My daughter (10th grade) is doing a science fair project on the toxicity of triclosan for the algae Selenastrum capricornutum. She is wondering the best way to measure the effect, given the limited resources at the school.

  • Should she use the school’s UV/Vis spectrophotometer or just do low-tech cell counting?
  • If she uses the UV/Vis, does she have to do a calibration curve where she manually counts the cells for some number of data points?
  • Or can she just compare %absorbance to the controls and before/after numbers?
  • Or is it already know for this particular algae at a particular wavelength that a certain %absorbance equals a certain number of cells?
  • If she uses the UV/Vis, does that indicate the total number of cells, dead or alive, or does it only measure live cells?

The trial is over 96 hours. Thanks!

  • $\begingroup$ I've asked some of my colleagues who work on algae to chip in... hopefully one of them will turn up :) $\endgroup$ Nov 8, 2013 at 9:34

1 Answer 1


I don't think it's wise to use a spectrophotometer for this experiment, as it is a total count, i.e. it can't distinguish between dead and alive cells very well, and so she might not see a clear difference between her treated and untreated algae. If she has access to a haemocytometer and a good microscope, then she can count the living cells directly, and calculate the cells/ml using a simple formula.

This paper has a nice overview of how to count algae using a haemocytometer, and if her species of algae can swim, then she would need to use the iodine solution mentioned in here:


I hope this helps!

  • 1
    $\begingroup$ I agree with your answer, but it got me thinking. You could use a spectrophotometer if you took identical samples of algae, treated with different concentrations of agent, then inoculated an aliquot of each into a fresh culture (with a fairly large dilution). By reading the OD of these secondary cultures at some later time you could relate the final OD to how many living cells were inoculated. This could easily be standardised by setting up a calibration with different amounts inoculated from an untreated control. $\endgroup$
    – Alan Boyd
    Nov 8, 2013 at 14:31

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