Heterogeneity in Nitrogen Sources Enhances Productivity and Nutrient Use Efficiency in Algal Polycultures

Algae hold much promise as a potential
feedstock for biofuels and
other products, but scaling up biomass production remains challenging.
We hypothesized that multispecies assemblages, or polycultures, could
improve crop yield when grown in media with mixed nitrogen sources,
as found in wastewater. We grew mono- and poly- cultures of algae
in four distinct growth media that differed in the form (i.e., nitrate,
ammonium, urea, plus a mixture of all three) but not the concentration
of nitrogen. We found that mean biomass productivity was positively
correlated with algal species richness, and that this relationship
was strongest in mixed nitrogen media (on average 88% greater biomass
production in 5-species polycultures than in monocultures in mixed
nitrogen treatment). We also found that the relationship between nutrient
use efficiency and species richness was positive across nitrogen treatments,
but greatest in mixed nitrogen media. While polycultures outperformed
the most productive monoculture only 0–14% of the time in this
experiment, they outperformed the average monoculture 26–52%
of the time. Our results suggest that algal polycultures have the
potential to be highly productive, and can be effective in recycling
nutrients and treating wastewater, offering a sustainable and cost-effective
solution for biofuel production.