Global Terrestrial Ecosystem Carbon Flux Inferred from TanSat XCO2 Retrievals

TanSat is China’s first greenhouse gases observing satellite. In recent years, substantial progresses have been achieved on retrieving column-averaged CO2 dry air mole fraction (XCO2). However, relatively few attempts have been made to estimate terrestrial net ecosystem exchange (NEE) using TanSat XCO2 retrievals. In this study, based on the GEOS-Chem 4D-Var data assimilation system, we infer the global NEE from April 2017 to March 2018 using TanSat XCO2. The inversion estimates global NEE at −3.46 PgC yr-1, evidently higher than prior estimate and giving rise to an improved estimate of global atmospheric CO2 growth rate. Regionally, our inversion greatly increases the carbon uptakes in northern mid-to-high latitudes and significantly enhances the carbon releases in tropical and southern lands, especially in Africa and India peninsula. The increase of posterior sinks in northern lands is mainly attributed to the decreased carbon release during the nongrowing season, and the decrease of carbon uptakes in tropical and southern lands basically occurs throughout the year. Evaluations against independent CO2 observations and comparison with previous estimates indicate that although the land sinks in the northern middle latitudes and southern temperate regions are improved to a certain extent, they are obviously overestimated in northern high latitudes and underestimated in tropical lands (mainly northern Africa), respectively. These results suggest that TanSat XCO2 retrievals may have systematic negative biases in northern high latitudes and large positive biases over northern Africa, and further efforts are required to remove bias in these regions for better estimates of global and regional NEE.

Synergetic Fermentation of Glucose and Glycerol for High-Yield N-Acetylglucosamine Production in Escherichia coli

N-acetylglucosamine (GlcNAc) is an amino sugar that has been widely used in the nutraceutical and pharmaceutical industries. Recently, microbial production of GlcNAc has been developed. One major challenge for efficient biosynthesis of GlcNAc is to achieve appropriate carbon flux distribution between growth and production. Here, a synergistic substrate co-utilization strategy was used to address this challenge. Specifically, glycerol was utilized to support cell growth and generate glutamine and acetyl-CoA, which are amino and acetyl donors, respectively, for GlcNAc biosynthesis, while glucose was retained for GlcNAc production. Thanks to deletion of the 6-phosphofructokinase (PfkA and PfkB) and glucose-6-phosphate dehydrogenase (ZWF) genes, the main glucose catabolism pathways of Escherichia coli were blocked. The resultant mutant showed a severe defect in glucose consumption. Then, the GlcNAc production module containing glucosamine-6-phosphate synthase (GlmS*), glucosamine-6-phosphate N-acetyltransferase (GNA1*) and GlcNAc-6-phosphate phosphatase (YqaB) expression cassettes was introduced into the mutant, to drive the carbon flux from glucose to GlcNAc. Furthermore, co-utilization of glucose and glycerol was achieved by overexpression of glycerol kinase (GlpK) gene. Using the optimized fermentation medium, the final strain produced GlcNAc with a high stoichiometric yield of 0.64 mol/mol glucose. This study offers a promising strategy to address the challenge of distributing carbon flux in GlcNAc production.

Media engineering in marine diatom Phaeodactylum tricornutum employing cost-effective substrates for sustainable production of high value renewables

Phaeodactylum tricornutum is a marine diatom, and well-studied model of unicellular microalga. This diatom contains a wide range of high-value renewables (HVRs) with high commercial relevance owing to their importance in human nutrition and health. In this study, we screened P. tricornutum for biomass, eicosapentaenoic acid (EPA) and fucoxanthin production under photoautotrophic and mixotrophic condition with various substrate combinations. Results highlights that culture supplemented with glycerol and urea lead to enhanced biomass, biochemical and HVR production. Further continuous feeding of urea in glycerol supplemented medium results in an increase in biomass yield (0.77 g L-1) by ~ 2-fold. Additionally, continuous feeding of urea channelizes the carbon flux towards biosynthesis of fatty acids increasing FAME content by ~2-fold as compared to the control conditions. Overall EPA and fucoxanthin production was 27 mg L-1 and 11 mg L-1 (~2 & 4 fold) in urea fed cultures respectively. Present study demonstrates efficient valorization of cost-effective substrates such as glycerol and urea for the production of high-value renewables in P. tricornutum.

Seasonal flux patterns and carbon transport from low-oxygen eddies at the Cape Verde Ocean Observatory: lessons learned from a time series sediment trap study (2009–2016)

Mesoscale eddies are abundant in the eastern tropical North Atlantic and act as oases for phytoplankton growth due to local enrichment of nutrients in otherwise oligotrophic waters. It is not clear whether these eddies can efficiently transfer organic carbon and other flux components to depth and if they are important for the marine carbon budget. Due to their transient and regionally restricted nature, measurements of eddies' contribution to bathypelagic particle flux are difficult to obtain. Rare observations of export flux associated with low-oxygen eddies have suggested efficient export from the surface to the deep ocean, indicating that organic carbon flux attenuation might be low. Here we report on particle flux dynamics north of the Cabo Verde islands at the oligotrophic Cape Verde Ocean Observatory (CVOO; approx. 17∘35′ N, 24∘15′ W). The CVOO site is located in the preferred pathways of highly productive eddies that ultimately originate from the Mauritanian upwelling region. Between 2009 and 2016, we collected biogenic and lithogenic particle fluxes with sediment traps moored at ca. 1 and 3 km water depths at the CVOO site. From concurrent hydrography and oxygen observations, we confirm earlier findings that highly productive eddies are characterized by colder and less saline waters and a low-oxygen signal as well. Overall, we observed quite consistent seasonal flux patterns during the passage of highly productive eddies in the winters of 2010, 2012 and 2016. We found flux increases at 3 km depth during October–November when the eddies approached CVOO and distinct flux peaks during February–March, clearly exceeding low oligotrophic background fluxes during winter 2011 and showing an enhanced particle flux seasonality. During spring, we observed a stepwise flux decrease leading to summer flux minima. The flux pattern of biogenic silicate (BSi) showed a stronger seasonality compared to organic carbon. Additionally, the deep fluxes of total mass showed an unusually higher seasonality compared to the 1 km traps. We assume that BSi and organic carbon/lithogenic material had different sources within the eddies. BSi-rich particles may originate at the eddy boundaries where large diatom aggregates are formed due to strong shear and turbulence, resulting in gravitational settling and, additionally, in an active local downward transport. Organic carbon associated with lithogenic material is assumed to originate from the interior of eddies or from mixed sources, both constituting smaller, dust-ballasted particles. Our findings suggest that the regularly passing highly productive eddies at CVOO repeatedly release characteristic flux signals to the bathypelagic zone during winter–spring seasons that are far above the oligotrophic background fluxes and sequester higher organic carbon than during oligotrophic settings. However, the reasons for a lower carbon flux attenuation below eddies remain elusive.