Using optical densities (O.D.) is a common measurement technique used to quantify the growth of cyanobacteria. For cyanobacteria, cell density is measured at a wavelength of 730 nm, typically using a spectrophotometer1. In the process of creating our biophotovoltaic cell hardware, we needed to develop a method to quantify the density of our screen printed cultures in terms of O.D.730, which we accomplished by using image analysis and our cyanobacterial growth model. While doing so, we realized that this could also be a novel method for measuring the optical densities of liquid cyanobacterial cultures and serve as a useful tool for other teams working with cyanobacteria with limited resources.
We developed our own calibration curve using the program ImageJ that can be used to determine the O.D. of Synechocystis sp. PCC 6803 samples without the use of a spectrophotometer. While the following measurement technique is based on Synechocystis sp. PCC 6803, it has the potential to be applied to other cyanobacterial strains and E. coli.
To serve as a standard for our calibration curve, we chose to use a KimWipe box as its shade of green is similar to that of cyanobacteria. A KimWipe box also serves as an accessible standard as it is a universally used material in all lab settings.
Figure 1: KimWipe box with Synechocystis sp. PCC 6803 samples
Figure 2: Calibration curve of liquid cultures of Synechocystis sp. PCC 6803
While our presented measurement technique is based on measuring the cell density of Synechocystis sp. PCC 6803, we believe this method can also be adapted to measure cyanobacterial health. By splitting the RGB channels of the photographs taken of samples, the ratios of mean gray values can be compared between different color channels to evaluate the health, similar to the way that a health analysis is possible by finding chlorophyll A concentration/O.D.7302 . If implemented, this measurement would drastically increase the accessibility of cyanobacteria work for labs with limited resources and allow for improved and increased research regarding cyanobacteria.