My research team and I are using a spectrometer to measure the OD of an algae sample. We are trying to answer the question "Is there a correlation between OD and the number of algae cells?"

What should we consider in our experiment? Is this an established technique?

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    $\begingroup$ are there any materials from your instructor that could help you? have you tried use google/web search? $\endgroup$ Commented Oct 19, 2018 at 19:54
  • $\begingroup$ No, we only have a spectrometer. Also, Google shows results for bacteria, not algae. $\endgroup$
    – AProNoob
    Commented Oct 20, 2018 at 19:47
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    $\begingroup$ just to be clear: you were told: "here is spectrometer, here's some algae. Go figure how many cells is there, don't ask me anything"? $\endgroup$ Commented Oct 20, 2018 at 22:26
  • $\begingroup$ No. We are doing a research project that we chose by ourselves. We are trying to find a way to measure the amount/ quantity of algae there is. We have a spectrometer but dont know how to correlate that to the number of cells. $\endgroup$
    – AProNoob
    Commented Oct 21, 2018 at 23:27
  • $\begingroup$ Hi there and welcome to Bio.SE. Although this is a weird way to count cells, I can see your line of thinking. If there is more absorbance, there are probably more algae. You'll need to control against the feed you add for the algae, get as close to uniform distribution of algae as possible, and consider that you are measuring algae per volume rather than a simple cell count that you'll presumably be doing at each OD measurement under a microscope for a tiny volume from the solution. If you could provide more details about your experimental setup and aims then you might get a better answer. $\endgroup$
    – James
    Commented Oct 24, 2018 at 11:19

1 Answer 1


As with bacterial cells, number of cell and OD are correlated. I am not a specialist, but it seems that you can use microscope to visually count number of cells, then correlate it with optical density of the solution you have. This will allow you to first establish how many cell correspond to 1 unit of OD, then use OD to estimate number of cells quicker than by counting.

Just a few details: counting of cell number is done by dropping given volume of culture on a special slide with grid, then manual counting using microscope.

If we assume that 100 cells/mL (or whatever) solution gives you OD of 1, then OD of 2 will correspond to 1000cells/mL concentration. That us due to logarithmic relationship between OD and light absorption by the cell culture.

  • $\begingroup$ What do you mean by 1 unit of OD? Also, how would we count the amount of cells in a liquid.... take a sample? $\endgroup$
    – AProNoob
    Commented Oct 22, 2018 at 18:08
  • $\begingroup$ @AProNoob added some details $\endgroup$ Commented Oct 22, 2018 at 18:35
  • $\begingroup$ Thanks so much! Wait, so the correlation is logarithmic for all cultures?! $\endgroup$
    – AProNoob
    Commented Oct 25, 2018 at 0:13

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