Dry Weight vs Cell Count
Dry weight is the weight of the product the water removed. Reed Mariculture measures dry weight in our microalgae products by placing 100 grams of algal concentrate into an 80°C drying oven and heating until a stable weight is reached. The weight of the residue is then used to calculate the dry weight. For Nanno 3600, 100 grams of concentrate will yield 18 grams of dry residue, hence it is an 18% dry weight product.
The reason we specify the concentration of our Instant Algae products by dry weight, and not by cell number, is because there is such a large difference in cell volume (and biomass) for a small difference in cell size. For example, Isochrysis is around 5 µm in diameter...
Diameter 5.0 µm, cell volume = 65.4 µm3
Diameter 5.5 µm, cell volume = 87.1 µm3
So an increase of just 10% in the diameter means a 33% increase in volume (and biomass)!
A 0.5 µm difference in dimension is difficult to measure by microscopy, especially in cells that are not perfect spheres. Algal cells in culture are growing and dividing, so there is a range of sizes from newly divided to just ready to divide. The 2 daughter cells from a new division have the same biomass as the mother cell, even though there are now twice as many cells. So cell number is of very limited value in specifying the feed value of algae, even for a single-species product. For Shellfish Diet, even just considering the roughly spherical cells (Thalassiosira weissflogii cells are cylindrical and can vary greatly in length):
Isochrysis at 5 µm diameter, cell volume = 65.4 µm3
Tetraselmis at 12 µm diameter, cell volume = 905 µm3
For comparison, Nannochloropsis cell sizes vary from 1-2 µm depending on growth conditions and where the cell is in its “cell cycle,” i.e. having just divided, growing, or ready to divide into two cells.
1 µm diameter, cell volume = 0.524 µm3
2 µm diameter, cell volume = 4.19 µm3
For this reason, cell number is not very informative of food value. A given biomass of filter feeders requires a certain biomass of phytoplankton to support metabolism and growth, not a certain number of algal cells. The only way you can reliably calculate your required feed rates is to consider the biomass of your animals and the biomass/dry weight of the algal feed.