About This Project

By 2050, the process of enhanced rock weathering can be capturing 0.3 Gt CO2/yr in the US alone [1], but limited weathering efficiency [2] shows scope for further bio-inspired improvements [3]. Microbial bio-mining [4] can speed weathering rates but the lack of organic C in rock grains limits its extent. Here, we propose a solution – developing lithogardens: autotrophy-powered microbiomes that rely on oxidation of ferrous iron, abundant in rock, to build up organic C and fuel faster weathering.

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Motivating Factor

Rock dressing has been practiced for at least two millenia with Theophrastus, as early as 350 BC, indicating that the act of supplying “earths” acts to “add heart to the soil” [1]. Over the last 3 decades, it was gradually realized that the weathering of silicate rocks in soil acts to generate alkalinity enabling the capture of atmospheric CO₂ that in turn cools Earth in the process of the global long-term C cycling [2]. Inspired by that, pioneering enhanced rock weathering (ERW) field trials in the heart of the US Corn Belt have recently revealed that spreading basalt rock powder to arable lands remineralizes the soil, boosts crop yields while simultaneously capturing up to 3.8 t CO₂ ha^-1 yr^-1 [3]. Consequently, ERW now represents one of few key negative-emission technologies with estimates showing that in the US, the adoption of ERW may remove up to 0.3Gt CO₂ yr^-1 by 2050 [4], making a substantial contribution to the decarbonization of agriculture.

Specific Bottleneck

Slow weathering rates require the use of large quantities of basalt rock powder to drive substantial CO₂ drawdown [3]. Thus, biology-based solutions are urgently needed to increase ERW rates and aid feasibility [5],[6]. Rock-colonizing microorganisms can speed this process [5],[7],[8] but low organic C limits bio-mining. Our results from a recent pilot studying basalt grain microbiomes from ERW trials show that weathered grains exhibit 41-fold lower organic C content relative to soil while only 6-fold lower microbial biomass. This higher C efficiency in rock microbiomes aligns with 4-fold greater abundance of lithoautotrophs whose metabolism does not require organic C. By fixing CO₂ into microbial biomass through oxidation of basalt-derived S and H₂, we show that lithoautotrophs on basalt grains build up the organic C sources for heterotrophs higher up the food chain to further advance bio-mining. However, S and H₂ are present in trace amounts and their contribution may be transient.

Actionable Goals

Although low in oxidizable S and H₂, basalt is a rich source of microbially oxidizable Fe(II), with each application adding ~2.5 t Fe(II) ha^-1 [5]. Our pilot data show that neutrophilic iron-oxidizing lithoautotrophs of the family Gallionellaceae [9] are 150-fold enriched on weathered grains relative to soil. However, their contribution to the community is small as most released Fe(II) is abiotically oxidized [10]. “Unlocking the gate” to these iron bacteria by re-routing Fe(II) away from abiotic processes may generate superior lithoautotrophic biomass running on ample, long-lasting Fe(II) supply. In turn, this could transform basalt into a natural self-sustainable microbial reactor of improved bio-mining and CO₂ drawdown. Thus, it is necessary to (a) investigate the gatekeepers to prolific microbial iron oxidation on basalt, (b) devise biotechnological solutions to promote this lithogarden weathering engine and (c) test its reproducibility on different soils and basalt feedstocks.