Which wavelength to use to determine algae using a colorimeter? I am doing a Biology project involving the measurements of the density of algae ( _Nannochloropsis_ ) using absorbance colorimetry. What wavelength should I set my colorimeter to? The issue is that there are two types of chlorophyll, each with a different absorbance optimum, there is spectral scatter to consider, as well as UV absorption when going to low wavelengths. What to do?

From **Biotek** I learned the following:

> Spectral scans of algae cultures demonstrate significant absorbance below 400 nm as well as two distinct absorbance peaks at 440 nm and 675 nm (Figure 1). Cell number determination is most consistent when light scatter is used rather than absorbance by cellular constituents. In order to avoid influence from absorbing material **600 nm** [is best] to monitor growth.

!spectral scan
Figure 1. Absorbance spectral curve of Microcystis aeruginosa suspension cultures.Source: Biotek