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

Abstract

Abstract. Anthropogenic greenhouse gas emissions are leading to global temperature increases, ocean acidification, and significant ecosystem impacts. Given current emissions trajectories, the IPCC's reports indicate that rapid abatement of CO2 emissions and development of carbon dioxide removal (CDR) strategies are needed to address legacy 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, which lie at the base of marine food webs. We grew six species representing several globally important phytoplankton species under elevated concentrations of olivine dissolution products via a synthetic olivine leachate (OL) based on olivine's elemental composition. We monitored their physiological and biogeochemical responses, which 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 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 physiological effects for any of the phytoplankton examined, even at high concentrations, and may support growth of particular taxa under some conditions. Future studies can shed light on long-term eco-evolutionary responses to olivine exposure and on the potential effects that marine microbes may in turn have on olivine dissolution rates and regional biogeochemistry.

Coastal enhanced olivine weathering for climate change mitigation: investigating the CO2 sequestration potential and ecotoxicological risks

“…The diatom Ditylum sp. did not outcompete the coccolithophore Emiliana huxleyi during co-culturing in contrast to the "green ocean" hypothesis by Bach et al ( 2019), but both species benefitted from the available olivine dissolution products (Hutchins et al, 2023). Finally, after a 21-day exposure to olivine particles (D10 -D90 = 22…”

“…Furthermore, the growth of the freshwater green microalgae Chlamydomonas reinhardtii and Chlorella pyrenoedosa was increased during a 9-day exposure to 8 g olivine L -1 as a result of the increased Mg availability (Xie et al, 2023). Neutral to positive physiological responses were observed for two silicifying diatoms, a calcifying coccolithophore, and three cyanobacteria species exposed to a range of olivine dissolution product concentrations (Hutchins et al, 2023).…”

“…Currently, only four studies have experimentally investigated the effects of olivine exposure on aquatic biota (Ren et al, 2021;Guo et al, 2023;Hutchins et al, 2023;Xie et al, 2023). Ren et al (2021) observed an increase in the abundance of marine biofilmforming particle attached microbes during a 10-day laboratory exposure to 1% w/w fine grained olivine (D50 = 30 µm) in natural seawater.…”

“…Silicifiers and calcifiers benefitted from additional Fe and Si availability or enhanced alkalinity, respectively, while metals such as Ni and Co promoted cyanobacterial growth (Hutchins et al, 2023). The diatom Ditylum sp.…”

“…additionally, little is known about the bioaccumulation potential and possible toxic effects of olivine and its dissolution products for marine biota (Ren et al, 2021;Hutchins et al, 2023). Robust ecological risks assessment of coastal enhanced olivine weathering is therefore currently not possible.…”