What this work is about, in plain words
Can the amount of dry biomass in a spirulina culture be estimated simply by measuring its turbidity? The authors showed that it can, but the conversion factor varies, and caution is needed with it in production.
Optical density is a quick way to estimate how many cells a culture contains. But living cells are not a dye solution. Cells clump, scatter light and change color, and the overall "turbidity–biomass" relationship is specific to each species, instrument and growth regime. The authors set out to test whether the dry weight of spirulina (Spirulina platensis) can be estimated from the optical density of the suspension.
In the experiments, the optical density of the culture at 750 nm on a common laboratory instrument and the dry weight of the cells were measured in parallel.
According to the authors, optical density and dry weight are directly proportional, but the conversion factor varies noticeably: from 0.66 to 1.1. In production it may fall outside these limits. Clumping of cells overestimates the estimate, and an admixture of other algae underestimates it.
The practical conclusion: optical density is suitable for a quick estimate, but the factor must be regularly checked by direct weighing.
Limitations. The work was done on one culture and one type of instrument. For other conditions, the relationship needs to be determined anew.