Wildfire ash impacts to photosynthesis and growth of marine phytoplankton cultures

Climate change includes increasing surface temperatures as well as extreme events—heatwaves, storms, floods, and droughts. In 2020, these factors produced a record 10,000 wildfires that burned 18,000 km ² in California USA. Air pollution, including airborne ash, from these fires was a widespread human health hazard. While the ecological effects of wildfires have been extensively documented in terrestrial and freshwater systems, impacts on ocean and coastal ecosystems are largely unexplored. Here, we describe the physical and chemical properties of ash from the CZU Lightning Complex fire and experimentally test its effects on the photosynthesis and growth of four unicellular marine phytoplankton. Sieved air-fall ash was primarily composed of particles 250-500 μm and contained ∼1 ‰ of Fe, Mn, and Ba. Diagnostic indices of polycyclic aromatic hydrocarbons indicated the air-fall ash originated from combusted wood, and the total concentration of the EPA 16 high-priority PAHs exceeded 2.7 ppm. Pulse Amplitude Modulation fluorometry documented various declines in the photosynthetic efficiency of Isochrysis and Dunaliella cultures dosed with ash, and the bulk cellular growth of these cultures was inhibited. While our study demonstrated the impacts of wildfire ash on marine producers, the precise mechanisms are unclear. We provide recommendations for how future studies may further resolve the impacts of ash on phytoplankton productivity, community diversity, and trophic transfer of toxins and describe the long-term impacts of wildfires on coastal marine ecosystems.