Native Soil Microbes Versus an Added Lab Strain: How a Riverbank Ureolytic Community Affected Sporosarcina pasteurii Biocementation (Murugan et al., 2021)

Most laboratory work on Microbially Induced Calcite Precipitation (MICP) uses one well-characterised bacterium, Sporosarcina pasteurii. In the field, though, that strain is added to ground that already holds its own microbes, some of which can also break down urea. A 2021 open-access paper by Raja Murugan, G K Suraishkumar, Abhijit Mukherjee and Navdeep K Dhami asks what those resident organisms do. Does a native ureolytic community slow down, help, or simply sit alongside an introduced S. pasteurii, and how do the crystals each makes compare? The paper was received on 5 July 2021 and accepted on 14 September 2021. It reports that the added strain precipitated calcium carbonate about six times faster and was not measurably affected by the native community. The slower native microbes grew larger crystals.

The two field strategies behind the question

The authors frame the study around two ways of getting calcifying bacteria into soil. Biostimulation feeds the organisms already present so they multiply and precipitate carbonate. Bioaugmentation introduces a strong urease producer, usually S. pasteurii. The paper describes augmentation as faster but costlier, because cultures must be grown and transported. Stimulation is presented as cheaper and as avoiding the possible ecological effects of introducing a non-indigenous organism, though it is generally slower. The authors say little work had examined how a native ureolytic microbial community (which they abbreviate NUMC) influences an added S. pasteurii, or how its own precipitation compares.

What was set up

The "native community" was a lab-assembled mixture of four ureolytic isolates (BS1 to BS4), combined in equal proportions. They came from soil on the Brahmaputra riverbank in India and had been described in an earlier study. By 16S rRNA comparison, BS1 and BS2 are close relatives of Sporosarcina siberiensis, BS3 of S. pasteurii and BS4 of Sporosarcina soli. The augmenting strain was S. pasteurii ATCC 11859.

The experiments ran in shaken liquid cultures of a urea and calcium chloride cementation medium, kept at 30 ºC and 180 rpm in a shaker incubator. No soil was involved. There were two series:

• Groups A to G: native community alone, at 0, 0.1, 0.2, 0.4, 0.8, 1.6 and 3.2 OD (optical density at 600 nm; the paper equates 1 OD with about 4.5 × 10⁸ cells/mL).
• Groups 1 to 7: a fixed 0.4 OD of S. pasteurii plus the native community at those same seven concentrations.

The team tracked dissolved calcium by titration and also measured pH. They fitted the calcium decline to a logistic curve to obtain a kinetic constant, K_cal, and used electron microscopy and X-ray diffraction to examine crystal size, shape and phase.

What was measured

Native community alone. Calcium fell in every group except A, the group with no cells. The more cells were present, the faster it fell. Group B took 96 hours to halve the calcium; groups F and G took 24 hours. K_cal climbed from 0.012 h⁻¹ at 0.1 OD to 0.078 h⁻¹ at 3.2 OD. The authors fitted these values with a Michaelis-Menten-type saturation curve, which gives a ceiling (K_cal,max) of 0.1 h⁻¹ and a half-saturation concentration of 1 OD. By the end of the 288-hour run, calcium was exhausted in groups B to G.

With S. pasteurii added. Calcium dropped immediately in all seven groups and was near zero by the sixth hour. Table 2 gives K_cal values between 0.64 and 0.78 h⁻¹. The authors found no significant differences among them, even in group 7, where the native community outnumbered S. pasteurii eight to one by optical density. They compare group 1's 0.64 h⁻¹ with the native community's fitted ceiling of 0.1 h⁻¹, and that comparison is where the "sixfold" figure comes from. At an equal 0.4 OD, the paper's own tables show a wider gap: 0.023 h⁻¹ for the native community against 0.64 h⁻¹ for S. pasteurii. The pH stayed between 6.5 and 8.3 in all groups. The paper says pH changed faster where S. pasteurii was present.

Crystals. Both series produced rhombohedral calcite. The only exception was group B, which contained 2.3% vaterite. The native-only precipitates, sampled at 288 hours, formed clusters 35–100 µm across. Crystal size shrank as cell concentration rose, from 80–100 µm in group B to 35–55 µm in group G. The augmented precipitates, sampled at 12 hours, measured 5–10 µm in groups 1 to 4, 15–25 µm in group 5 and 30–40 µm in groups 6 and 7. The authors invoke classical nucleation theory to explain this: rapid ureolysis drives the solution to supersaturation quickly, which favours many small nuclei, while slow precipitation lets fewer crystals grow larger. They describe the slow, large-crystal outcome as "a positive impact on the quality of crystals."

What the evidence does and does not show

The paper's title and conclusions refer to soil microbes, but no soil was cemented. Everything happened in well-mixed liquid, so the results say nothing directly about strength, stiffness, permeability or how evenly the binder spreads through ground. The native community was not a natural soil population. It was four enriched isolates, one of them a close relative of S. pasteurii itself. The run lasted about two weeks, and no competition or survival of the two populations was measured, only overall calcium removal. Larger crystals are treated as preferable on the strength of cited literature, not measurements made here. The crystal-size comparison also mixes sampling times (12 hours against 288 hours). The authors acknowledge that their result conflicts with studies that favour biostimulation, and they suggest different native communities and conditions as the reason. A few internal inconsistencies remain. The text lists eight K_cal values for seven augmented groups, and the error bars are described as coming from three trials in one figure caption but two trials in the tables.

Connections within the dossier

This result sits awkwardly beside Not Always Calcite: How Three Sporosarcina Strains Produced Different Calcium Carbonate Phases (Clarà Saracho et al., 2020). In that study, the same S. pasteurii ATCC 11859 produced amorphous calcium carbonate rather than calcite, under different solution strengths and conditions. In this one it gave calcite throughout. Read side by side, the two papers hint that the polymorph may depend on the treatment recipe and not only on the organism, though neither tested that directly. The cost and logistics of augmentation, which the authors weigh against its speed, echo the constraints listed under Bio-Cementation. The paper's ureolysis chemistry is the route described in Ureolysis-Driven Carbonate Precipitation. The work was funded by Australian Research Council Linkage Project LP180100132, and the authors declare no competing interests.