Photobioreactor engineering: High-density algal cultures using light-emitting diodes.

Abstract

Detailed light requirement for photosynthesis and cell growth can be evaluated using solid state light technology. Advanced gallium aluminum arsenide light-emitting diodes (LEDs) were examined for their ability to support mass culture of a unicellular eucaryotic green alga, Chlorella vulgaris. LEDs with peak emittance of 680 nm were used as a sole light source for cultivation. The final biomass and specific cellular growth rate under LEDs were comparable to those obtained under full spectrum from a fluorescent light (FL). The narrow spectrum monochromatic red light was found to affect the average cell volume (reducing it from 60 $\mu$m$\sp3$ to 30 $\mu$m$\sp3$), and make the size distribution and the per cell DNA distribution narrower, but did not affect the total biomass production. Based on two parametric flow cytometric analyses, it was found that the cells grown under red LED light had rather uniform amounts of DNA in all cell sizes, unlike those grown under FL. This result implies that the critical cell size for division under red light may be smaller than that under white light. A novel photobioreactor (PBR) system using the LEDs was designed and constructed. Direct internal illumination by 680 nm LEDs could deliver as high as 50 mW$\cdot$cm$\sp{-2}$ of light into the culture medium. Gas transfer by internal sparging had the capacity to transfer up to 1 mol O$\sb2{\cdot}$L$\sp{-1}{\cdot}$hr$\sp{-1}$. Nutritional and biological limitations could be overcome by continuous perfusion. When the PBR operated with a continuous perfusion rate of 6 reactor volumes a day, it could support ultra high-density algal cultures up to cell concentrations of 4$\cdot$10$\sp9$ cell$\cdot$mL$\sp{-1}$ and total biomass of 9.4% v/v (about 25 g$\cdot$L$\sp{-1}$ dry weight). The volumetric productivity was 3.15 g dry weight$\cdot$L$\sp{-1}{\cdot}$day$\sp{-1}$ (or 44 g dry weight$\cdot$m$\sp{-2}{\cdot}$day$\sp{-1}$). The oxygen production rate at its peak was 13-15 mmol$\cdot$L culture$\sp{-1}{\cdot}$hr$\sp{-1}$. This performance represents the highest reported cell densities attained in photoautotrophic cultures. Continuous perfusion allowed for long-term stable oxygen production. All the limiting factors in PBR design were discussed and analyzed for their characteristics. The data presented here demonstrates that PBR technology can be significantly improved and will provide a basis in future PBR designs.