ELEMENTAL BALANCING OF BIOMASS AND MEDIUM COMPOSITION ENHANCES GROWTHCAPACITY IN HIGH-DENSITY CHLORELLA-VULGARIS CULTURES

The basic requirements for high-density photoautotrophic microalgal cu ltures in enclosed photobioreactors are a powerful light source and pr oper distribution of light, efficient gas exchange, and suitable mediu m composition. This article introduces the concept of balancing the el emental composition of growth medium with biomass composition to obtai n high-density cultures. N-8 medium, commonly used for culturing Chlor ella vulgaris was evaluated for its capacity to support high-density c ultures on the basis of elemental stoichiometric composition of C. vul garis. This analysis showed that the N-8 medium is deficient in iron, magnesium, sulfur, and nitrogen at high cell densities. N-8 medium was redesigned to contain stoichiometrically balanced quantities of the f our deficient elements to support a biomass concentration of 2% (v/v). The redesigned medium, called M-8 medium, resulted in up to three- to fivefold increase in total chlorophyll content per volume of culture as compared to N-8 medium. Further experiments showed that addition of each of the four elements separately to N-8 medium did not improve cu lture performance and that balanced supplementation of all four defici ent elements was required to yield the improved performance. Long-term (24 d) C. vulgaris culture in M-8 medium showed continuous increase i n chlorophyll content and biomass throughout the period of cultivation . In contrast, the increase in chlorophyll content and biomass ceased after 7 and 12 d, respectively in N-8 medium, demonstrating the higher capacity of M-8 medium to produce biomass. Thus, the performance of h igh cell density photobioreactors can be significantly enhanced by pro per medium design. The elemental composition of the biomass generated is an appropriate basis for medium design. (C) 1998 John Wiley & Sons, Inc. Biotechnol Bioeng 59: 605-611, 1998.