Responses of globally important phytoplankton groups to olivine dissolution products and implications for carbon dioxide removal via ocean alkalinity enhancement

Abstract. Anthropogenic greenhouse gas emissions are leading to global temperature increases, ocean acidification, and significant ecosystem impacts. Given current emissions trajectories, the IPCC calls for both the rapid abatement of CO₂ emissions and development of carbon dioxide removal (CDR) strategies that can address legacy emissions and difficult to abate emissions sources. These CDR methods must efficiently and safely sequester gigatons of atmospheric CO₂. Coastal Enhanced Weathering (CEW) via the addition of the common mineral olivine to coastal waters is one promising approach to enhance ocean alkalinity for large-scale CDR. As olivine weathers, it releases several biologically active dissolution products, including alkalinity, trace metals, and the nutrient silicate. Released trace metals can serve as micronutrients but may also be toxic at high concentrations to marine biota including phytoplankton that lie at the base of marine food webs. We grew several globally important phytoplankton functional groups under elevated concentrations of olivine dissolution products using a synthetic olivine leachate (OL) based on olivine elemental composition, and monitored their physiological and biogeochemical responses. This allowed us to determine physiological impacts and thresholds at elevated olivine leachate concentrations, in addition to individual effects of specific constituents. We found both positive and neutral responses but no evident toxic effects for two silicifying diatoms, a calcifying coccolithophore, and three cyanobacteria. In both single and competitive co-cultures, silicifiers and calcifiers benefited from olivine dissolution products like iron and silicate or enhanced alkalinity, respectively. The non-N₂-fixing picocyanobacterium could use synthetic olivine-derived iron for growth, while N₂-fixing cyanobacteria could not. However, other trace metals like nickel and cobalt supported cyanobacterial growth across both groups. Growth benefits to particular phytoplankton groups in situ will depend on species-specific responses and ambient concentrations of other required nutrients. Results suggest olivine dissolution products appear unlikely to cause negative effects for marine phytoplankton, even at high concentrations, and may support growth of particular taxa under some conditions. Future studies can shed light on long-term evolutionary responses to olivine exposure, and on the potential effects that marine microbes may in turn have on olivine dissolution rates and regional biogeochemistry.