John F. Cahill, Thomas K. Darlington, Christine Fitzgerald, Nathan G. Schoepp, Joris Beld, Michael D. Burkart, Kimberly A. Prather

Metabolomics studies typically perform measurements on populations of whole cells which provide the average representation of a collection of many cells. However key mechanistic information can be lost using this approach. Investigating chemistry at the single cell level yields a more accurate representation of the diversity of populations within a cell sample, however this approach has many analytical challenges. In this study, an aerosol time-of-flight mass spectrometer (ATOFMS) was used for rapid analysis of single algae and cyanobacteria cells with diameters ranging from 1-8 µm. Cells were aerosolized by nebulization and directly transmitted into the ATOFMS. Whole cells were determined to remain intact inside the instrument through a combination of particle sizing and imaging measurements. Differences in cell populations were observed after perturbing Chlamydomonas reinhardtii cells via nitrogen deprivation. Thousands of single cells were measured over a period of 4 days for nitrogen-replete and nitrogen-limited conditions. A comparison of the single cell mass spectra of the cells sampled under the two conditions revealed an increase in the dipalmitic acid sulfolipid sulfoquinovosyldiacylglycerol (SQDG), a chloroplast membrane lipid, under nitrogen-limited conditions. Single cell peak intensity distributions demonstrate the ability of the ATOFMS to measure metabolic differences of single cells. The ATOFMS provides an unprecedented maximum throughput of 50 Hz, enabling the rapid on-line measurement of thousands of single cell mass spectra.