Cofactor symbiosis for enhanced algal growth, biofuel production, and wastewater treatment

Algae have gained attention for production of fuels and chemicals, and treatment of wastewater. The high cost of algae cultivation, however, has limited industry adoption for these applications. Developing methods to increase algal growth rates and lipid content has emerged as an important strategy toward reducing production costs, and significant research effort has been exerted in this area. We have reported previously that co-culturing the green alga, Auxenochlorella protothecoides, with Escherichia coli under mixotrophic conditions led to 2–6 fold increases in algal growth, doubling of neutral lipid content, and elevated nutrient removal rates compared to axenic growth, indicative of a symbiotic relationship. In the present work, we reveal that symbiosis stems largely from E. coli's provision of thiamine derivatives and degradation products to A. protothecoides. LCMS showed that residual cell-free medium obtained from axenic E. coli culture contained roughly 1.15 nM thiamine pyrophosphate and 4.0–9.1 nM of the thiamine precursor and degradation product, 4-amino-5-hydroxymethyl-2-methylpyrimidine (HMP). These compounds were found to promote the growth, lipid content, and glucose uptake of A. protothecoides, while dramatically improving substrate utilization efficiency. Due to widespread cofactor auxotrophy among algae, the co-culture results presented here likely extend to a large number of microbial community systems. We show that algal-algal symbiosis based on cofactor exchange is also possible, opening a new frontier in algae cultivation management. These findings highlight the potential of engineered microbial communities for improved algal biofuel production and wastewater treatment.