Terrestrial carbon export via inland aquatic systems is a key process in the global carbon cycle. It includes loss of carbon to the atmosphere via outgassing from rivers, lakes, or reservoirs and carbon fixation in the water column as well as in sediments. This review focuses on headwater streams that are important because their stream biogeochemistry directly reflects carbon input from soils and groundwaters. Major drivers of carbon dioxide partial pressures (pCO2) in streams and mechanisms of terrestrial dissolved inorganic, organic and particulate organic carbon (DIC, DOC, and POC) influxes are summarized in this work. Our analysis indicates that the global river average pCO2 of 3100 ppmV is more often exceeded by contributions from small streams when compared to rivers with larger catchments (> 500 km2). Because of their large proportion in global river networks (> 96% of the total number of streams), headwaters contribute large—but still poorly quantified—amounts of CO2 to the atmosphere. Conservative estimates imply that globally 36% (i.e., 0.93 Pg C yr−1) of total CO2 outgassing from rivers and streams originate from headwaters. We also discuss challenges in determination of CO2 sources, concentrations, and fluxes. To overcome uncertainties of CO2 sources and its outgassing from headwater streams on the global scale, new investigations are needed that should include groundwater data. Such studies would also benefit from applications of integral CO2 outgassing isotope approaches and multiscale geophysical imaging techniques.

Light stable isotope analyses of hydrogen (2H/1H) and oxygen (18O/16O) of water are used in many terrestrial and marine aquatic studies. The advantage of using stable isotope ratios is that water molecules serve as ubiquitous and already present natural tracers. Within recent years, these analyses have been revolutionized by the development of new isotope ratio laser spectroscopy (IRIS) systems that are cheaper, more robust, and mobile compared with traditional isotope ratio mass spectrometry (IRMS). Although easier to operate, laser systems also need thorough calibration with international reference materials, and raw data need correction for analytical effects (i.e., memory and drift). This study presents modifications to the hardware for liquid water injection, an optimized sequence layout and a simple post‐run correction procedure. These protocols will maximize precision, accuracy, and sample throughput via an efficient memory correction. The number of injections per unknown sample can be reduced to 4 or less. This procedure meets the demands of faster throughput with reduced costs per analysis. Procedures presented here are based on real analyses. They were also verified by an international proficiency test and traditional IRMS techniques.

No abstract

In plants and algae, the serine/threonine kinase STN7/STT7, orthologous protein kinases in Chlamydomonas reinhardtii and Arabidopsis (Arabidopsis thaliana), respectively, is an important regulator in acclimation to changing light environments. In this work, we assessed STT7-dependent protein phosphorylation under high light in C. reinhardtii, known to fully induce the expression of LIGHT-HARVESTING COMPLEX STRESS-RELATED PROTEIN3 (LHCSR3) and a nonphotochemical quenching mechanism, in relationship to anoxia where the activity of cyclic electron flow is stimulated. Our quantitative proteomics data revealed numerous unique STT7 protein substrates and STT7-dependent protein phosphorylation variations that were reliant on the environmental condition. These results indicate that STT7-dependent phosphorylation is modulated by the environment and point to an intricate chloroplast phosphorylation network responding in a highly sensitive and dynamic manner to environmental cues and alterations in kinase function. Functionally, the absence of the STT7 kinase triggered changes in protein expression and photoinhibition of photosystem I (PSI) and resulted in the remodeling of photosynthetic complexes. This remodeling initiated a pronounced association of LHCSR3 with PSI-LIGHT HARVESTING COMPLEX I (LHCI)-ferredoxin-NADPH oxidoreductase supercomplexes. Lack of STT7 kinase strongly diminished PSII-LHCII supercomplexes, while PSII core complex phosphorylation and accumulation were significantly enhanced. In conclusion, our study provides strong evidence that the regulation of protein phosphorylation is critical for driving successful acclimation to high light and anoxic growth environments and gives new insights into acclimation strategies to these environmental conditions. Oxygenic photosynthesis converts solar energy into chemical energy. This energy is utilized for carbon dioxide assimilation, allowing the formation of complex organic material. Plant photosynthesis is performed by a series of reactions in and at the thylakoid membrane, resulting in light-dependent water oxidation, NADP reduction, and ATP formation (Whatley et al., 1963). These light reactions are catalyzed by two photosystems (PSI and PSII). A third multiprotein complex, also embedded in the thylakoid membrane, is the cytochrome b 6 f (cyt b 6 f) complex that links photosynthetic electron transfer processes between the two photosystems and functions in proton translocation. The ATP synthase takes advantage of the proton-motive force that is generated by the light reactions (Mitchell, 1961) to produce ATP. ATP and NADPH, generated through linear electron flow from PSII to PSI, drive the CalvinBenson-Bassham cycle (Bassham et al., 1950) to fix CO 2 . Alternatively, cyclic electron flow (CEF) between PSI and the cyt b 6 f complex solely produces ATP (Arnon, 1959).Under normal growth conditions, CEF provides additionally required ATP for CO 2 fixation (Lucker and Kramer, 2013), counteracts overreduction of the PSI acceptor side under stressful environmental cues,...

The scaffold protein Vac14 acts in a complex with the lipid kinase PIKfyve and its counteracting phosphatase FIG4, regulating the interconversion of phosphatidylinositol-3-phosphate to phosphatidylinositol-3,5-bisphosphate. Dysfunctional Vac14 mutants, a deficiency of one of the Vac14 complex components, or inhibition of PIKfyve enzymatic activity results in the formation of large vacuoles in cells. How these vacuoles are generated and which processes are involved are only poorly understood. Here we show that ectopic overexpression of wild-type Vac14 as well as of the PIKfyve-binding deficient Vac14 L156R mutant causes vacuoles. Vac14-dependent vacuoles and PIKfyve inhibitor-dependent vacuoles resulted in elevated levels of late endosomal, lysosomal, and autophagy-associated proteins. However, only late endosomal marker proteins were bound to the membranes of these enlarged vacuoles. In order to decipher the linkage between the Vac14 complex and regulators of the endolysosomal pathway, a protein affinity approach combined with multidimensional protein identification technology was conducted, and novel molecular links were unraveled. We found and verified the interaction of Rab9 and the Rab7 GAP TBC1D15 with Vac14. The identified Rab-related interaction partners support the theory that the regulation of vesicular transport processes and phosphatidylinositolmodifying enzymes are tightly interconnected. Molecular