Hi,
The photosynthetic efficiency of microalgae varies based on the Solar irradiation, CO2 uptake...According to FAO for microalgae it is at a rate of 3 - 6% ( See the calculation here:
http://www.fao.org/docrep/w7241e/w7241e05.htm). Which could be futher increased by cultivation in photobioreactor in which Flow rate of CO2 could be increased.