I played with algae for a couple of years and concluded the same. At a minimum, about 40-50% of an algae-derived fuel is needed to run the generators for the farm [or bioreactors]. And the exhaust from that provides a ready supply of not only CO2, but by using a diesel engine, nitrogen, which represents a significant savings in the cost of production. In fact it might be possible to modify a diesel engine to provide 100% of the required nitrogen [beginning as NOx and then reacting it with water]. In principle it seemed that you should be able to recover the phosphorous and potassium as well. But we never addressed that possibility directly.
Also, my working hypothesis was that the diesel engine acts as an air purifier [due to the high temps and pressures], which provides significant cost savings, as opposed to HEPA filters and the energy required to run an air supply system. Just the energy cost of the aeration system can be a killer! In a typical engine, you have about 3 psi to work with by removing the muffler, which is more than enough for the aeration.