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

The lack of a dilution effect in this study contrasts with a body of literature from the field of disease ecology that has shown that more diverse communities are often less susceptible to dis- ease than are less diverse communities [28, 45–47]. Biodiversity can ‘dilute’ the prevalence of disease via several mechanisms. One proposed mechanism for disease dilution occurs when less susceptible host species reduce the relative abundance of more susceptible host species in a community [32]. In turn, a greater relative abundance of less susceptible host species ‘dilute’ the risk of disease establishment and spread throughout the community. In this study system, this mechanism did not operate because its assumptions were not met. To the contrary, it was found that the competitively dominant algae (S. capricornutum) was also least susceptible to fungal disease. Therefore, when S. capricornutum came to dominate the algal polycultures, it increased the relative abundance of less susceptible hosts, and decreased the relative abun- dance of more susceptible host species (C. sorokiniana and S. obliquus). In turn, there was no reduction in disease risk in the algal polycultures compared to a monoculture of S. capricornu- tum (Fig 3). Essentially, the polycultures behaved as a monoculture with disease resistance driven by the biology of S. capricornutum. It is possible that the application of pesticides con- tributed to the reduced biomass production in both polycultures and monocultures in the After-infection period. However, biomass production metrics began decreasing immediately

following infection, prior to application of pesticides, suggesting that initial decline in biomass production was driven by fungal infection stress. Of course, these results are limited in scope because of the limited species pool of algae and that they were exposed to just one type of pathogen. The species that were the focus of this study were chosen based on several years of prior work that specifically sought to identify the most productive and stable species monoculture (S. capricornutum) and polyculture from a species list that started with 55 Chlorophycean and Charophycean green algae that are com- monly used in biofuel research [12, 16, 17, 35, 36]. The goal was to pit the single best mono- verses polyculture against each other in typical outdoor cultivation setting as a test of disease resistance to determine if species consortia have advantages over traditional monoculture feed- stocks that are exposed to and treated for common fungal infections (i.e. with pesticides). However, because the species pool was limited, this experiment should be interpreted as a sin- gle case study, rather than as general evidence against the operation of dilution effects in algal feedstocks. It is quite possible, perhaps even likely, that dilution effects will be identified in future studies that use different species pools that conform more to the assumptions of models of dilution effects. Accordingly, it may still be possible to identify algal species that have com- plimentary levels of disease susceptibility spanning a range of pathogens that might improve

temporal yields and stability in outdoor cultivation ponds. In addition to finding no benefit of algal diversity for disease resistance, no evidence was found that algal polycultures outperform monocultures in terms of algal production, stability, and response to pesticides. Such results also contrast with certain prior studies, such as those by Shurin et al. [10, 14] and Stockenreiter et al. [15, 18], which have found that the best per- forming polyculture obtains higher biomass yields than that of the component species grown in monocultures. However, it is emphasized that most prior work has been performed in highly controlled laboratory conditions, and it is presently unclear if such results can be repli- cated in the less controlled, more variable conditions of outdoor production. These results are more comparable to those of Godwin et al. [12], who found that under outdoor cultivation, polycultures obtained lower biomass yield and stability compared to the best performing com- ponent species grown in monoculture. This one case study emphasizes that diverse algal poly- cultures can maintain similar levels of productivity and have equal disease resistance to that of certain monoculture. Whether or not this study represents a generality will require that addi- tional outdoor pond experiments using a greater variety of species are performed. PLOS ONE Biodiversity, biofuel and disease: Ecological engineering in algal feedstocks PLOS ONE | https://doi.org/10.1371/journal.pone.0267674 April 28, 2022 12 / 15