Organic carbon preservation in wetlands: Iron oxide protection vs. thermodynamic limitation.

The sequestration of organic carbon (OC) in wetland sediments is influenced by the presence of oxygen or lack thereof. The mechanisms of OC sequestration under redox fluctuations, particularly by the co-mediation of reactive iron (Fe) protection and thermodynamic limitation by the energetics of the OC itself, remain unclear. Over the past 26 years, a combination of field surveys and remote sensing images had revealed a strong decline in both natural and constructed wetland areas in Tianjin. This decline could be attributed to anthropogenic landfill practices and agricultural reclamation efforts, which may have significant impacts on the oxidation-reduction conditions for sedimentary OC. The Fe-bound OC (CBD extraction) decreased by 2 to 10-fold (from 8.3 to 10% to 0.7-4.5%) with increasing sediment depth at three sites with varying water depths (WD). The high-resolution spectro-microscopy analysis demonstrated that Fe (oxyhydr)oxides were colocalized with sedimentary OC. Corresponding to lower redox potential, the nominal oxidation state of C (NOSC), which corresponds to the energy content in OC, became more negative (energy content increased) with increasing sediment depth. Taken together, the preservation of sedimentary OC is contingent on the prevailing redox conditions: In environments where oxygen availability is high, reactive Fe provides protection for OC, while in anoxic environments, thermodynamic constraints (i.e., energetic constraints) limit the oxidation of OC.

Carbon­sulfur coupling in a seasonally hypoxic, high-sulfate reservoir in SW China: Evidence from stable CS isotopes and sulfate-reducing bacteria.

Anthropogenic input of sulfate (SO42-) in reservoirs may enhance bacterial sulfate reduction (BSR) under seasonally hypoxic conditions in the water column. However, factors that control BSR and its coupling to organic carbon (OC) mineralization in seasonally hypoxic reservoirs remain unclear. The present study elucidates the coupling processes by analyzing the concentrations and isotopic composition of dissolved inorganic carbon (DIC) and sulfur (SO42-, sulfide) species, and the microbial community in water of the Aha reservoir, SW China, which has high SO42- concentration due to the inputs from acid mine drainage about twenty years ago. The water column at two sites in July and October revealed significant thermal stratification. In the hypoxic bottom water, the δ13C-DIC decreased while the δ34S-SO42- increased, implying organic carbon mineralization due to BSR. The magnitude of S isotope fractionation (Δ34S, obtained from δ34Ssulfate-δ34Ssulfide) during the process of BSR fell in the range of 3.4‰ to 27.0‰ in July and 21.6‰ to 31.8‰ in October, suggesting a change in the community of sulfate-reducing bacteria (SRB). The relatively low water column stability in October compared to that in July weakened the difference of water chemistry and ultimately affected the SRB diversity. The production of DIC (ΔDIC) scaled a strong positive relationship with the Δ34S in July (p < 0.01), indicating that high OC availability favored the survival of incomplete oxidizers of SRB. However, in October, Δ13C-DIC was correlated with the Δ34S in the bottom hypoxic water (p < 0.01), implying that newly degraded OC depleted in 13C could favor the dominance of complete oxidizers of SRB which caused greater S isotope fractionation. Moreover, the sulfide supplied by BSR might stimulate the reductive dissolution of Fe and Mn oxides (Fe(O)OH and MnO2). The present study helps to understand the coupling of C and S in seasonally hypoxic reservoirs characterized by high SO42- concentration.

Direct Air Capture of CO2 through Carbonate Alkalinity Generated by Phytoplankton Nitrate Assimilation.

Despite the consensus that keeping global temperature rise within 1.5 °C above pre-industrial level by 2100 reduces the chance for climate change to reach the point of no return, the newest Intergovernmental Panel on Climate Change (IPCC) report warns that the existing commitment of greenhouse gas emission reduction is only enough to contain the warming to 3-4 °C by 2100. The harsh reality not only calls for speedier deployment of existing CO2 reduction technologies but demands development of more cost-efficient carbon removal strategies. Here we report an ocean alkalinity-based CO2 sequestration scheme, taking advantage of proton consumption during nitrate assimilation by marine photosynthetic microbes, and the ensuing enhancement of seawater CO2 absorption. Benchtop experiments using a native marine phytoplankton community confirmed pH elevation from ~8.2 to ~10.2 in seawater, within 3-5 days of microbial culture in nitrate-containing media. The alkaline condition was able to sustain at continued nutrient supply but reverted to normalcy (pH ~8.2-8.4) once the biomass was removed. Measurements of δ13C in the dissolved inorganic carbon revealed a significant atmospheric CO2 contribution to the carbonate alkalinity in the experimental seawater, confirming the occurrence of direct carbon dioxide capture from the air. Thermodynamic calculation shows a theoretical carbon removal rate of ~0.13 mol CO2/L seawater, if the seawater pH is allowed to decrease from 10.2 to 8.2. A cost analysis (using a standard bioreactor wastewater treatment plant as a template for CO2 trapping, and a modified moving-bed biofilm reactor for nitrate recycling) indicated that a 1 Mt CO2/year operation is able to perform at a cost of ~$40/tCO2, 2.5-5.5 times cheaper than that offered by any of the currently available direct air capture technologies, and more in line with the price of $25-30/tCO2 suggested for rapid deployment of large-scale CCS systems.

Fungal Nanophase Particles Catalyze Iron Transformation for Oxidative Stress Removal and Iron Acquisition.

Microbe-mineral interactions have shaped the surface of the Earth and impacted the evolution of plants and animals. Although more than two-thirds of known mineral species have biological imprints, how the biotransformation of minerals may have benefited microbial development, beyond nutritional and energetic use, remains enigmatic. In this research, we have shown that biogenic ferrihydrite nanoparticles are extensively formed at the interface between an actively growing fungus and an iron-containing mineral, hematite. These biogenic nanoparticles formed through the fungus-hematite interactions can behave as mimetic catalysts, similar to nanozymes that imitate peroxidase, which scavenges hydrogen peroxide for the mitigation of potential cytotoxicity. Evidence from various X-ray spectroscopic analyses indicated that non-lattice oxygen in the nanomaterials was chiefly responsible for this catalytic activity, rather than through the conventional mechanisms of iron redox chemistry. Cryo-scanning electron microscopy, high-resolution (∼30 nm) 3D volume rendering, and biomass analyses further confirmed that the organism was active and capable of mediating the catalytic reactions. We therefore hypothesize that this confers an advantage to the organism in terms of protection from oxidative stress and ensuring the acquisition of essential iron. This work raises new questions about the roles of biogenic nanomaterials in the coevolution of the lithosphere and biosphere and provides a step toward understanding the feedback pathways controlling the evolution of biogenic mineral formation.