An experimental test in outdoor ponds using a before-after-control- impact (BACI) design

For outdoor cultivation of algal feedstocks to become a commercially viable and sustainable option for biofuel production, algal cultivation must maintain high yields and temporal stability in environmentally variable outdoor ponds. One of the main challenges is mitigating disease outbreaks that lead to culture crashes. Drawing on predictions from the “dilution effect” hypothesis, in which increased biodiversity is thought to reduce disease risk in a community, we tested whether algal polycultures would reduce disease risk and improve feedstock production efficiencies compared to monocultures. While the positive benefits of biodiversity on disease risk have been demonstrated in various systems, to the best of our knowledge this is the first test in an algal biofuel system. Here, we present the results of a before-after-control-impact (BACI) experimental design to compare mean monoculture (control) and polyculture (impact) yield, stability, and productivity before and after fungal infection when grown in 400-L outdoor raceway ponds. We found that polycultures did not experience a reduction in disease risk compared to monocultures or differ in production efficiencies throughout the course of the 43-day experiment. Our results show that polyculture feedstocks did not improve productivity, stability, and disease resistance compared to that of a monoculture. Determining whether these results are generalizable or represent one case study requires additional outdoor experiments using a larger variety of host and pathogen species.