Microalgae have been studied intensively in the past decade because they have great potential in simultaneous production of biofuels and other high-value products [1]. For example, microalgae extracts have shown great antioxidant and anti-cancer effects [2] and many of the antioxidant pigments have already been commercialized. However, the production of microalgae biomass and their cellular contents strongly depends on the kind of microalgae, the cultivation condition, and the stress for inducing the accumulation of specific molecules. Conventional analyses for the cellular components of microalgae are multi-step and time-consuming, making the optimization of cultivation strategy challenging and prolonged. Therefore, a rapid and high-throughput platform for assessing the quality of microalgae culture is in great need.   

To rapidly investigate the effects of cultivation conditions and stresses on microalgae, micro-bioreactors have been developed and applied in enhancing the production of lipids [3] and astaxanthin [4]. The accumulation of lipids and antioxidant pigments is induced by nutrient starvation, high irradiation, high temperature, or extreme pH values. However, nutrient starvation creates a changing stress that is challenge to track and control. Oxidative stresses created by adverse environment can arrest the growth of microalgae. On the other hand, a weak electric field is reported to enhance the production of both chlorophyll and carotenoids in microalgae. Therefore, we design a micro-bioreactor integrated with microelectrodes to investigate the improvement of production of microalgal biomass and pigments by the electrical stimulus.  

The micro-bioreactor is composed of a glass slide containing microelectrode and multiple layers of PDMS, including the inlet layer (containing inlet microchannel), the bioreactor layer, the microelectrode layer, the outlet layer (containing outlet microchannel, and the cover layer. Two microalgae, C. vulgaris and S. abundans, are inoculated in the micro-bioreactor and fresh medium is supplied into the micro-bioreactor using a syringe pump. Twelve micro-bioreactors are operated simultaneously for the same combination of nutrient compositions and electric field to obtain statistically reliable outcomes. The biomass and total pigments are quantified by the optical density at 682 nm and 440 nm, respectively.

The effect of electric field on the production of microalgal biomass is first investigated and the results are shown in Fig. 1. An electric field higher than 5 V/cm promotes the production of biomass for both microalgae. The increment of biomass is most significant in 10 V/cm and the biomass of C. vulgaris and S. abundans increases to more than 200% of the untreated culture. The combined effect of nutrient supply and electric field is also investigated. The electric field has the highest promoting effects on the biomass of C. vulgaris cultured in glucose and sucrose and S. abundans cultured in glucose (Fig. 2). Finally, the ratio of pigment per cell (OD₄₄₀/OD₆₈₂) is investigated and Fig. 3 shows that electric field increases the ratio of pigment per cell to higher than 150% for S. abundans cultured in sucrose. In conclusion, the micro-bioreactor can serve as an effective tool in searching suitable nutrient compositions and stresses for the improved production of microalgal biomass and pigments.