Botryococcus braunii reduces algal grazing losses to Daphnia and Poterioochromonas through both chemical and physical interference

Crop protection from algal grazers is a key area of concern, as grazing zooplankton and flagellates can decimate algal crops and impede economic viability of cultivation for biofuels and bioproducts. Inhibition of grazing by chemical and physical interference is one promising solution; however, there have been few empirical tests of this approach that use defense traits innate to algal crop species. Here we conduct an experiment to test whether the hydrocarbon excreting alga Botryococcus braunii can mitigate losses to grazing by two distinct grazers, Daphnia magna and Poterioochromonas malhamensis, due to both chemical inhibition and physical interference linked to large/inedible colonies. We show that chemical and physical defenses interactively reduce the total effect of grazing, thus significantly increasing the biomass of cultures of B. braunii and Nannochloropsis limnetica when either grazer is present. Specifically, B. braunii filtrate alone inhibits grazing and thus weakens top-down control of N. limnetica growth rates and final biomass by both grazers; B. braunii colonies alone also inhibit biomass losses; and the combination of filtrate and B. braunii colonies reveals an interactive effect of both chemical and physical defenses on grazing. Our study demonstrates how community engineering can identify synergies arising from algal cocultivation (e.g., by using industrially relevant strains for crop protection). Such ecological discoveries may help to reduce the costs of large-scale deployment of algal cultivation for sustainable foods, fuels, bioproducts (e.g., bioplastics), and carbon capture.