Commercially viable algal biofuel production requires discovery of new strains, genetic engineering for higher productivity and optimization of growth conditions. To accelerate research in these areas, we developed a microfluidic cytometer that measures forward light scatter, chlorophyll fluorescence induction and lipophilic stain fluorescence at a rate of 100 cells s−1. The chlorophyll fluorescence data is processed in real-time to measure the fluorescence-based maximum quantum yield, reported as Fv/Fm, to quantify the photochemical energy conversion of each cell. To demonstrate instrument performance, Fv/Fm measurements are obtained for unstressed (nutrient-replete) and stressed (nutrient-limited) cultures of the marine diatom Phaeodactylum tricornutum and are correlated to values obtained in bulk samples using traditional pulse-amplitude-modulating fluorometry. We then use the cytometer to characterize unstressed and stressed P. tricornutum and show that lipid content (as measured by Nile Red fluorescence) is inversely correlated with Fv/Fm. We believe these findings to be the first time that both photosynthetic efficiency and lipid accumulation have been simultaneously evaluated at the single cell level, and that in doing so, the diversity within these populations was revealed.
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