Researchers have engineered the marine bacterium Alteromonas macleodii to continuously accelerate the dissolution of olivine, increasing mineral weathering rates 2.6-fold to capture atmospheric carbon dioxide, according to a study published in Nature Biotechnology.
Silicate minerals such as olivine generate stable bicarbonate alkalinity when they dissolve in seawater, drawing carbon dioxide out of the air. Natural dissolution stalls when oxidized iron precipitates back onto the rock surface, creating a passivating barrier. While bacteria naturally secrete iron-chelating molecules called siderophores that solubilize this iron, endogenous genetic regulation shuts down siderophore production once iron becomes abundant in the surrounding medium.
Bacterial engineering
To overcome this regulatory shutoff, the research team placed the petrobactin siderophore synthesis operon, asb, under the control of synthetic constitutive promoters. In continuous chemostats where olivine sand was retained by settling, the engineered plasmid strain, designated asb+P, maintained petrobactin concentrations more than 100-fold higher than wild-type cultures under iron-replete conditions.
The continuous secretion of petrobactin increased the rate of nickel dissolution from the mineral substrate by a factor of 2.6 compared to abiotic controls. The researchers also integrated the synthetic promoter J23101 directly into the bacterial genome, creating an asb+G strain that sustains petrobactin production without requiring continuous antibiotic selection.
Feedstock selection
Life-cycle analysis revealed that the choice of carbon feedstock determines whether biologically accelerated weathering achieves net carbon removal. Glucose feeds generated 2.8 kilograms of carbon dioxide emissions per kilogram of feedstock and produced acidic metabolic byproducts that reduced alkalinity by more than 3 milligrams of calcium carbonate equivalent per gram of glucose.
Switching to electrochemically generated acetate produced from captured carbon dioxide by Lectrolyst eliminated acid byproduct generation and yielded an upward shift in pH. In pilot-scale continuous bioreactors containing 4 kilograms of olivine and unprocessed seawater, engineered bacteria fed with renewable acetate sustained a direct removal rate of 0.50 grams of carbon dioxide per day from the air.
Industrial-scale modeling of a reactor holding 150 tons of olivine and 24,000 liters of seawater projected net carbon dioxide removal of 1.13 kilograms per day with engineered bacteria and renewable acetate, compared to 0.65 kilograms per day for a seawater-only control. The model indicated that engineered cells must function in unprocessed seawater for at least one week, or roughly seven hydraulic residence times, to optimize net carbon removal efficiency.
