Carbon Fixation Rates Research Articles

Atmospheric Trace Gas Oxidizers Contribute to Soil Carbon Fixation Driven by Key Soil Conditions in Terrestrial Ecosystems.

Microbial oxidizers of trace gases such as hydrogen (H2) and carbon monoxide (CO) are widely distributed in soil microbial communities and play a vital role in modulating biogeochemical cycles. However, the contribution of trace gas oxidizers to soil carbon fixation and the driving environmental factors remain unclear, especially on large scales. Here, we utilized biogeochemical and genome-resolved metagenomic profiling, assisted by machine learning analysis, to estimate the contributions of trace gas oxidizers to soil carbon fixation and to predict the key environmental factors driving this process in soils from five distinct ecosystems. The results showed that phylogenetically and physiologically diverse H2 and CO oxidizers and chemosynthetic carbon-fixing microbes are present in the soil in different terrestrial ecosystems. The large-scale variations in soil carbon fixation were highly positively correlated with both the abundance and the activity of H2 and CO oxidizers (p < 0.05-0.001). Furthermore, soil pH and moisture-induced shifts in the abundance of H2 and CO oxidizers partially explained the variation in soil carbon fixation (55%). The contributions of trace gas oxidizers to soil carbon fixation in the different terrestrial ecosystems were estimated to range from 1.1% to 35.0%. The estimated rate of trace gas carbon fixation varied from 0.04 to 1.56 mg kg-1 d-1. These findings reveal that atmospheric trace gas oxidizers may contribute to soil carbon fixation driven by key soil environmental factors, highlighting the non-negligible contribution of these microbes to terrestrial carbon cycling.

Online monitoring of methane transfer rates unveils nitrogen fixation dynamics in Methylococcus capsulatus.

This study explores methane utilization by the methanotrophic microorganism Methylococcus capsulatus (Bath) for biomass production, presenting a promising approach to mitigate methane emissions and foster the development sustainable biomaterials. Traditional screening methods for gas cultivations involve either serum flasks without online monitoring or costly, low-throughput fermenters. To address these limitations, the Respiration Activity MOnitoring System was augmented with methane sensors for real-time methane transfer rate (MTR) monitoring in shake flasks. Utilizing online monitoring of the MTR in shake flasks results in enhanced throughput and cost-effectiveness for cultivating M. capsulatus. Simultaneous monitoring of transfer rates for oxygen, methane, and carbon dioxide was conducted in up to eight shake flasks, ensuring the success of the cultivation process. Alterations in methane-to-oxygen transfer rate ratios and carbon fixation rates reveal the impact of transfer limitations on microbial growth. Detection of gas transfer limitations, exploration of process parameter influences, and investigations of medium components were enabled by the introduced method. Optimal nitrogen concentrations could be determined to ensure optimal growth. This streamlined approach accelerates the screening process, offering efficient investigations into metabolic effects, limitations, and parameter influences in gas fermentations without the need for elaborate offline sampling, significantly reducing costs and enhanced reproducibility.

Isolation and Whole-genome analysis of Desmodesmus sp. SZ-1: Novel acid-tolerant carbon-fixing microalga

Utilizing microalgae to capture flue gas pollutants is an effective strategy for mitigating greenhouse gas emissions. However, existing carbon-fixing microalgae exhibit poor tolerance towards acidic flue gas. In this study, the Desmodesmus sp. SZ-1, which can thrive in acidic environments and efficiently sequester CO2, was isolated. Desmodesmus sp. SZ-1 exhibited strong acid tolerance ability, with an average carbon fixation rate of 497.6 mg/L/d under 10 % CO2 and pH 3.5. Physiological analysis revealed that SZ-1 responded to high CO2 by increasing chlorophyll levels while coping with acidic stress by activating antioxidant enzymes. Genome analysis revealed a large number of carbon fixation and acid adaptation genes, involved in membrane lipid biosynthesis, H+ pumps, molecular chaperones, peroxidase system, amino acid synthesis, and carbonic anhydrase. This study provides a novel algal resource for mitigating acid gas emissions and a comprehensive genetic database for genetically modifying microalgae.

Hydrothermal vents supporting persistent plumes and microbial chemoautotrophy at Gakkel Ridge (Arctic Ocean).

Hydrothermal vents emit hot fluids enriched in energy sources for microbial life. Here, we compare the ecological and biogeochemical effects of hydrothermal venting of two recently discovered volcanic seamounts, Polaris and Aurora of the Gakkel Ridge, in the ice-covered Central Arctic Ocean. At both sites, persistent hydrothermal plumes increased up to 800 m into the deep Arctic Ocean. In the two non-buoyant plumes, rates of microbial carbon fixation were strongly elevated compared to background values of 0.5-1 μmol m-3 day-1 in the Arctic deep water, which suggests increased chemoautotrophy on vent-derived energy sources. In the Polaris plume, free sulfide and up to 360 nM hydrogen enabled microorganisms to fix up to 46 μmol inorganic carbon (IC) m-3 day-1. This energy pulse resulted in a strong increase in the relative abundance of SUP05 by 25% and Candidatus Sulfurimonas pluma by 7% of all bacteria. At Aurora, microorganisms fixed up to 35 μmol IC m-3 day-1. Here, metal sulfides limited the bioavailability of reduced sulfur species, and the putative hydrogen oxidizer Ca. S. pluma constituted 35% and SUP05 10% of all bacteria. In accordance with this data, transcriptomic analysis showed a high enrichment of hydrogenase-coding transcripts in Aurora and an enrichment of transcripts coding for sulfur oxidation in Polaris. There was neither evidence for methane consumption nor a substantial increase in the abundance of putative methanotrophs or their transcripts in either plume. Together, our results demonstrate the dominance of hydrogen and sulfide as energy sources in Arctic hydrothermal vent plumes.

A novel approach for improving carbon fixation of Chlorella sp. by elements in converter steel slag using machine learning

This study investigates the biomass and carbon fixation rate of Chlorella sp. by the main valuable elements in converter steel slag in F/2 seawater medium. The results indicate that Ca, Mg, P, Si, Fe, and Mn can increase the biomass and carbon fixation rate of Chlorella sp., while Cu, Zn, Cr, and Al can decrease the biomass and carbon fixation rate of Chlorella sp.. Three machine learning methods known as Back Propagation neural network (BPNN), decision tree (DT), and random forest (RF) were applied to construct the prediction model for the carbon fixation rate of Chlorella sp. based on real-life experimental data obtained from single factor experiments. The overall results exhibited that the BPNN model is better than the DT model and RF model to predict the carbon fixation rate of Chlorella sp.. Finally, the maximum carbon fixation rate for Chlorella sp. predicted by BPNN model is 50.86 mg/(L·d), which was 2.46 times that of the control group, under the optimum conditions of Ca 5.77 g/L, Mg 4.74 g/L, P 1.27 g/L, Si 6.31×10−1 g/L, Fe 6.50×10−4 g/L, Mn 5.00×10−5 g/L, Cu 2.51×10−6 g/L, Zn 4.98×10−6 g/L, Cr 0 g/L, and Al 0 g/L in F/2 seawater medium.

Shedding light on blue-green photosynthesis: A wavelength-dependent mathematical model of photosynthesis in Synechocystis sp. PCC 6803.

Cyanobacteria hold great potential to revolutionize conventional industries and farming practices with their light-driven chemical production. To fully exploit their photosynthetic capacity and enhance product yield, it is crucial to investigate their intricate interplay with the environment including the light intensity and spectrum. Mathematical models provide valuable insights for optimizing strategies in this pursuit. In this study, we present an ordinary differential equation-based model for the cyanobacterium Synechocystis sp. PCC 6803 to assess its performance under various light sources, including monochromatic light. Our model can reproduce a variety of physiologically measured quantities, e.g. experimentally reported partitioning of electrons through four main pathways, O2 evolution, and the rate of carbon fixation for ambient and saturated CO2. By capturing the interactions between different components of a photosynthetic system, our model helps in understanding the underlying mechanisms driving system behavior. Our model qualitatively reproduces fluorescence emitted under various light regimes, replicating Pulse-amplitude modulation (PAM) fluorometry experiments with saturating pulses. Using our model, we test four hypothesized mechanisms of cyanobacterial state transitions for ensemble of parameter sets and found no physiological benefit of a model assuming phycobilisome detachment. Moreover, we evaluate metabolic control for biotechnological production under diverse light colors and irradiances. We suggest gene targets for overexpression under different illuminations to increase the yield. By offering a comprehensive computational model of cyanobacterial photosynthesis, our work enhances the basic understanding of light-dependent cyanobacterial behavior and sets the first wavelength-dependent framework to systematically test their producing capacity for biocatalysis.

Studying the spatiotemporal variations of the ecological network and carbon utilization efficiency in Southeast Tibet based on complex network theory

Currently, with the gradual advancement of urbanization and industrialization, ecological patches are severely fragmented and corridors are cut off, leading to the construction of regional ecological networks that can connect fragmented patches and corridors with regional characteristics. However, there is limited research on the relationship between ecological networks and Carbon Utilization Efficiency (CUE) based on complex network theory. This paper takes the ecologically fragile Southeast Tibet as the research area and investigates the variation patterns of CUE through the establishment of ecological spatial networks. The Graphab Model is employed to extract the ecological space network of Southeast Tibet. The Graphab Model integrates the complex network with the ecological space network to compute the topological index of ecological nodes and the overall ecological network. Later, MOD17A2HGF GPP and MOD17A2HGF PSNnet data from 2013 to 2021 are used as the primary data for calculating the CUE of ecological nodes. The connection between topological index and ecological nodes is analysed. The results indicate that, with interannual variations, ecological nodes dropped by 15, and the number of ecological galleries decreased by 7. However, the average values of various topological indices showed an increasing trend, suggesting that the ecological remediation items in Southeast Tibet have made some achievements. Nevertheless, the analysis of CUE in Southeast Tibet reveals a downward trend in the carbon fixation rate of vegetation in recent years. Through correlation analysis, it was found that the correlation between various topological indices and CUE was the lowest and insignificant in 2013, while from 2015 to 2021, all topological indices were significantly positively correlated with CUE. Through Principal Component Analysis (PCA), It is recommended to reduce and shorten redundant ecological corridors by aggregating small ecological patches and increasing stepping stone patches, and increase ecological galleries to enhance the carbon fixation ability of plants. This research lays a scientific foundation for addressing ecological issues, landscape patterns, and developing ecological networks in Southeast Tibet.

Calcification increases carbon supply, photosynthesis, and growth in a globally distributed coccolithophore

AbstractCoccolithophores fix organic carbon and produce calcite plates (coccoliths) that ballast organic matter and facilitate carbon export. Photosynthesis consumes carbon dioxide, while calcification produces it, raising questions about whether coccolithophores are a net sink or source of carbon. We characterized the physiology of calcified and noncalcified (“naked”) phenotypes of Emiliania huxleyi (CCMP374) and investigated the relationship between calcification and photosynthesis across a gradient of light (25–2000 μmol photons m−2 s−1) spanning the euphotic zone. Growth and photophysiological parameters increased with light until reaching a mid‐light (150 μmol photons m−2 s−1) maximum for both phenotypes. Calcified cells were characterized by enhanced photophysiology and less photoinhibition. Further, enhanced bicarbonate transport in calcified cells led to higher rates of particulate organic carbon fixation and growth compared to naked cells at mid‐light to high light (150–2000 μmol photons m−2 s−1). Coccolith production was similarly high at mid and high light, but the rate of coccolith shedding was &gt;3‐fold lower at high‐light (1.2 vs. 0.35 coccoliths cell−1 h−1). The cellular mechanims(s) of this differential shedding remain unknown and underly light‐related controls on coccosphere maintenance. Our data suggest coccoliths shade cells at high light and that enhanced bicarbonate transport associated with calcification increases internal carbon supplies available for organic carbon fixation.

Using eddy covariance observations to determine the carbon sequestration characteristics of subalpine forests in the Qinghai–Tibet Plateau

Abstract. Subalpine forests are a crucial component of the carbon cycling system in the Qinghai–Tibet Plateau (QTP). However, there are currently significant data gaps in the QTP, and it is essential to enhance continuous monitoring of forest carbon absorption processes in the future. This study investigates 2 years' carbon exchange dynamics of a subalpine forest on the QTP using an eddy covariance method. We first characterized the seasonal carbon dynamics of the subalpine forest, revealing the higher carbon dioxide (CO2) exchange rates in summer and autumn and lower rates in winter and spring, and found that autumn is the peak period for carbon sequestration in this subalpine forest, with the maximum measured value of CO2 absorption reaching 10.70 µmol m−2 s−1. Subsequently, we examined the environmental factors influencing the carbon sequestration function. Principal component analysis (PCA) shows that photosynthetically active radiation (PAR) was the major environmental factor driving the net ecosystem exchange (NEE) of CO2, significantly influencing forest carbon absorption, and the increase in relative humidity decreases the rate of carbon fixation. In addition, we explored NEE and its influencing factors at the regional scale and found that air temperature promotes carbon dioxide absorption (negative NEE values), while the average annual precipitation shows a minor effect on NEE. At the annual scale, the subalpine forest functions as a strong carbon sink, with an average NEE of −332 to −351 g C m−2 (from November 2020 to October 2022). Despite the challenges of climate change, forests remain robust carbon sinks with the highest carbon sequestration capacity in the QTP, with an average annual CO2 absorption rate of 368 g C m−2. This study provides valuable insights into the carbon cycling mechanism in subalpine ecosystems and the global carbon balance.

Inorganic carbon migration and transformation in groundwater evaporation discharge area

During groundwater evaporation discharge, a series of carbon-related water-rock interactions potentially impact the terrestrial carbon cycle significantly. However, the migration and transformation of carbon in groundwater evaporation discharge area remain inadequately understood. Using the Tumochuan Plain in Inner Mongolia as a case study, this paper constructs a carbon balance equation for groundwater evaporation discharge area by employing mass balance principles and hydrogeochemical simulation methods, thereby analyzing the mechanisms of carbon diversion during groundwater evaporation. The result showed that evaporation discharge area of Tumochuan Plain was a ‘carbon sink’. Carbon emission rate to atmosphere in study area was 7.35 g/(m2·a), while carbon fixation rate by calcite precipitation and dissolved inorganic carbon (DIC) into groundwater was 37.15 g/(m2·a). The precipitation of calcite and the dissolution of dolomite were the main water-rock interactions controlling the migration and transformation of DIC. The carbon absorbed by dolomite dissolution reached 21,698.02 t/a (30.56 g/(m2·a)), offsetting a significant portion of the CO2 emitted during calcite precipitation. In addition, the calcium released by the dissolution of dolomite and anorthite effectively promoted the precipitation of calcite, which was the primary factor for groundwater to become a carbon sink in this area.

Steel slag powder-CO2 contact state in static or rotary process of carbon fixation

The carbon fixation of steel slag can effectively improve its soundness and activity, increase its utilization rate, and reduce the emission of CO2, which has a significant development prospect. Currently, the effects of different carbon fixation processes for steel slag need further exploration. In this study, the ‘static-pressure’ and the’rotary’ process of the carbon fixation by steel slag powder were simulated, using the gas–solid contact between slag and CO2 as the indicator. The influence of different parameters was investigated, and experimental validation was conducted under optimized conditions. The results indicate that in the ‘static-pressure’ process, reducing the stack thickness and applying pressure to accelerate CO2 transfer are beneficial for gas–solid contact. In the ‘rotary’ process, reducing the filling coefficient to below 20.0 % and increasing the rotation speed to 1.0 rad/s are conducive to gas–solid interaction. Under the optimized conditions, the carbon fixation rate for the ‘static-pressure’ process after 2 h is approximately 6.4 %, while the carbon fixation rate for the ‘rotary’ process is approximately 9 %, indicating better carbon fixation performance in the latter. The addition of microorganisms can further accelerate carbon fixation and enhance its effectiveness. Under the ‘rotary’ process, the carbon fixation rate of microorganism-treated samples reaches 10.21 % after 1 h, surpassing the carbon fixation rate of 7.8 % in the’static-pressure’ process. A satisfied carbon fixation effect is achieved when the contact duration between steel slag particles and CO2 reaches 12.6 min.

Photosynthetic processes in Antarctic sea ice during the spring melt

AbstractHigh‐latitude oceans experience strong seasonality where low light limits photosynthetic activity most of the year. This limitation is pronounced for algae within and underlying sea ice, and these algae are uniquely acclimated to low light levels. During spring melt, however, light intensity and daylength increase drastically, triggering blooms of ice algae that play important roles in carbon cycling and ecosystem productivity. How the algae acclimate to this dynamic and heterogeneous environment is poorly understood. Here, we measured 14C‐carbon fixation rates, photophysiology, and ribulose 1,5‐bisphosphate carboxylase oxygenase (Rubisco) content of sea‐ice algae in coastal waters near the western Antarctic Peninsula during spring, ranging from a low‐light‐acclimated, bottom community to a light‐saturated bloom. Carbon fixation rates by sea‐ice algae were similar to other Antarctic sea‐ice measurements (2–49 mg C m−2 d−1), and there was little phytoplankton biomass in the underlying water at the time of sampling. Net‐to‐gross ratios of carbon fixation were generally high and showed no relationship with ice type. We found algal photophysiology and Rubisco concentrations varied in relation to the different types of ice, altering the balance between the photochemical and biochemical processes that constrain carbon fixation rates. For algae inhabiting the bottom layers of sea ice, rates of carbon fixation were largely constrained by light availability whereas in surface seawater, interior and rotten/brash ice, carbon fixation rates could be calculated with reasonable accuracy from measurements of Rubisco concentrations. This work provides additional insight and means to evaluate carbon fixation rates as sea ice continues to change in future.

Hydrogen isotope fractionation is controlled by CO2 in coccolithophore lipids

Hydrogen isotope ratios (δ2H) represent an important natural tracer of metabolic processes, but quantitative models of processes controlling H-fractionation in aquatic photosynthetic organisms are lacking. Here, we elucidate the underlying physiological controls of 2H/1H fractionation in algal lipids by systematically manipulating temperature, light, and CO2(aq) in continuous cultures of the haptophyte Gephyrocapsa oceanica. We analyze the hydrogen isotope fractionation in alkenones (αalkenone), a class of acyl lipids specific to this species and other haptophyte algae. We find a strong decrease in the αalkenone with increasing CO2(aq) and confirm αalkenone correlates with temperature and light. Based on the known biosynthesis pathways, we develop a cellular model of the δ2H of algal acyl lipids to evaluate processes contributing to these controls on fractionation. Simulations show that longer residence times of NADPH in the chloroplast favor a greater exchange of NADPH with 2H-richer intracellular water, increasing αalkenone. Higher chloroplast CO2(aq) and temperature shorten NADPH residence time by enhancing the carbon fixation and lipid synthesis rates. The inverse correlation of αalkenone to CO2(aq) in our cultures suggests that carbon concentrating mechanisms (CCM) do not achieve a constant saturation of CO2 at the Rubisco site, but rather that chloroplast CO2 varies with external CO2(aq). The pervasive inverse correlation of αalkenone with CO2(aq) in the modern and preindustrial ocean also suggests that natural populations may not attain a constant saturation of Rubisco with the CCM. Rather than reconstructing growth water, αalkenone may be a powerful tool to elucidate the carbon limitation of photosynthesis.

Co-expression analysis reveals distinct alliances around two carbon fixation pathways in hydrothermal vent symbionts

Most autotrophic organisms possess a single carbon fixation pathway. The chemoautotrophic symbionts of the hydrothermal vent tubeworm Riftia pachyptila, however, possess two functional pathways: the Calvin–Benson–Bassham (CBB) and the reductive tricarboxylic acid (rTCA) cycles. How these two pathways are coordinated is unknown. Here we measured net carbon fixation rates, transcriptional/metabolic responses and transcriptional co-expression patterns of Riftia pachyptila endosymbionts by incubating tubeworms collected from the East Pacific Rise at environmental pressures, temperature and geochemistry. Results showed that rTCA and CBB transcriptional patterns varied in response to different geochemical regimes and that each pathway is allied to specific metabolic processes; the rTCA is allied to hydrogenases and dissimilatory nitrate reduction, whereas the CBB is allied to sulfide oxidation and assimilatory nitrate reduction, suggesting distinctive yet complementary roles in metabolic function. Furthermore, our network analysis implicates the rTCA and a group 1e hydrogenase as key players in the physiological response to limitation of sulfide and oxygen. Net carbon fixation rates were also exemplary, and accordingly, we propose that co-activity of CBB and rTCA may be an adaptation for maintaining high carbon fixation rates, conferring a fitness advantage in dynamic vent environments.

Interference of microplastics on autotrophic microbiome in paddy soils: Shifts in carbon fixation rate, structure, abundance, co-occurrence, and assembly process

Autotrophic microorganisms play a crucial role in soil CO2 assimilation. Although microplastic pollution is recognized as a significant global concern, its precise impact on carbon sequestration by autotrophic microorganisms in agroecosystem soil remains poorly understood. This study conducted microcosm experiments to explore how conventional polystyrene (PS) and biodegradable poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) microplastics affect carbon fixation rates (CFRs) and the community characteristics of soil autotrophic microorganisms in paddy agroecosystems. The results showed that compared with the control groups, 0.5 % and 1 % microplastic treatments significantly reduced soil CFRs by 11.8 − 24.5 % and 18.7 − 32.3 %, respectively. PS microplastics exerted a stronger inhibition effect on CFRs than PHBV microplastics in bulk soil. However, no significant difference was observed in the inhibition of CFRs by both types of microplastics in rhizosphere soils. Additionally, PS and PHBV microplastics altered the structure of autotrophic microbial communities, resulting in more stochastically dominated assembly and looser, more fragile coexistence networks compared to control groups. Moreover, microplastics drove the changes in autotrophic microbial carbon fixation primarily through their direct interference and the indirect effect by increasing soil organic carbon levels. Our findings enhance the understanding and predictive capabilities regarding the impacts of microplastic pollution on carbon sinks in agricultural soils.

The carbon footprint of livestock farms under conventional management and silvopastoral systems in Jalisco, Chiapas, and Campeche (Mexico)

Livestock production is one of the most important agricultural sources of greenhouses gasses (GHG). Transforming the livestock sector is a critical task to mitigate the effects of climate change, and the implementation of silvopastoral systems (SPS) may be a way to do so. Carbon footprint (CF) estimation was carried out for 131 conventional livestock farms (CONF) and SPS farms (SPSF) selected in the states of Jalisco, Chiapas and Campeche (Mexico). GHG emissions were estimated based on interviews with the farmers and farm monitoring to collect information about management, milk production and the live weight gain (LWG) of calves (for meat production) using national emission factors. Emissions from animals, fossil fuels, electricity and nitrogen fertilizers were estimated using Tier I and II criteria. Land use distribution was recorded for each farm, and their carbon fixation in total biomass and soils was estimated using studies from tropical Latin America. CF was estimated for area and for production of milk and the LWG of calves. SPSF had lower GHG emissions, higher carbon fixation rates and a better CF than CONF (5.7 vs. 8.0 t CO2e/ha/year; 6.9 vs. 5.5 t C/ha/year and − 5.0 vs. −2.9 t CO2e/ha/year, respectively). The CF of milk production and calf LWG were − 68.6 to −4.6 kg CO2e/kg and − 3.2 to −0.1 t CO2e/kg, respectively. Our results highlight the importance of implementing SPS to mitigate climate change in livestock production, given its contribution to GHG emissions and the importance of capturing atmospheric carbon in biomass and soils. National and regional policies should focus on promoting implementing SPS as an active and effective strategy for improving CF in livestock farms.

Greenness of moss biocrusts derived from RGB image is a reliable indicator to estimate biocrust carbon-fixation rates in drylands

Biocrusts are common living covers found across drylands worldwide, and their photosynthesis substantially contributes to carbon input in these ecosystems. However, direct monitoring of biocrusts’ photosynthetic carbon fixation is challenging due to their scattered distribution, dark pigments, and lower CO2 exchange rate. Current studies have limited monitoring frequency and the results are difficult to extend to a broader spatial scale. Greenness typically functions as an indicator of both vegetation cover and plant photosynthesis. Monitoring biocrust greenness is expected to be able to provide insights into their environmental responses and estimate carbon fixation rates, particularly for moss biocrusts. Here, we monitored greenness, soil moisture and temperature, and gross primary productivity (GPP) of moss biocrusts, as well as climatic factors over two years in the northern China’s Loess Plateau. The results indicated that moss biocrust greenness was highly sensitive to changing environmental conditions and exhibited significant temporal variability. Moss biocrust greenness exhibited a notably higher value in wet season (0.384) than in dry season (0.259). According to these greenness values, we proposed moss biocrust activity and classified it into high (>0.403), medium (0.334–0.403), low (0.271–0.333), and dormancy (<0.270). Soil moisture, relative humidity, and wind speed were the three main determinates of moss biocrust greenness. Higher soil moisture and relative humidity were linked to increased greenness, whereas elevated wind speed led to a reduction. In dry seasons, soil moisture contributed 23.4 % to the variation in moss biocrust greenness, while in wet seasons relative humidity contributed 16.1 %. A significant and positive correlation between moss biocrust greenness and GPP was found, and annual GPP prediction based on greenness using machine learning performed well. Our study highlights the significance of moss biocrust greenness and their potential as indicators for moss biocrust photosynthetic carbon fixation rates and possibly other ecological functions in drylands.

Mechanisms of hormetic effects of ofloxacin on Chlorella pyrenoidosa under environmental-relevant concentration and long-term exposure

Antibiotics are frequently detected in surface water and pose potential threats to organisms in aquatic ecosystem such as microalgae. The occurrence of biphasic dose responses raised the possibility of stimulation of microalgal biomass by antibiotics at environmental-relevant concentration and caused potential ecological risk such as algal bloom. However, the underlying mechanisms of low concentration-induced hormetic effects are not well understood. In this study, we evaluated the hormesis of ofloxacin on Chlorella pyrenoidosa under environmental-relevant concentration and long-term exposure. Results showed the hormetic effects of ofloxacin on cell density and carbon fixation rate (RC). The predicted maximum promotion was 17.45 % by 16.84 μg/L and 20.08 % by 15.78 μg/L at 21 d, respectively. The predicted maximum concentration of non-effect on cell density and RC at 21 d was 3.24 mg/L and 1.44 mg/L, respectively. Ofloxacin induced the mobilization of pigments and antioxidant enzymes to deal with oxidative stress. PCA analysis revealed Chl-a/Chl-b could act as a more sensitive biomarker under acute exposure while chlorophyll fluorescence parameters were in favor of monitoring long-term implication. The hormesis in increased secretion of extracellular organic matters was regarded as a defensive mechanism and accelerated indirect photodegradation of ofloxacin. Bioremoval was dominant and related to biomass accumulation in the total dissipation while abiotic removal appeared slight contributions. This study provided new insights into the understanding of hormesis of microalgae induced by antibiotics.

Effect of excess CO2 on semi-continuous microalgae systems: Carbon biofixation

Microalgae-based CO2 fixation is a promising solution for achieving carbon neutrality. However, the low rate of carbon fixation is a significant issue due to gas transfer. A closed cultivation system was employed to overcome this challenge, incorporating headspace micro-positive pressure and elevated total CO2 (TCO2) levels to minimize CO2 loss and improve CO2 mass transfer. Microalgae could thrive and maintain a higher level of system pH within the TCO2 range of 0.12–0.49 mol CO2/L absorbent, and high TCO2 levels over 0.36 mol CO2/L absorbent were the optimal range. The semi-continuous microalgae system removed up to 8.72 ± 0.85 g CO2/L/day. Simultaneously, it exhibited phosphate and nitrate removal rates of 50.95 ± 10.71 and 182.14 ± 30.71 mg/L/day, respectively. The study found that 80% of the removed carbon was fixed as organic biomass. An increased level of TCO2 resulted in the accumulation of more lipids within the cells. Moreover, microalgae released carbon fixation products into the extracellular environment, and the dissolved organic carbon fixation rate reached as high as 14.58 ± 1.89 mmol/L/day. These findings suggest that microalgae use multiple pathways for carbon fixation, including biomass organic carbon accumulation, extracellular organic carbon secretion, and inorganic carbon precipitation. Improving carbon fixation rates may benefit the industrial application of microalgae technology, which aims to mitigate carbon emissions.

Numerical study of structural optimization for improving gas-liquid flow and microalgae carbon fixation rate in air-lift photobioreactors

In this paper, the gas-liquid flow pattern and CO2 dissolution transport process of microalgae solution in an air-lift photobioreactor (PBR) are investigated by numerical simulations. In order to promote the gas-liquid mass transfer efficiency to enhance the carbon fixation, we propose different optimization schemes for the structure. Firstly, a two-phase flow model and a mass transfer model are employed to investigate the fluid flow characteristics. The flow state is evaluated by velocity in light direction, turbulent kinetic energy (TKE), gas hold-up and CO2 mass transfer coefficient. The results show that the gas flow rate of 0.2 V/V/min is the optimum value for the inlet. Secondly, CO2 mass transfer and carbon fixation rates are analyzed for different inlet CO2 volume fractions. With the increase of CO2 volume fraction, the carbon fixation rate first increases and then decreases, the highest fixation rate reaching 2.21 × 10−4 mol m−3 s−1 at 6 %. Finally, based on numerical simulations, three optimized structures with different spacing of baffles are proposed and evaluated the performance in comparison with the original. The results show that the gas distribution and CO2 mass transfer are significantly improved, with the one-sided baffle structure is the best. In summary, this study provides a new and useful reference to evaluate the gas-liquid mixing and improve the microalgae carbon fixation rate.