Biodiversity and disease risk in an algal biofuel system: An experimental test in outdoor ponds using a before-after-control-impact (BACI) design

perform more functions at a higher levels compared to the component species grown in monoculture [12, 17]. However, to our knowledge, no study has tested whether diverse algal cultures help feedstocks resist the impacts of disease. Invasion from pest species in outdoor algal cultivation can rapidly result in culture crashes and complete loss of feedstocks, which presents one of the main limiting factors for successful cultivation [6, 20, 44]. Thus, developing effective disease mitigation strategies will improve the feasibility of large-scale algal biofuel cultivation. Here the first empirical test of whether multi-species feedstocks increase resistance to a fun- gal pathogen is provided. Fungal pathogens, especially aphelid and chytrid fungal strains, rep- resent one of the greatest challenges to algal cultivation, as they can quickly proliferate through cultures and result in high host mortality [22, 23]. This study hypothesized that more diverse algal cultures would reduce the prevalence of fungal disease by generating a dilution effect. The dilution effect occurs when an increase in host species diversity leads to a reduction in the risk of disease within an entire community [32]. Contrary to the hypothesis, it has been found that polycultures and monocultures did not differ in their disease risk, thus biodiversity did not confer greater disease resistance through a dilution effect. Because the competitively domi- nant species (S. capricornutum) was also the most resistant to the parasite, it quickly domi- nated the monocultures after infection.

Fig 3. Time series of algal cell densities and infection metrics. Panel A shows the grand mean proportion of total cell density that is composed of each species in polyculture. Panel B shows the grand mean proportion of each species infected with the fungus in both polycultures and monocultures Bars represent the standard error. Highlighted yellow section of both graphs represents the experimental period post infection. Vertical light-grey dotted lines indicate harvest days. Data collected on harvest days are pre-harvest. Vertical red dashed line represents the day of infection (day 23). Infection proportions in panel B are only shown for the After Infection Period because no significant signs of infection were detected prior to day 24. https://doi.org/10.1371/journal.pone.0267674.g003 PLOS ONE Biodiversity, biofuel and disease: Ecological engineering in algal feedstocks PLOS ONE | https://doi.org/10.1371/journal.pone.0267674 April 28, 2022 11 / 15