Algal Diversity and Traits Predict Biomass Yield and Grazing Resistance in Wastewater Cultivation

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Increased community diversity has been proposed as a strategy to increase productivity of algal cultures and reduce the risk of major pond collapses caused by grazing pests, which would enhance economics of wastewater-based mass algal cultivation. We tested whether higher algal diversity would increase biomass yield when grown in dairy wastewater, and whether polycultures would exceed the biomass of the highest-yielding monocultures. To test these hypotheses, we employed 28 species combinations and manipulated species richness across three species richness levels. Greater species richness significantly increased yield (up to 41%) compared to the average across monocultures, but no polyculture exceeded the biomass of the most productive monoculture. When exposed to grazing pressure, we observed higher grazing resistance among polycultures, though this varied by species combination. To identify causes of this variation, we then used a trait-based model selection approach to find the best predictors of grazing resistance; traits including cell size, stoichiometry, biomass yield, growth rate, and cell size variation significantly explained grazing resistance, although the best predictors were different for monocultures vs. polycultures. Finally, we observed evidence that polycultures may be less prone to trade-offs between desirable functions for crops (like grazing resistance and growth rate), supporting the concept that diversity can support more functions at once. These results may help inform polyculture selection strategies and provide insight into how ecologically informed engineering approaches could further optimize the productivity and stability of wastewater-based algal cultivation.