You can use a spectrometer to measure bacterial, yeast or algal cell growth.

This is typically done by taking a sample from a liquid culture of cells and putting the sample in a cuvette. The absorbance of the sample is then measured at 600 nm. This is often referred to as taking the OD 600, or the optical density of the culture at 600 nm. The OD 600 is a measure of the density of the cells in culture. It does not tell you if the cells are alive or dead, it just gives you an indication of how may cells are in the sample.

There is nothing special about using 600 nm. You could use almost any visible wavelength to measure cell density, but the industry standard is to use 600 nm. Please note that there are much better ways to measure algal cell growth. Please see the notes below.

The OD 600 does not tell you how many cells are in the sample. To know the true number of cells in the culture, you would need to create a standard curve that correlates cell number to a set of absorbance readings at 600 nm. Creating a standard curve would require doing cell counts with a microscope. Please be advised that you cannot just get an OD 600 reading from a Vernier branded spectrometer and convert that to cell numbers.

If you want to measure cell growth over time it is important to make sure that you start with a sample that reads between 0.1-0.2 Abs at 600 nm. Please note that you can dilute the sample from your culture to make sure the spectrometer is in this range. Just make sure you dilute each sample by the same amount as you track cell growth over time. For example, if you diluted your sample by 50% on the first day to get a reading between 0.1-0.2, make sure you dilute your sample by the same percentage when you measure on day 2 etc. Always use an appropriate blank. If you dilute your samples in buffer, make sure your blank is the growth media diluted in buffer at the same percentage.

Measuring cell growth of unicellular algae and cyanobacteria

You can measure algal cell growth by tracking the OD 600, but another way to measure cell growth is to measure the absorbance of the algal culture at 430 or 450 nm over time. These wavelengths correspond to the peak absorbance of chlorophyll A and chlorophyll B. Please note that some ‘algae’ will only have chlorophyll A (cyanobacteria and red algae) while most unicellular green algae will have both chlorophyll A and B. Just make sure you choose the appropriate wavelength to track your cultures. Once again, make sure that your starting sample has an absorbance between 0.1-0.2 Abs at the chosen wavelength. Once again, always use an appropriate blank. Then just track the Abs over a period of days to measure algal cell growth.

You can also track the growth of an algal culture using fluorescence spectroscopy. The Go Direct SpectroVis Plus has a 405 nm LED that can be used for this purpose. A set of basic instructions for measuring fluorescence can be found below.

• Launch Spectral Analysis. Connect the spectrometer to your Chromebook, computer, or mobile device. Select Fluorescence vs. Wavelength.
• Place the cuvette containing the sample into the cuvette slot of the spectrometer. 
• Change the excitation wavelength to 405 nm in the Collection Settings menu.
• Change Integration Time to 150 ms in the Collection Settings menu.
• Start data collection. A full spectrum graph of the solution will be displayed. Note that one area of the graph contains a peak at approximately 675 nm. This peak is from chlorophyll. Stop data collection.
• The starting height of the peak should be between 0.1 and 0.2. Adjust the integration time to increase or decrease the size of the fluorescent peak.
• Write down the integration time you used / found for your sample.
• Measure the fluorescence of a sample from your culture on consecutive days using the same dilution and integration time.

Another option would be to take samples from your culture, aliquot into 2 mL centrifuge tubes, spin then down in a mini-centrifuge, then remove the supernatant and extract any chlorophyll by adding 1-0.5 mL of 70% isopropanol to each tube. Transfer the chlorophyll extract to a cuvette and measure the absorbance using a spectrometer. You could also use this same technique to look at changes in fluorescence over time. If you choose to use this method, make sure you use 70% isopropanol as your blank.