High dimethylsulfide photolysis rates in nitrate-rich Antarctic waters

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 31 (2004): L11307, doi:10.1029/2004GL019863.The photochemistry of dimethylsulfide (DMS) was examined in the Southern Ocean to assess its impact on the biogeochemical dynamics of DMS in Antarctic waters. Very high DMS photolysis rate constants (0.16–0.23 h−1) were observed in surface waters exposed to full sunlight. DMS photolysis rates increased linearly with added nitrate concentrations, and 35% of the DMS loss in unamended samples was attributed to the photochemistry of ambient nitrate (29 μM). Experiments with optical filters showed that the UV-A band of sunlight (320–400 nm) accounted for ~65% of DMS photolysis suggesting that dissolved organic matter was the main photosensitizer for DMS photolysis. During the austral spring, DMS photolysis was the dominant loss mechanism under non-bloom and non-ice cover conditions owing to the high doses and deep penetration of UV radiation in the water column, low observed microbial consumption rates, and high in situ nitrate concentrations.This work was supported by NSF (OPP- 0230499, DJK; OPP-0230497, RPK)

Control of defect-mediated tunneling barrier heights in ultrathin MgO films

The impact of oxygen vacancies on local tunneling properties across rf-sputtered MgO thin films was investigated by optical absorption spectroscopy and conducting atomic force microscopy. Adding O$_2$ to the Ar plasma during MgO growth alters the oxygen defect populations, leading to improved local tunneling characteristics such as a lower density of current hotspots and a lower tunnel current amplitude. We discuss a defect-based potential landscape across ultrathin MgO barriers.Comment: 4 pages, 4 figure

Absorption Spectral Slopes and Slope Ratios as Indicators of Molecular Weight, Source, and Photobleaching of Chromophoric Dissolved Organic Matter

A new approach for parameterizing dissolved organic matter ( DOM) ultraviolet-visible absorption spectra is presented. Two distinct spectral slope regions ( 275-295 nm and 350-400 nm) within log-transformed absorption spectra were used to compare DOM from contrasting water types, ranging from wetlands (Great Dismal Swamp and Suwannee River) to photobleached oceanic water ( Atlantic Ocean). On the basis of DOM size-fractionation studies ( ultrafiltration and gel filtration chromatography), the slope of the 275-295- nm region and the ratio of these slopes (SR; 275-295- nm slope : 350-400- nm slope) were related to DOM molecular weight ( MW) and to photochemically induced shifts in MW. Dark aerobic microbial alteration of chromophoric DOM ( CDOM) resulted in spectral slope changes opposite of those caused by photochemistry. Along an axial transect in the Delaware Estuary, large variations in SR were measured, probably due to mixing, photodegradation, and microbial alteration of CDOM as terrestrially derived DOM transited through the estuary. Further, SR varied by over a factor of 13 between DOM-rich wetland waters and Sargasso Sea surface waters. Currently, there is no consensus on a wavelength range for log-transformed absorption spectra. We propose that the 275-295- nm slope be routinely reported in future DOM studies, as it can be measured with high precision, it facilitates comparison among dissimilar water types including CDOM-rich wetland and CDOM-poor marine waters, and it appears to be a good proxy for DOM MW. © 2008, by the American Society of Limnology and Oceanography, Inc

Seasonal ITCZ migration dynamically controls the location of the (sub)tropical Atlantic biogeochemical divide

Inorganic nitrogen depletion restricts productivity in much of the low-latitude oceans, generating a selective advantage for diazotrophic organisms capable of fixing atmospheric dinitrogen (N2). However, the abundance and activity of diazotrophs can in turn be controlled by the availability of other potentially limiting nutrients, including phosphorus (P) and iron (Fe). Here we present high-resolution data (∼0.3°) for dissolved iron, aluminum, and inorganic phosphorus that confirm the existence of a sharp north–south biogeochemical boundary in the surface nutrient concentrations of the (sub)tropical Atlantic Ocean. Combining satellite-based precipitation data with results from a previous study, we here demonstrate that wet deposition in the region of the intertropical convergence zone acts as the major dissolved iron source to surface waters. Moreover, corresponding observations of N2 fixation and the distribution of diazotrophic Trichodesmium spp. indicate that movement in the region of elevated dissolved iron as a result of the seasonal migration of the intertropical convergence zone drives a shift in the latitudinal distribution of diazotrophy and corresponding dissolved inorganic phosphorus depletion. These conclusions are consistent with the results of an idealized numerical model of the system. The boundary between the distinct biogeochemical systems of the (sub)tropical Atlantic thus appears to be defined by the diazotrophic response to spatial–temporal variability in external Fe inputs. Consequently, in addition to demonstrating a unique seasonal cycle forced by atmospheric nutrient inputs, we suggest that the underlying biogeochemical mechanisms would likely characterize the response of oligotrophic systems to altered environmental forcing over longer timescales

Variability of atmospheric dimethylsulphide over the southern Indian Ocean due to changes in ultraviolet radiation

Dimethylsulphide (DMS) is a climatically important component of global biogeochemical cycles, through its role in the sulphur cycle. Changes in ultraviolet radiation (UV) exhibit both positive and negative forcings on the dynamics of production and turnover of DMS and its precursor dimethylsulphoniopropionate (DMSP). In this study we investigate the net forcing of UV on atmospheric DMS. The work is based on a 10-year record of observed DMS at Amsterdam Island in the southern Indian Ocean, and satellite-based retrievals of surface UV and photosynthetically active radiation (PAR). The results show an inverse relationship between UV radiation and atmospheric DMS associated with extreme changes (defined as the greatest 5%) in daily UV, independent of changes in wind speed, sea surface temperature, and PAR

The BTB ubiquitin ligases ETO1, EOL1 and EOL2 act collectively to regulate ethylene biosynthesis in Arabidopsis by controlling type-2 ACC synthase levels

Ethylene biosynthesis is directed by a family of 1-aminocyclopropane-1-carboxylic acid (ACC) synthases (ACS) that convert S-adenosyl-l-methionine to the immediate precursor ACC. Members of the type-2 ACS subfamily are strongly regulated by proteolysis with various signals stabilizing the proteins to increase ethylene production. In Arabidopsis, this turnover is mediated by the ubiquitin/26 S proteasome system, using a broad complex/tramtrack/bric-a-brac (BTB) E3 assembled with the ETHYLENE OVERPRODUCER 1 (ETO1) BTB protein for target recognition. Here, we show that two Arabidopsis BTB proteins closely related to ETO1, designated ETO1-like (EOL1) and EOL2, also negatively regulate ethylene synthesis via their ability to target ACSs for breakdown. Like ETO1, EOL1 interacts with type-2 ACSs (ACS4, ACS5 and ACS9), but not with type-1 or type-3 ACSs, or with type-2 ACS mutants that stabilize the corresponding proteins in planta. Whereas single and double mutants affecting EOL1 and EOL2 do not show an ethylene-related phenotype, they exaggerate the effects caused by inactivation of ETO1, and further increase ethylene production and the accumulation of ACS5 in eto1 plants. The triple eto1 eol1 eol2 mutant phenotype can be effectively rescued by the ACS inhibitor aminoethoxyvinylglycine, and by silver, which antagonizes ethylene perception. Together with hypocotyl growth assays showing that the sensitivity and response kinetics to ethylene are normal, it appears that ethylene synthesis, but not signaling, is compromised in the triple mutant. Collectively, the data indicate that the Arabidopsis BTB E3s assembled with ETO1, EOL1 and EOL2 work together to negatively regulate ethylene synthesis by directing the degradation of type-2 ACS proteins

Localization of the Raf-like Kinase CTR1 to the Endoplasmic Reticulum of Arabidopsis through Participation in Ethylene Receptor Signaling Complexes

The plant hormone ethylene is perceived by a five-member family of receptors related to the bacterial histidine kinases. The Raf-like kinase CTR1 functions downstream of the ethylene receptors as a negative regulator of ethylene signal transduction. CTR1 is shown here to be associated with membranes of the endoplasmic reticulum in Arabidopsis as a result of its interactions with ethylene receptors. Membrane association of CTR1 is reduced by mutations that eliminate ethylene receptors and by a mutation in CTR1 that reduces its ability to bind to the ethylene receptor ETR1. Direct evidence that CTR1 is part of an ethylene receptor signaling complex was obtained by co-purification of the ethylene receptor ETR1 with a tagged version of CTR1 from an Arabidopsis membrane extract. The histidine kinase activity of ETR1 is not required for its association with CTR1, based on co-purification of tagged ETR1 mutants and CTR1 after expression in a transgenic yeast system. These data demonstrate that CTR1 is part of an ethylene receptor signaling complex in Arabidopsis and support a model in which localization of CTR1 to the endoplasmic reticulum is necessary for its function. Additional data that demonstrate a post-transcriptional effect of ethylene upon the expression of CTR1 suggest that production of ethylene receptor signaling complexes may be coordinately regulated

The Arabidopsis Histidine Phosphotransfer Proteins Are Redundant Positive Regulators of Cytokinin Signaling

Arabidopsis thaliana histidine phosphotransfer proteins (AHPs) are similar to bacterial and yeast histidine phosphotransfer proteins (HPts), which act in multistep phosphorelay signaling pathways. A phosphorelay pathway is the current model for cytokinin signaling. To assess the role of AHPs in cytokinin signaling, we isolated T-DNA insertions in the five AHP genes that are predicted to encode functional HPts and constructed multiple insertion mutants, including an ahp1,2,3,4,5 quintuple mutant. Single ahp mutants were indistinguishable from wild-type seedlings in cytokinin response assays. However, various higher-order mutants displayed reduced sensitivity to cytokinin in diverse cytokinin assays, indicating both a positive role for AHPs in cytokinin signaling and functional overlap among the AHPs. In contrast with the other four AHPs, AHP4 may play a negative role in some cytokinin responses. The quintuple ahp mutant showed various abnormalities in growth and development, including reduced fertility, increased seed size, reduced vascular development, and a shortened primary root. These data indicate that most of the AHPs are redundant, positive regulators of cytokinin signaling and affect multiple aspects of plant development

RNA Sequencing Identifies Transcriptionally Viable Gene Fusions in Esophageal Adenocarcinomas

Esophageal adenocarcinoma (EAC) is a deadly cancer with increasing incidence in the U.S., but mechanisms underlying pathogenesis are still mostly elusive. In addressing this question, we assessed gene-fusion landscapes by comprehensive RNA sequencing (RNAseq) of 55 pre-treatment EAC and 49 non-malignant biopsy tissues from patients undergoing endoscopy for Barrett’s esophagus. In this cohort, we identified 21 novel candidate EAC-associated fusions occurring in 3.33%-11.67% of EACs. Two candidate fusions were selected for validation by PCR and Sanger sequencing in an independent set of pre-treatment EAC (N=115) and non-malignant (N=183) biopsy tissues. In particular, we observed RPS6KB1–VMP1 gene fusion as a recurrent event occurring in ~10% of EAC cases. Notably, EAC cases harboring RPS6KB1–VMP1 fusions exhibited significantly poorer overall survival as compared to fusion-negative cases. Mechanistic investigations established that the RPS6KB1–VMP1 transcript coded for a fusion protein which significantly enhanced the growth rate of non-dysplastic Barrett’s esophagus cells. Compared to the wild-type VMP1 protein, which mediates normal cellular autophagy, RPS6KB1–VMP1 fusion exhibited aberrant subcellular localization and was relatively ineffective in triggering autophagy. Overall, our findings identified RPS6KB1–VMP1 as a genetic fusion that promotes EAC by modulating autophagy-related processes, offering new insights into the molecular pathogenesis of esophageal adenocarcinomas

CTR1 phosphorylates the central regulator EIN2 to control ethylene hormone signaling from the ER membrane to the nucleus in Arabidopsis

The gaseous phytohormone ethylene C2H4 mediates numerous aspects of growth and development. Genetic analysis has identified a number of critical elements in ethylene signaling, but how these elements interact biochemically to transduce the signal from the ethylene receptor complex at the endoplasmic reticulum (ER) membrane to transcription factors in the nucleus is unknown. To close this gap in our understanding of the ethylene signaling pathway, the challenge has been to identify the target of the CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) Raf-like protein kinase, as well as the molecular events surrounding ETHYLENE-INSENSITIVE2 (EIN2), an ER membrane-localized Nramp homolog that positively regulates ethylene responses. Here we demonstrate that CTR1 interacts with and directly phosphorylates the cytosolic C-terminal domain of EIN2. Mutations that block the EIN2 phosphorylation sites result in constitutive nuclear localization of the EIN2 C terminus, concomitant with constitutive activation of ethylene responses in Arabidopsis. Our results suggest that phosphorylation of EIN2 by CTR1 prevents EIN2 from signaling in the absence of ethylene, whereas inhibition of CTR1 upon ethylene perception is a signal for cleavage and nuclear localization of the EIN2 C terminus, allowing the ethylene signal to reach the downstream transcription factors. These findings significantly advance our understanding of the mechanisms underlying ethylene signal transduction