotation frequency on growth phase of the microalgae and on optical trapping power is investigated. To study these effects, each cell is held in the optical trap, and the laser light transmitted by the sample is collected with a microscope objective. Then, the back focal plane of the collection objective is imaged onto a quadrant photodiode. The voltage outputs of the photodiode are then recorded with a computer through the use of a custom Arduino circuit, and written to a text file for post-processing.
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