UC San Diego

Phytoplankton offer great potential as a bioenergy crop; however, technological advances are needed to intensify their yield and reduce their footprints for water, nutrients and land. One approach to enhance productivity is to grow polycultures of mixed species, which convert abiotic resources into biomass more efficiently than any single taxon. We measured traits related to nutrient and light use, growth rate, biomass production, stoichiometry and neutral lipid concentration in 16 diverse microalgal taxa. Species with large cells (primarily Chlorophyta) showed rapid growth, high asymptotic biomass, low minimum nutrient demands, and high cellular C: N and C: P ratios. These same species also exhibited high minimum demands for light and low lipid concentrations. We grew all 119 possible species pairs and found that biomass yield exceeded the component monocultures in polycultures consisting of species with highly divergent traits. However, underyielding occurred frequently as many pairs produced less biomass than either the mean or the maximum of the two component monocultures. In terms of ecological trade-offs, competitive ability for N and P were positively correlated, but negatively related to ability to grow at low light. In terms of bioenergy production, the species with high cellular lipid concentrations showed both slow growth and high demands for nutrients. Synthesis and applications. Our results identify trade-offs among functional traits that determine the suitability of different algal species as biofuel feedstocks and narrow the search for productive and robust species combinations to maximize bioenergy productivity. An approach based on the ecology of species traits will be more effective in optimizing yield in bioenergy communities than promoting high species diversity per se. © 2014 British Ecological Society.