This is a difficult question. It depends on the type of microalgal research you are completing. In many fundamental studies it is nice to do cell counts because it allows you to see morphological changes and look at doubling time. If you are looking at large scale research, dry weight, specifically ash free dry weight, is valuable for looking at biomass production. In most instances at least 20 mL per filter and 2 filters per sample are required for accurate measurement. Cell counts do not provide a good relationship to cell dry weight due to cell size variation over time. Optical density, typically 750 nm for algae, is a way to quickly determine the approximate dry weight of a sample. It is important to note that you will need to create your own correlation curve based on the algae being used.

This is a difficult question. It depends on the type of microalgal research you are completing. In many fundamental studies it is nice to do cell counts because it allows you to see morphological changes and look at doubling time. If you are looking at large scale research, dry weight, specifically ash free dry weight, is valuable for looking at biomass production. In most instances at least 20 mL per filter and 2 filters per sample are required for accurate measurement. Cell counts do not provide a good relationship to cell dry weight due to cell size variation over time. Optical density, typically 750 nm for algae, is a way to quickly determine the approximate dry weight of a sample. It is important to note that you will need to create your own correlation curve based on the algae being used.

I second Everett's "depends . . ."

Are you looking at multispecies assemblies or are you looking a single species culture? Counting will give you the ability to partition biomass by species which you won't be able to do with the other methods. You can also get at least some estimate of biomass with cell counts by measuring biovolumes (or surface area if you want to factor in a large vacuole).

Which type of algae are you looking at? With things like diatoms and other silicaceous types, the frustules etc. will be included in dry mass measurement. Can you do ash-free dry mass? This would at least allow you to separate out the organic from the inorganic components of dry mass.

I would second Everett. Unless you're primarily interested in biomass cell counts are probably the highest quality data overall, but haemocytometer counts take quite a bit of time. If you have very thin or uniform samples (clean, non-clumpy cultures or perhaps seawater samples) then a Coulter counter works quite well, once you tune it to the cell size in which you're interested.

thank you all! This definitely clear things up, I been going through tons of publications looking for a clear answer

Dear Joseph,

I think that dry weight is exactest method, and others are just tentative. You see, the gravimetric method can be performed with an exactness of 0.01 mg. Regretfully, the optical density is not one and same in the whole volume, for example, when work with filamentous algae or cyanobacteria. Besides, the optical density characterizes the system of conjugated double bonds. At oscillations of the amount of optically non-visible substances the error is ready. Cell count is exact just in case of more than 95 % synchronous culture.

It depends on the algae and your purpose. If you are working with mixed cultures or from the wild samples it will be practical to do cell count to avoid errors in the weight and optical density due to some particulate and suspended solids. For cultures, either dry weight or optical density is okay depending on the type of algae, time and availability of equipment (centrifuge,suction pump, filtering system and spectrophotometer). Filamentous and bigger microalgae are best done in dry weight/wet weight but for small, unicellular algae (Chlorella) in logarithmic phase optical density.