Comparative Assessment of Growth Efficiency of the Microalga Scenedesmus rubescens in Open and Closed Photobioreactors

A comparative study was conducted to evaluate the growth dynamics, productivity, pigment composition, and economic parameters of the unialgal green microalga Scenedesmus rubescens (Dangeard) Kessler, strain IBSS-91, cultivated in open and closed photobioreactors. Cultivation in open systems was performed under natural solar radiation with a diurnal light–dark cycle in greenhouse conditions, whereas closed photobioreactors operated under strictly controlled environmental parameters and continuous artificial illumination. The results demonstrated that closed photobioreactors provided higher biomass yield, specific growth rate, and maximum productivity. The maximum productivity of S. rubescens in closed systems reached 0.35 g/L day on Tamiya medium and 0.31 g/L day on BBM, which is approximately 2.2-fold higher than in open photobioreactors. Higher productivity in closed systems was observed despite lower cumulative irradiance and was attributed to the continuity of the light regime and the stability of cultivation conditions. Tamiya medium supported the highest biomass accumulation and photosynthetic pigment content in both reactor types. Economic analysis revealed that the lowest biomass production cost (12.38 USD/kg dry weight) was achieved in open photobioreactors operated with BBM, whereas closed systems were characterized by substantially higher energy consumption. Thus, the choice between open and closed photobioreactors represents a compromise between technological reliability, biological productivity, and economic feasibility and should be determined by the specific goals and conditions of industrial application.