Wildfire ash inhibits the growth and photosynthetic efficiency of marine phytoplankton

While the ecological effects of wildfires have been documented in terrestrial and aquatic environments, impacts on coastal and ocean ecosystems are largely unexplored. Here, we describe the physical and chemical properties of ash from the CZU Lightning Complex fire (California, USA) and assess its experimental impacts on unicellular marine phytoplankton, using model monocultures. Air‐fall ash samples were primarily composed of particles 250–500 μ m and contained ~ 1‰ of Fe, Mn, and Ba. Diagnostic indices indicated the air‐fall ash originated from combusted wood, and the total concentration of the EPA 16 high‐priority polycyclic aromatic hydrocarbons approached 3 ppm. Pulse Amplitude Modulation fluorometry documented 0%–18% reductions in photosynthetic efficiency of Isochrysis and Dunaliella cultures dosed with ash, and culture extracts showed 20%–84% declines in bulk cellular growth. While our study demonstrated the impacts of wildfire ash on marine producers, the precise physiological mechanisms are unclear, and we make several recommendations for future studies.