Cofactor symbiosis enhances growth and biofuel production in Chlorella

Studies were completed to elucidate our prior discovery that algae growth and triacylglycerol content significantly increase when Chlorella minutissima is co-cultured with Escherichia coli. Cultivation of C. minutissima on residual E. coli media resulted in an 18-fold increase in algal growth compared to control cultures grown on fresh medium. The growth rate on residual media was nearly 1 g/L/day, an order of magnitude higher than many results reported for mixotrophic algae in the literature. In separate experiments, exogenous thiamine enhanced C. minutissima growth by 10 and 36 fold for mixotrophic and autotrophic cultures, respectively. Additional experiments have revealed that C. minutissima has the ability to salvage thiamine from degradation products, something that has never been reported in algae. In a survey of 306 algae species, 22% were found to require thiamine, roughly 5% require biotin, and nearly half require cyanocobalamin. This suggests that our co-culture findings could extend broadly across algae species. We also cultured C. minutissima on residual medium obtained from another Chlorella species, which had no apparent thiamine requirement. The result was an 8.5 fold increase in growth suggesting that there is also potential for algal-algal symbiosis based on co-factor exchange. This is particularly significant if both algal species can produce valuable products. Our findings suggest the importance of designing systems that enhance the growth of mutually beneficial organisms.