Cross-species Comparison of Ultramafic Rock Bio-accelerated Weathering Performance

Abstract

Carbon mineralization is a natural process that sequesters atmospheric CO2 by reacting it with cations (e.g., Mg2+, Ca2+) released by weathering of rocks to form solid carbonates. However, this process is too slow to capture the excess CO2 resulting from anthropogenic emissions in time to prevent significant warming of the atmosphere1-3. Additionally, accelerating carbon mineralization process by chemical or mechanical methods has proven prohibitively expensive to date4. Microbial rock-dissolution processes, including acidolysis, redoxolysis, and complexolysis, have the potential to accelerate weathering with low energy input5. However, there is no industrially useful microbe capable of dissolving ultramafic (rich in ferromagnesian minerals) rocks, suggesting that one will need to be discovered or built with synthetic biology. While microbes are known to dissolve ultramafic minerals6,7, the performance envelopes for these processes remain uncharacterized, and significant gaps exist in our knowledge of microbe-mineral interaction processes. Here, we make a normalized performance comparison of the dissolution of the ultramafic rock dunite (> 90% olivine ((Mg, Fe)2SiO4)) by three well-known mineral-dissolving microbes: Gluconobacter oxydans7-9, Sphingomonas desiccabilis10,11, and Penicillium simplicissimum12-14. We show that G. oxydans outperformed P. simplicissimum and S. desiccabilis, producing the most acidic biolixiviant (pH 2.15 when leaching 1% pulp density), and extracting the most magnesium (3,130 mg/L when leaching at 3% pulp density). Additionally, G. oxydans co-dissolves nine other metals, eight of which are critical for energy technologies (Cr, Mn, Co, Ni, Cu, and Zn)15,16 with a maximum dissolved concentration of 33 mg/L for Ni. While increasing the pulp density of the dunite (solid to liquid ratio) resulted in higher metal dissolution by G. oxydans and S. desiccabilis, notably pulp densities above 2% inhibited mineral dissolution by P. simplicissimum. Our results provide evidence that the gap in performance between G. oxydans and the other two microbes increases with pulp density, and thus, G. oxydans is best suited for process and genetic engineering to maximize performance and minimize costs and environmental impacts of bio-accelerated weathering. Finally, we propose that G. oxydans and P. simplicissimum can use cellulosic hydrolysate as a cost-effective substitute for glucose for biolixiviant production.