Adjusting the light supply to microalgae cultures at high cell density can enhance photosynthetic efficiency at latter stages of cultivation providing extra biomass growth and production. First we investigate inoculum cultivation based on physical and developmental characteristics. Chlorella sorokiniana cultures inoculated with inoculum at three different physiological stages (lag, exponential and stationary) were cultivated under three different CO2 concentrations (0.038%, 5% or 10% CO2 v/v). Samples inoculated with lag phase inoculum supplied with 5% CO2 achieved the maximum biomass production whereas samples supplied with 0.038% CO2 never reached exponential growth. The better growth of samples inoculated with lag phase inoculum was attributed to its increased number of cells compared to the other two inocula. In another set of experiments, we investigate changes in the light supply to optimize biomass growth at high culture cell densities. First, using chlorophyll fluorescence measurements we evaluated the effects of 6 wavelengths (627nm, 617nm, 590nm, 530nm, 505nm, 470nm) and a full spectrum neutral white LED at three different light intensities on the quantum yield of photosystem II (PSII) and non-photochemical quenching (NPQ) in the microalgae Chlorella sorokiniana at three different cell densities (OD 0.5, 1.0 and 1.5). An inverted correlation between PSII and light intensity was found across the whole experiment. As C. sorokiniana cell density increased a decrease in PSII values measured under the green light was observed. NPQ had a noticeable decrease under all light sources as the culture density increased from OD 0.5 to 1.0. To confirm the indications found in the previous experiment, in a second experiment photosynthetic activity and biomass production induced by 4 different LEDs (470nm, 530nm, 655nm, and white-3000K) were analyzed on high-density cultures of Scenedesmus bijuga. As culture density increased, the weakly absorbed green light became more photosynthetically efficient than the red light, thereby inducing significantly higher oxygen evolution at culture concentration of 1.45 g/L. High-density culture (2.19 g/L) cultivated under the green light showed higher biomass production rate (30 mg/L/d) with a 8.43% dry biomass growth in a 6-day period compared to the red light that induced 4.35% dry biomass growth during the same period.