A single-cell analytical technology was developed for evaluating fast-growing cultures of green algae. The main part of the single-cell analysis is an epifluorescence microscopy-based cytometric approach combined with an automated image analysis algorithm and a single-threshold discrimination procedure. The reliability of the technique in terms of object recognition, evaluating particle size, and determining chlorophyll was successfully proven via reference analyses. The microscopy technique was used to determine the size of single cells, the amount of chlorophyll, and the density of chlorophyll in a model algal culture (Acutodesmus o.). The algal cells showed unexpected heterogeneity in all single-cell parameters, and exhibited a high correlation between cell size and amount of chlorophyll but a very low correlation between cell size and chlorophyll density. For a given cell size, the cell-to-cell heterogeneity of the relative chlorophyll density showed a spread of 0.02-0.08. This points to large variations in the architecture and the physiological state of the photosynthetic apparatus in the cells. This complex situation should be considered in future systems biology approaches focusing on the relationships between biomass accumulation, photosynthetic activity, and central carbon metabolism. Graphical abstract Analysis of cell-to-cell heterogeneity obtained from microscopic images.
Keywords: Chlorophyll; Multidimensional single-cell analysis; Object recognition; Wide-field fluorescence microscopy.