Carbon Fixation Rates Research Articles

A Two-Stage Culture Strategy for Scenedesmus sp. FSP3 for CO2 Fixation and the Simultaneous Production of Lutein under Light and Salt Stress.

In this study, Scenedesmus sp. FSP3 was cultured using a two-stage culture strategy for CO2 fixation and lutein production. During the first stage, propylene carbonate was added to the medium, with 5% CO2 introduced to promote the rapid growth and CO2 fixation of the microalgae. During the second stage of cultivation, a NaCl concentration of 156 mmol L-1 and a light intensity of 160 μmol m-2 s-1 were used to stimulate the accumulation of lutein in the microalgal cells. By using this culture method, high lutein production and CO2 fixation were simultaneously achieved. The biomass productivity and carbon fixation rate of Scenedesmus sp. FSP3 reached 0.58 g L-1 d-1 and 1.09 g L-1 d-1, with a lutein content and yield as high as 6.45 mg g-1 and 2.30 mg L-1 d-1, respectively. The results reveal a commercially feasible way to integrate microalgal lutein production with CO2 fixation processes.

Study on treatment of city tail water by constructed wetland with corn straw biochar substrate

Constructed wetland (CW) has obvious advantages in city tail water treatment. The biochar preparation from corn straw is an effective measure for improving the straw utilization. In order to improve the pollutant removal rate of CW, the city tail water treatment based on straw biochar substrate was studied. The thermogravimetric experiment of straw, characteristics test (scanning electron microscope, pore structure, element, adsorption) of biochar and city tail water treatment experiment by CW was carried out. The results show that the carbon fixation rate of straw biochar prepared at 450 °C (10 °C ⋅ min −1 , 2 h, N 2 ) was 67.72%, and the adsorption capacity of biochar exceeded 40% of the requirements of wood activated carbon adsorption standard. When the proportion of biochar in substrate was 2.95%, the mean removal rates of ammonia nitrogen, total nitrogen and nitrate nitrogen was more than 85%. This study will be conducive to reducing straw carbon emissions and improve CW technical level. • The carbon fixation rate of biochar prepared at 450 °C (10 °C ⋅ min −1 , 2 h,) was 67.72% • The adsorption of straw biochar for NH4+-N was greater than that of TP and NO3–N • Elovich equation was good to simulate the dynamic adsorption of NH4+-N, NO3–N and TP • The nitrogen removal rate was over 85% when the mass ratio of biochar was 2.95% in CW.

Alkalihalophilic alga Picocystis salinarum SLJS6 from Sambhar Salt Lake: Potential for bicarbonate-based biomass production and carbon capture

An extremely alkalihalophilic picoalgal strain, Picocystis salinarum SLJS6 isolated from Sambhar salt lake, Rajasthan, India, was evaluated for its suitability for bicarbonate based carbon capture and biomass production. The isolate was characterized in terms of growth attributes, tolerance to salinity, high dissolved inorganic carbon (DIC) concentration and carbon fixation rates. The isolate grew robustly in a medium containing mainly Na2CO3, 50 g/L, NaHCO3, 50 g/L, NaCl, 50 g/L (salinity ≈150 ‰, DIC ≈ 1 M NaHCO3/Na2CO3) at pH 10, achieving a growth rate of 0.2282 day−1, biomass productivity of 0.498 g/L/day (>35 folds as compared to low DIC and salinity) and CO2 fixation rate of 0.729 g/L/day. Based on a rapid colorimetric method, the average total lipid content of SLJS6 was found to be 226.9 mg/g DW. Thus, P. salinarum SLJS6 is a promising candidate for bicarbonate-based CO2 capture and recycling.

Single and interactive effects of variables associated with climate change on wheat metabolome.

One of the key challenges linked with future food and nutritional security is to evaluate the interactive effect of climate variables on plants’ growth, fitness, and yield parameters. These interactions may lead to unique shifts in the morphological, physiological, gene expression, or metabolite accumulation patterns, leading to an adaptation response that is specific to future climate scenarios. To understand such changes, we exposed spring wheat to 7 regimes (3 single and 4 combined climate treatments) composed of elevated temperature, the enhanced concentration of CO2, and progressive drought stress corresponding to the predicted climate of the year 2100. The physiological and metabolic responses were then compared with the current climate represented by the year 2020. We found that the elevated CO2 (eC) mitigated some of the effects of elevated temperature (eT) on physiological performance and metabolism. The metabolite profiling of leaves revealed 44 key metabolites, including saccharides, amino acids, and phenolics, accumulating contrastingly under individual regimes. These metabolites belong to the central metabolic pathways that are essential for cellular energy, production of biosynthetic pathways precursors, and oxidative balance. The interaction of eC alleviated the negative effect of eT possibly by maintaining the rate of carbon fixation and accumulation of key metabolites and intermediates linked with the Krebs cycle and synthesis of phenolics. Our study for the first time revealed the influence of a specific climate factor on the accumulation of metabolic compounds in wheat. The current work could assist in the understanding and development of climate resilient wheat by utilizing the identified metabolites as breeding targets for food and nutritional security.

Bioconversion of carbon dioxide to succinate by Citrobacter

Carbon dioxide as a feedstock for synthesizing chemicals and fuels attracts numerous efforts via biological or chemical approaches. A newly Citrobacter BD11 was detected with distribution of 71.31 % in reservoir samples by metagenomic sequencing, and can convert CO2 to succinic acid with hydrogen production under anaerobic condition. The pathway of hydrogen production and the succinic acid synthesis was explored by whole-genome sequencing and real-time quantitative polymerase chain reaction. The results showed the positive influences of pH, hydrogen pressure and magnesium ions on succinic acid production. The stage transition of hydrogen production and utilization was detected, and the nonlinear relationship between succinic acid production and exogenous carbon dioxide was proved. Under optimized conditions, the carbon fixation rate of 1.77 g/L*d and the succinate production rate of 7.51 g/L*d were finally achieved at pH = 7.0 with 50 KPa of hydrogen pressure and 4.0 g/L of MgCl2. This study demonstrated that this Citrobacter could be a promising platform for hydrogen production and convert CO2 to diverse acids.

Chem-bio interface design for rapid conversion of CO2 to bioplastics in an integrated system

Chem-bio interface design for rapid conversion of CO2 to bioplastics in an integrated system

Photoacclimation of the polar diatom Chaetoceros neogracilis at low temperature.

Polar microalgae face two major challenges: 1- growing at temperatures (-1.7 to 5°C) that limit enzyme kinetics; and 2- surviving and exploiting a wide range of irradiance. The objective of this study is to understand the adaptation of an Arctic diatom to its environment by studying its ability to acclimate to changes in light and temperature. We acclimated the polar diatom Chaetoceros neogracilis to various light levels at two different temperatures and studied its growth and photosynthetic properties using semi-continuous cultures. Rubisco content was high, to compensate for low catalytic rates, but did not change detectably with growth temperature. Contrary to what is observed in temperate species, in C. neogracilis, carbon fixation rate (20 min 14C incorporation) equaled net growth rate (μ) suggesting very low or very rapid (<20 min) re-oxidation of the newly fixed carbon. The comparison of saturation irradiances for electron transport, oxygen net production and carbon fixation revealed alternative electron pathways that could provide energy and reducing power to the cell without consuming organic carbon which is a very limiting product at low temperatures. High protein contents, low re-oxidation of newly fixed carbon and the use of electron pathways alternative to carbon fixation may be important characteristics allowing efficient growth under those extreme environmental conditions.

Combined Influences of Light and Nitrogen Enrichment on the Physiological Performance of a Golden Tide Alga (Sargassum horneri)

Sargassum golden tides (GT) are common in numerous coastal areas all over the world, and it adversely affects local marine life. Eutrophication is critical for Sargassum GT development. However, its physiological and ecological mechanism remains unclear. To investigate the responses of drifting Sargassum horneri, the species causing GT in the western Pacific, to light and enriched nitrogen, we set three light conditions (Low-light (LL), 10 μmol photons m−2 s−1; Middle-light (ML), 60 μmol photons m−2 s−1; and High-light (HL), 300 μmol photons m−2 s−1) and three nitrogen conditions (Natural seawater, the final concentration of N was 31.8 μmol L−1, including 30.5 μmol L−1 of NO3− and 1.3 μmol L−1 of NH4+; Enrichment of NO3−, final concentration of N was 200 μmol L−1; and Enrichment of NH4+, the final concentration of N was 200 μmol L−1), and grew the thalli under varying conditions for 10 days before determining the growth and utilization of carbon and nitrogen. Based on the accumulated data, the elevated light level led to a higher growth rate of alga. In the LL culture, the higher capacity for carbon utilization, which was reflected by the higher maximum photosynthetic carbon fixation rate (Vmax), resulted in the elevated growth rates of thalli in the nitrogen-enriched media as compared with the natural seawater. Furthermore, a higher growth rate was found in the enrichment of NH4+ despite a low affinity for inorganic carbon indicated by a higher value of the half-saturation constant (K0.5). In the ML treatment, an insignificant difference in growth rate was found in three nitrogen cultures, except for a slight increase in the enrichment of NH4+ than the enrichment of NO3−. In the HL treatment, compared with natural seawater culture, enrichment of NO3− or NH4+ accelerated the growth of alga, with no significant difference between the two nitrogen sources. Such enhancement in growth was related to the more photosynthetic carbon fixation, indicated by the higher value of Vmax and soluble carbohydrates content of alga cultured with NO3− and NH4+ enrichments. Additionally, the uptake and assimilation products of nitrogen, such as pigments and soluble proteins, remained unaffected by nitrogen source enrichment of NO3− or NH4+ at all three light levels. In conclusion, enrichment of NO3− and NH4+ exhibited different influences on the growth of S. horneri at different light levels, which was mainly associated with the capacity and efficiency of photosynthetic carbon utilization. At the HL level, both the enrichment of NO3− and NH4+ dramatically accelerate the growth of alga by stimulating the photosynthetic carbon fixation. Accordingly, we speculated that drifting S. horneri, exposed to HL level on the surface of the sea, were likely to develop rapidly to form GT in eutrophic oceanic areas with upwelled and river plume NO3− or NH4+ nutrients.

Carbon dioxide mitigation and biodiesel production by a marine microalga under mixotrophic mode by using transesterification by-product crude glycerol: A synergy of biofuels and waste valorization

The present work integrates carbon sequestration and waste reclamation with biofuel production into a single dynamic for encouraging eco-friendlier approaches ensuring environmental sustainability. To achieve improved carbon capturing and lipid production, a marine microalga Picochlorum soloecismus was selected. The maximum productivities of biomass and lipid were asserted to be 0.24 ± 0.02 gL−1d−1 and 58.2 ± 0.2 mgL−1d−1 under a CO2 supply of 17.0% at 0.7 vvm aeration in a 5 L photobioreactor under photoautotrophic condition. A low-priced mixotrophic medium was designed using CO2 with the transesterification by-product, i.e., crude glycerol. This produced maximum biomass and lipid productivities of 0.67 ± 0.04 and 0.13 ± 0.01 gL−1d−1 at a carbon fixation rate of 0.77 ± 0.06 gL−1d−1. The effect of mixotrophy was also investigated on a modified culture medium, i.e. Optimized ASN (Artificial Seawater Nutrient) prepared in our recent study generating a maximum lipid productivity of 0.31 ± 0.02 gL−1d−1at a carbon fixation rate of 0.80 ± 0.07 gL−1d−1. The use of crude glycerol in place of commercial glycerol for mixotrophy can reduce the medium cost by 22-fold. The methyl ester profile of biodiesel obtained under mixotrophy exhibited higher proportion of saturated fats and lower fraction of unsaturated fats. The fuel parameters lied within the established biodiesel standards.

Estimates of diurnal and daily net primary productivity using the Geostationary Ocean Color Imager (GOCI) data

For the past three decades, polar-orbiting ocean color satellites have provided local, regional to global scale estimates of oceanic net primary production that have greatly aided studies of ocean carbon cycling, food web dynamics and climate change. Despite considerable progress, accurate estimates of daily ocean productivity from space have not been realized because polar-orbiting satellites are unable to account for variations in phytoplankton physiology and carbon fixation rates from changing incident irradiance levels over the course of the day. Here we have attempted to exploit the unique short-temporal measurements provided by the Korean Geostationary Ocean Color Imager (GOCI), to obtain for the first time, diurnal and daily measurements of surface- and euphotic-column integrated Net Primary Productivity (NPP). These estimates are based on the Absorption Based Productivity Model (AbPM), and are calibrated with a bio-optical database of measurements made at different times of the day during the Korea-US Ocean Color (KORUS-OC) cruise in May–June 2016. Photo-physiological rate estimates, varied across different water types encountered around the Korean Peninsula, but on account of their limited number, were regionally scaled through the use of dynamic optical-biogeochemical (O-BGC) Biomes. Comparisons of GOCI derived estimates of daily and weekly integrated NPP against in-situ measurements, clearly underscore the superiority of geostationary over polar-orbiting ocean color satellites, which consider the strong short-term fluctuations in phytoplankton light absorption and incident irradiance fields over the course of the day.

Elevated pCO2 Induced Physiological, Molecular and Metabolic Changes in Nannochloropsis Oceanica and Its Effects on Trophic Transfer

The rise of dissolution of anthropogenic CO2 into the ocean alters marine carbonate chemistry and then results in ocean acidification (OA). It has been observed that OA induced different effects on different microalgae. In this study, we explored the physiological and biochemical changes in Nannochloropsis oceanica in response to increased atmospheric carbon dioxide and tested the effect of ocean acidification (OA) on the food web through animal feeding experiments at a laboratory scale. We found that the levels of C, N, C/N, Fv/Fm, and photosynthetic carbon fixation rate of algae cells were increased under high carbon dioxide concentration. Under short-term acidification, soluble carbohydrate, protein, and proportion of unsaturated fatty acids in cells were significantly increased. Under long-term acidification, the proportion of polyunsaturated fatty acids (PUFAs) (~33.83%) increased compared with that in control (~30.89%), but total protein decreased significantly compared with the control. Transcriptome and metabonomics analysis showed that the differential expression of genes in some metabolic pathways was not significant in short-term acidification, but most genes in the Calvin cycle were significantly downregulated. Under long-term acidification, the Calvin cycle, fatty acid biosynthesis, TAG synthesis, and nitrogen assimilation pathways were significantly downregulated, but the fatty acid β-oxidation pathway was significantly upregulated. Metabolome results showed that under long-term acidification, the levels of some amino acids increased significantly, while carbohydrates decreased, and the proportion of PUFAs increased. The rotifer Brachionus plicatilis grew slowly when fed on N. oceanica grown under short and long-term acidification conditions, and fatty acid profile analysis indicated that eicosapentaenoic acid (EPA) levels increased significantly under long-term acidification in both N. oceanica (~9.48%) and its consumer B. Plicatilis (~27.67%). It can be seen that N. oceanica formed a specific adaptation mechanism to OA by regulating carbon and nitrogen metabolism, and at the same time caused changes of cellular metabolic components. Although PUFAs were increased, they still had adverse effects on downstream consumers.

Carbon fixation rates in groundwater similar to those in oligotrophic marine systems

The terrestrial subsurface contains nearly all of Earth’s freshwater reserves and harbours the majority of our planet’s total prokaryotic biomass. Although genetic surveys suggest these organisms rely on in situ carbon fixation, rather than the photosynthetically derived organic carbon transported from surface environments, direct measurements of carbon fixation in the subsurface are absent. Using an ultra-low level 14C-labelling technique, we estimate in situ carbon fixation rates in a carbonate aquifer. We find these rates are similar to those measured in oligotrophic marine surface waters and up to six-fold greater than those observed in the lower euphotic zone. Our empirical carbon fixation rates agree with nitrification rate data. Metagenomic analyses reveal abundant putative chemolithoautotrophic members of an uncharacterized order of Nitrospiria that may be behind the carbon fixation. On the basis of our determined carbon fixation rates, we conservatively extrapolate global primary production in carbonate groundwaters (10% of global reserves) to be 0.11 Pg carbon per year. These rates fall within the range found for oligotrophic marine surface waters, indicating a substantial contribution of in situ primary production to subsurface ecosystem processes. We further suggest that, just as phototrophs are for marine biogeochemical cycling, such subsurface carbon fixation is potentially foundational to subsurface trophic webs.

Characterizing the Interplay of Rubisco and Nitrogenase Enzymes in Anaerobic-Photoheterotrophically Grown Rhodopseudomonas palustris CGA009 through a Genome-Scale Metabolic and Expression Model.

ABSTRACTRhodopseudomonas palustris CGA009 is a Gram-negative purple nonsulfur bacterium that grows phototrophically by fixing carbon dioxide and nitrogen or chemotrophically by fixing or catabolizing a wide array of substrates, including lignin breakdown products for its carbon and fixing nitrogen for its nitrogen requirements. It can grow aerobically or anaerobically and can use light, inorganic, and organic compounds for energy production. Due to its ability to convert different carbon sources into useful products during anaerobic growth, this study reconstructed a metabolic and expression (ME) model of R. palustris to investigate its anaerobic-photoheterotrophic growth. Unlike metabolic (M) models, ME models include transcription and translation reactions along with macromolecules synthesis and couple these reactions with growth rate. This unique feature of the ME model led to nonlinear growth curve predictions, which matched closely with experimental growth rate data. At the theoretical maximum growth rate, the ME model suggested a diminishing rate of carbon fixation and predicted malate dehydrogenase and glycerol-3 phosphate dehydrogenase as alternate electron sinks. Moreover, the ME model also identified ferredoxin as a key regulator in distributing electrons between major redox balancing pathways. Because ME models include the turnover rate for each metabolic reaction, it was used to successfully capture experimentally observed temperature regulation of different nitrogenases. Overall, these unique features of the ME model demonstrated the influence of nitrogenases and rubiscos on R. palustris growth and predicted a key regulator in distributing electrons between major redox balancing pathways, thus establishing a platform for in silico investigation of R. palustris metabolism from a multiomics perspective.IMPORTANCE In this work, we reconstructed the first ME model for a purple nonsulfur bacterium (PNSB). Using the ME model, different aspects of R. palustris metabolism were examined. First, the ME model was used to analyze how reducing power entering the R. palustris cell through organic carbon sources gets partitioned into biomass, carbon dioxide fixation, and nitrogen fixation. Furthermore, the ME model predicted electron flux through ferredoxin as a major bottleneck in distributing electrons to nitrogenase enzymes. Next, the ME model characterized different nitrogenase enzymes and successfully recapitulated experimentally observed temperature regulations of those enzymes. Identifying the bottleneck responsible for transferring an electron to nitrogenase enzymes and recapitulating the temperature regulation of different nitrogenase enzymes can have profound implications in metabolic engineering, such as hydrogen production from R. palustris. Another interesting application of this ME model can be to take advantage of its redox balancing strategy to gain an understanding of the regulatory mechanism of biodegradable plastic production precursors, such as polyhydroxybutyrate (PHB).

Comparative analyses of nutritional strategies among the species within the genus Paragymnodinium (Gymnodiniales, Dinophyceae).

The genus Paragymnodinium exhibits various nutritional strategies despite the fact that each species has pigmented plastids. This provides an excellent opportunity to investigate the evolution of the mode of nutrition and the function of the plastids through comparative studies. In this study, we analyzed the growth, pigment composition, absorption spectra, variable chlorophyll a fluorescence, and photosynthetic carbon fixation capabilities of P. stigmaticum, P. asymmetricum, and P. inerme. The autotrophic species P. asymmetricum and P. inerme without resorting to any nutrition from prey organisms displayed high PSII activity and carbon fixation rates. The pigment compositions of these two species were identical to those of other typical peridinin-containing type dinoflagellates. On the contrary, the phagotrophic species P. stigmaticum showed heterotrophic growth, and the variable chlorophyll a fluorescence properties and carbon fixation rates indicated significantly lower photosynthetic competence relative to those of the above two species. Paragymnodinium stigmaticum also contained peridinin, but pigment content ratios of peridinin, chlorophyll c2, and β-carotene were significantly different from those of other two species. The absorption spectrum analysis revealed a redshift in the peak of the Qy band of chlorophyll a in P. stigmaticum, presumably due to a change in chlorophyll-protein complexes. We conclude that such distinct differences in nutritional strategies between members of the genus Paragymnodinium would provide a platform for the hypothetical loss of photosynthetic function leading to colorless dinoflagellates.

The influence of mussel restoration on coastal carbon cycling.

Increasing responsiveness to anthropogenic climate change and the loss of global shellfish ecosystems has heightened interest in the carbon storage and sequestration potential of bivalve‐dominated systems. While coastal ecosystems are dynamic zones of carbon transformation and change, current uncertainties and notable heterogeneity in the benthic environment make it difficult to ascertain the climate change mitigation capacity of ongoing coastal restoration projects aimed at revitalizing benthic bivalve populations. In this study we sought to distinguish between direct and indirect effects of subtidal green‐lipped mussels (Perna canaliculus) on carbon cycling, and combined published literature with in‐situ experiments from restored beds to create a carbon budget for New Zealand's shellfish restoration efforts. A direct summation of biogenic calcification, community respiration, and sediment processes suggests a moderate carbon efflux (+100.1 to 179.6 g C m−2 year−1) occurs as a result of recent restoration efforts, largely reflective of the heterotrophic nature of bivalves. However, an examination of indirect effects of restoration on benthic community metabolism and sediment dynamics suggests that beds achieve greater carbon fixation rates and support enhanced carbon burial compared to nearby sediments devoid of mussels. We discuss limitations to our first‐order approximation and postulate how the significance of mussel restoration to carbon‐related outcomes likely increases over longer timescales. Coastal restoration is often conducted to support the provisioning of many ecosystem services, and we propose here that shellfish restoration not be used as a single measure to offset carbon dioxide emissions, but rather used in tandem with other initiatives to recover a bundle of valued ecosystem services.

Dark Diazotrophy during the Late Summer in Surface Waters of Chile Bay, West Antarctic Peninsula.

Although crucial for the addition of new nitrogen in marine ecosystems, dinitrogen (N2) fixation remains an understudied process, especially under dark conditions and in polar coastal areas, such as the West Antarctic Peninsula (WAP). New measurements of light and dark N2 fixation rates in parallel with carbon (C) fixation rates, as well as analysis of the genetic marker nifH for diazotrophic organisms, were conducted during the late summer in the coastal waters of Chile Bay, South Shetland Islands, WAP. During six late summers (February 2013 to 2019), Chile Bay was characterized by high NO3− concentrations (~20 µM) and an NH4+ content that remained stable near 0.5 µM. The N:P ratio was approximately 14.1, thus close to that of the Redfield ratio (16:1). The presence of Cluster I and Cluster III nifH gene sequences closely related to Alpha-, Delta- and, to a lesser extent, Gammaproteobacteria, suggests that chemosynthetic and heterotrophic bacteria are primarily responsible for N2 fixation in the bay. Photosynthetic carbon assimilation ranged from 51.18 to 1471 nmol C L−1 d−1, while dark chemosynthesis ranged from 9.24 to 805 nmol C L−1 d−1. N2 fixation rates were higher under dark conditions (up to 45.40 nmol N L−1 d−1) than under light conditions (up to 7.70 nmol N L−1 d−1), possibly contributing more than 37% to new nitrogen-based production (≥2.5 g N m−2 y−1). Of all the environmental factors measured, only PO43- exhibited a significant correlation with C and N2 rates, being negatively correlated (p < 0.05) with dark chemosynthesis and N2 fixation under the light condition, revealing the importance of the N:P ratio for these processes in Chile Bay. This significant contribution of N2 fixation expands the ubiquity and biological potential of these marine chemosynthetic diazotrophs. As such, this process should be considered along with the entire N cycle when further reviewing highly productive Antarctic coastal waters and the diazotrophic potential of the global marine ecosystem.

Uncertainties of soil organic carbon stock estimation caused by paleoclimate and human footprint on the Qinghai Plateau

BackgroundQuantifying the stock of soil organic carbon (SOC) and evaluating its potential impact factors is important to evaluating global climate change. Human disturbances and past climate are known to influence the rates of carbon fixation, soil physiochemical properties, soil microbial diversity and plant functional traits, which ultimately affect the current SOC storage. However, whether and how the paleoclimate and human disturbances affect the distribution of SOC storage on the high-altitude Tibetan Plateau remain largely unknown. Here, we took the Qinghai Plateau, the main component of the Tibetan Plateau, as our study region and applied three machine learning models (random forest, gradient boosting machine and support vector machine) to estimate the spatial and vertical distributions of the SOC stock and then evaluated the effects of the paleoclimate during the Last Glacial Maximum and the mid-Holocene periods as well as the human footprint on SOC stock at 0 to 200 cm depth by synthesizing 827 soil observations and 71 environmental factors.ResultsOur results indicate that the vegetation and modern climate are the determinant factors of SOC stocks, while paleoclimate (i.e., paleotemperature and paleoprecipitation) is more important than modern temperature, modern precipitation and the human footprint in shaping current SOC stock distributions. Specifically, the SOC stock was deeply underestimated in near natural ecosystems and overestimated in the strongly human disturbance ecosystems if the model did not consider the paleoclimate. Overall, the total SOC stock of the Qinghai Plateau was underestimated by 4.69%, 12.25% and 6.67% at depths of 0 to 100 cm, 100 to 200 cm and 0 to 200 cm, respectively. In addition, the human footprint had a weak influence on the distributions of the SOC stock. We finally estimated that the total and mean SOC stock at 200 cm depth by including the paleoclimate effects was 11.36 Pg C and 16.31 kg C m−2, respectively, and nearly 40% SOC was distributed in the top 30 cm.ConclusionThe paleoclimate is relatively important for the accurate modeling of current SOC stocks. Overall, our study provides a benchmark for predicting SOC stock patterns at depth and emphasizes that terrestrial carbon cycle models should incorporate information on how the paleoclimate has influenced SOC stocks.

Carbon Assimilation by the Picoplanktonic Community Inhabiting the Secondary Chlorophyll Maximum of the Anoxic Marine Zones of the Eastern Tropical North and South Pacific

Anoxic marine zones (AMZs) constitute pelagic systems distinguished from the oxygen minimum zones (OMZs) by the complete absence of detectable oxygen and the accumulation of nitrite in mid-waters. At the top of the oxygen-depleted layer and below the oxycline, nutrients are abundant; light intensity is very much reduced (&amp;lt;1% of incident light) and a secondary chlorophyll maximum (SCM) is developed. The shoaling of the oxygen-depleted layer, product of the AMZ expansion, could enhance this SCM, which has little-known biogeochemical effects. Here, we show that the SCM is contributing a measurable signal in the particulate organic carbon (POC), enough to alter the δ13CPOC in the top of the oxygen-depleted layer. This data showed significant differences among stations with and without the development of a SCM, being 3.0‰ heavier when a SCM is developed, and indicating photosynthetic activity and/or remineralization in the top of the AMZ. More depleted δ13CPOC values were also found when no SCM was present indicating stronger chemoautotrophic activity, potentially driven by anammox and sulfur-oxidizing bacteria activity. Assimilation rate data show that when sufficient light and Prochlorococcus are present, photosynthesis exceeds chemoautotrophic carbon fixation, and can exceed heterotrophic assimilation of glucose or acetate. However, in the majority of the stations, assimilation rates of both glucose and acetate exceeded carbon fixation rates under light stimulation, suggesting that often the SCM is still a net heterotrophic system.

Exploring Variability of Trichodesmium Photophysiology Using Multi-Excitation Wavelength Fast Repetition Rate Fluorometry.

Fast repetition rate fluorometry (FRRf) allows for rapid non-destructive assessment of phytoplankton photophysiology in situ yet has rarely been applied to Trichodesmium. This gap reflects long-standing concerns that Trichodesmium (and other cyanobacteria) contain pigments that are less effective at absorbing blue light which is often used as the sole excitation source in FRR fluorometers—potentially leading to underestimation of key fluorescence parameters. In this study, we use a multi-excitation FRR fluorometer (equipped with blue, green, and orange LEDs) to investigate photophysiological variability in Trichodesmium assemblages from two sites. Using a multi-LED measurement protocol (447+519+634 nm combined), we assessed maximum photochemical efficiency (Fv/Fm), functional absorption cross section of PSII (σPSII), and electron transport rates (ETRs) for Trichodesmium assemblages in both the Northwest Pacific (NWP) and North Indian Ocean in the vicinity of Sri Lanka (NIO-SL). Evaluating fluorometer performance, we showed that use of a multi-LED measuring protocol yields a significant increase of Fv/Fm for Trichodesmium compared to blue-only excitation. We found distinct photophysiological differences for Trichodesmium at both locations with higher average Fv/Fm as well as lower σPSII and non-photochemical quenching (NPQNSV) observed in the NWP compared to the NIO-SL (Kruskal–Wallis t-test df = 1, p < 0.05). Fluorescence light response curves (FLCs) further revealed differences in ETR response with a lower initial slope (αETR) and higher maximum electron turnover rate () observed for Trichodesmium in the NWP compared to the NIO-SL, translating to a higher averaged light saturation EK (= /αETR) for cells at this location. Spatial variations in physiological parameters were both observed between and within regions, likely linked to nutrient supply and physiological stress. Finally, we applied an algorithm to estimate primary productivity of Trichodesmium using FRRf-derived fluorescence parameters, yielding an estimated carbon-fixation rate ranging from 7.8 to 21.1 mgC mg Chl-a–1 h–1 across this dataset. Overall, our findings demonstrate that capacity of multi-excitation FRRf to advance the application of Chl-a fluorescence techniques in phytoplankton assemblages dominated by cyanobacteria and reveals novel insight into environmental regulation of photoacclimation in natural Trichodesmium populations.

Effects of desiccation and rehydration on carbon fixation and DOC release in Sargassum thunbergii

Although macroalgae is regarded as the emerging fourth category of “blue carbon”, few studies are available on its carbon sequestration and DOC (dissolved organic carbon) release mechanism at present. In addition, tidal action is an important environmental factor in the intertidal zone, which affects the carbon sequestration and DOC release of macroalgae. Therefore, we set 4-h desiccation and 1-h rehydration to examine the effects of drying-rewetting on the photosynthetic carbon fixation capacity and DOC release mechanism of Sargassum thunbergii. The flowing seawater can provide the nutrients for the growth of marine plants, which plays a vital role in the growth and reproduction of marine plants, so a measurement system that can adjust the flow speed was designed to study the effects of 1 h-rehydration on S. thunbergii. The results showed the desiccation decreased carbon fixation capacity, but increased the protection of PSII by down-regulating Fv/Fm and ФPSII and up-regulating NPQ, thereby promoting the recovery of carbon fixation capacity during 1-h rehydration period. It was found that the DOC release rate of S. thunbergii decreased with the carbon fixation rate increase and it was much greater than the carbon fixation rate in the 1-h rehydration period. The DOC release after rehydration was somehow a passive release, which was related to the increase of osmotic pressure, cell membrane damage, and carbon accumulation caused by desiccation. This measurement system can provide a practical reference for the study of carbon sink of macroalgae.