Algae are a promising feedstock for biofuels due to their high productivity. But rapidly growing algae are rich in nitrogen (N) and phosphorus (P), which means that large-scale cultivation would require massive amounts of fertilizers. In order for algal biofuels to be economically sustainable and avoid exacerbating nutrient pollution, algal cultivation and processing must maximize rates of production (as biofuel carbon, C) while simultaneously minimizing the consumption of N and P. Engineering has increased efficiency of biofuel systems, but ecology has much to offer from decades of studying what influences growth and elemental content of algae. In our study, we determine if polycultures of algae could improve N and P nutrient use efficiency compared to monocultures of those same species, and potentially balance tradeoffs between nutrient use efficiency and biocrude production. For six species of green algae grown as mono- and polycultures (2 to 6 species), we analyzed flows of N and P through the laboratory-scale processes of biomass cultivation, conversion to biocrude by hydrothermal liquefaction, and nutrient recycling. We used the concept of net nutrient use efficiency (nNUE, C harvested per unit N or P lost) to estimate the N and P demand of biomass cultivation using mono- and polycultures.
Results/Conclusions
As monocultures, algae exhibited tradeoffs among N- and P- nNUE and biocrude production, such that no single species exhibited strong performance in all three functions simultaneously (i.e. > 25th percentile). Among species with the highest biocrude productivity, one was efficient at N (nNUE=90) and inefficient at P (nNUE=201) and the other was inefficient at N (nNUE=21) and efficient at P (nNUE=464). Polycultures did not outperform the best single species in terms of biocrude production or net nutrient use efficiency when quantified separately. However, certain polycultures performed all three functions at higher levels simultaneously (>60th percentile) than any of the six species did as monocultures. Despite the strong performance tradeoffs exhibited by monocultures, certain polycultures came closer to performing all three functions at the overall maximum and polycultures exhibited more balanced performance in the three functions. These results illustrate the potential for strong tradeoffs among species that have similar potential for growth rates and biomass yields. Although polycultures of algae had lower performance than the best species for any single function, they were better at balancing tradeoffs between N- and P-use efficiency, which could be used to simultaneously reduce the demand for both N and P fertilizer.