A marine biologist used a haemocytometer to estimate the population density of the unicellular microalga Dunaliella salina in a bioreactor.
The grid on the haemocytometer slide has medium-sized squares, each measuring 0.2 mm×0.2 mm0.2\text{ mm} \times 0.2\text{ mm}0.2 mm×0.2 mm. The depth of the liquid chamber beneath the coverslip is 0.1 mm.
Before counting, the biologist prepared a dilution by mixing 1 cm3 of the bioreactor culture with 399 cm3 of sterile seawater (resulting in a dilution factor of 400).
After loading the diluted sample onto the haemocytometer, the biologist counted the algae cells in several medium-sized squares and calculated a mean of 18 cells per square.
Calculate the concentration of Dunaliella salina cells in the original bioreactor culture, in cells per mm3\text{mm}^3mm3. Give your answer in standard form.
