Periodic Reporting for period 2 - Mobidic (Are microbes the ultimate drivers of subsurface weathering? Rates, actors, mechanisms and nanoscale imprints of the bioweathering of fresh and aged silicates)
Reporting period: 2023-03-01 to 2024-08-31
The ambition of this project is to provide the very first realistic assessment of the contribution of microbes to the chemical alteration of silicate rocks, a process that contributes to the geochemical cycles of most chemical elements, and partly controls atmospheric CO2 uptake and Earth’s climate over geologic timescales. If successful, the knowledge collected in this project will help improve existing models of rock alteration at the Earth’s surface, and ultimately contribute provide a more reliable estimate of the feedback between global warming and rock weathering.
The motivation for such a research project is simple: ~70% of Earth’s microbes live underground, and microbes have long been suspected to impact the alteration rate of silicate rocks. However, the rates and mechanisms of microbially-mediated silicate alteration essentially remain unknown and not accounted for in existing models of rock alteration. This project particularly focuses on basalt settings, as these environments are suggested to be a potential host for early life and represent prime targets for massive injections of CO2 to fight against global warming, whose success strongly relies on silicate reactivity. Having a deep insight into the respective biotic and abiotic contributions to subsurface silicate weathering rates in these settings is therefore both fundamental and urgent.
Providing such estimates requires to overcome a twofold challenge: 1) to quantify silicate dissolution rates directly in the environment (as opposed to in vitro estimates from laboratory experiments) and at the same time and 2) to identify features at the mineral surface that can be unambiguously be attributed to the impact of microbial life, to eventually estimate the respective contribution of abiotic and biotic processes.
This proposal offers a solution: An interdisciplinary and non-conventional approach to assess the contribution of microbes to silicate weathering rates in complex environmental media. It consists in measuring dissolution rates using non-invasive nanotopography measurements of the silicate substrates reacted in soil profiles and/or environmental fluids. These substrates are treated beforehand to get surface properties that mimic various stages of aging. These measurements are combined with studies of the microbial diversity associated with the substrates, innovative nanoscale characterizations of the reacted surfaces, and modeling of the dissolution process based on parameters derived independently from quantum mechanics. If successful, this strategy will not only provide an unprecedented and timely picture of the functioning and rates of microbially-mediated subsurface silicate bioweathering: it will also pave the way to the definition of criteria for biosignatures of microbially-mediated silicate dissolution, of prime interest for the search for life in the Earth’s geological record and beyond.
The motivation for such a research project is simple: ~70% of Earth’s microbes live underground, and microbes have long been suspected to impact the alteration rate of silicate rocks. However, the rates and mechanisms of microbially-mediated silicate alteration essentially remain unknown and not accounted for in existing models of rock alteration. This project particularly focuses on basalt settings, as these environments are suggested to be a potential host for early life and represent prime targets for massive injections of CO2 to fight against global warming, whose success strongly relies on silicate reactivity. Having a deep insight into the respective biotic and abiotic contributions to subsurface silicate weathering rates in these settings is therefore both fundamental and urgent.
Providing such estimates requires to overcome a twofold challenge: 1) to quantify silicate dissolution rates directly in the environment (as opposed to in vitro estimates from laboratory experiments) and at the same time and 2) to identify features at the mineral surface that can be unambiguously be attributed to the impact of microbial life, to eventually estimate the respective contribution of abiotic and biotic processes.
This proposal offers a solution: An interdisciplinary and non-conventional approach to assess the contribution of microbes to silicate weathering rates in complex environmental media. It consists in measuring dissolution rates using non-invasive nanotopography measurements of the silicate substrates reacted in soil profiles and/or environmental fluids. These substrates are treated beforehand to get surface properties that mimic various stages of aging. These measurements are combined with studies of the microbial diversity associated with the substrates, innovative nanoscale characterizations of the reacted surfaces, and modeling of the dissolution process based on parameters derived independently from quantum mechanics. If successful, this strategy will not only provide an unprecedented and timely picture of the functioning and rates of microbially-mediated subsurface silicate bioweathering: it will also pave the way to the definition of criteria for biosignatures of microbially-mediated silicate dissolution, of prime interest for the search for life in the Earth’s geological record and beyond.
Mobidic aims to quantify the magnitude of microbially-mediated silicate reactivity on Earth, with a specific focus on basaltic (volcanic) provinces. To this end, the project is organized following a decreasing complexity of the studied system, starting with the collection of environmental samples (rocks, soils and fluids) and their microbial content (Fig. 1). Then fresh and aged mineral coupons and powders of interest are inserted into the collected samples such as soil columns (Fig. 2) or environmental solutions, which are maintained under physicochemical conditions close to those that prevailed in the field. After different incubation durations, the coupons and powders are retrieved and analyzed for their microbial content and weathering imprints, including those resulting from microbial contribution (Figs. 3 and 4). This approach provides first clues about the microorganisms that enhance (or inhibit) mineral alteration. These microorganisms are then isolated and put in contact with mineral coupons and powders in simplified experimental set-ups to verify the contribution to mineral alteration expected from the experiments conducted with complex environmental matrices. Finally, the alteration features characterized under similar conditions in abiotic synthetic solutions coupled to atomic-scale modeling of the dissolution process allows for tackling the molecular-scale mechanisms of weathering, and defining biosignatures of alteration.
The work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far in the various workpackages (WP) are as follows:
* WP1: Contribution of deep microbial communities to silicate weathering
(1) Collection of groundwater samples from basaltic aquifers with contrasted physicochemical properties;
(2) Characterization of microbial diversity associated with those samples and determination of the environmental parameters (temperature, pH, salinity) that mainly control microbial diversity;
(3) Collection of a subset of groundwater samples representative of contrasted microbial diversities to quantify their biomass in view of laboratory experiments;
(4) Conduction of preliminary fluid-mineral interaction experiments with collected environmental fluid to quantify the abiotic and biotic contribution to mineral alteration.
* WP2: Contribution of basaltic soils microbial communities to silicate weathering
(1) Collection of samples from soils developed on basaltic bedrock with contrasted physicochemical properties;
(2) Characterization of microbial diversity associated with those soil samples;
(3) Conduction of weathering experiments in the lab columns filled with collected soils.
* WP3: Investigation of the dissolution silicates mediated by microbial strains
(1) Development of statistical and modeling tools required to evidence microtopographic imprints of bacterial weathering using a model bacterial strain and a model mineral;
(2) Development of a fluid cell for in situ vertical scanning interferometry (VSI) imaging.
WP4: Physicochemical properties of silicate surfaces altered in synthetic abiotic solutions
(1) Quantification of the abiotic reactivity of fresh and aged minerals in abiotic conditions relevant for WP1+2;
(2) Characterization of the surface properties of some of the reacted minerals.
The work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far in the various workpackages (WP) are as follows:
* WP1: Contribution of deep microbial communities to silicate weathering
(1) Collection of groundwater samples from basaltic aquifers with contrasted physicochemical properties;
(2) Characterization of microbial diversity associated with those samples and determination of the environmental parameters (temperature, pH, salinity) that mainly control microbial diversity;
(3) Collection of a subset of groundwater samples representative of contrasted microbial diversities to quantify their biomass in view of laboratory experiments;
(4) Conduction of preliminary fluid-mineral interaction experiments with collected environmental fluid to quantify the abiotic and biotic contribution to mineral alteration.
* WP2: Contribution of basaltic soils microbial communities to silicate weathering
(1) Collection of samples from soils developed on basaltic bedrock with contrasted physicochemical properties;
(2) Characterization of microbial diversity associated with those soil samples;
(3) Conduction of weathering experiments in the lab columns filled with collected soils.
* WP3: Investigation of the dissolution silicates mediated by microbial strains
(1) Development of statistical and modeling tools required to evidence microtopographic imprints of bacterial weathering using a model bacterial strain and a model mineral;
(2) Development of a fluid cell for in situ vertical scanning interferometry (VSI) imaging.
WP4: Physicochemical properties of silicate surfaces altered in synthetic abiotic solutions
(1) Quantification of the abiotic reactivity of fresh and aged minerals in abiotic conditions relevant for WP1+2;
(2) Characterization of the surface properties of some of the reacted minerals.
Until now, accounting for the contribution of life in quantitative models of rock alteration has been restricted to the indirect impact of living organisms (limited to vascular plants in the vast majority of modeling exercises) on the physical and chemical parameters that control the chemical weathering rates of minerals. Although microorganisms essentially occupy niches on Earth that are home to fluid-rock interactions, their direct contribution to this process is absent from such models, thereby resulting in a potentially distorted assessment of the evolution of rock weathering in response to external forcings, including those associated to climate change.Defining the hot spots and hot moments of microbial activity with respect to rock weathering basaltic environments, which are amongst the most reactive silicate materials on Earth, thus represents a major scientific challenge that the project aims to tackle. Progress going beyond the state of the art and expected results until the end of the project include:
(i) (completed) The rigorous identification of microtopographic signatures of microbially-mediated mineral alteration derived from quantitative statistical parameters and confirmed by an independent mechanistic support. This represents a pre-requisite to the quantification of microbial contribution to chemical weathering, and opens new avenues for the detection of life in the geological record, of prime interest for astrobiology concerns;
(ii) (ongoing) An inventory of microorganisms (bacteria, archea and fungi) thriving in contrasted basaltic environments, from the rocky bedrock to the topsoils;
(iii) (ongoing) A direct measurement of the impact of complex microbial communities in vivo, paving the way to the determination of conditions where microbial activity significantly contributes to rock alteration;
(iv) (upcoming) The identification of specific genera/species with rock-alteration abilities and gene markers of microbially-mediated mineral alteration.
(i) (completed) The rigorous identification of microtopographic signatures of microbially-mediated mineral alteration derived from quantitative statistical parameters and confirmed by an independent mechanistic support. This represents a pre-requisite to the quantification of microbial contribution to chemical weathering, and opens new avenues for the detection of life in the geological record, of prime interest for astrobiology concerns;
(ii) (ongoing) An inventory of microorganisms (bacteria, archea and fungi) thriving in contrasted basaltic environments, from the rocky bedrock to the topsoils;
(iii) (ongoing) A direct measurement of the impact of complex microbial communities in vivo, paving the way to the determination of conditions where microbial activity significantly contributes to rock alteration;
(iv) (upcoming) The identification of specific genera/species with rock-alteration abilities and gene markers of microbially-mediated mineral alteration.