University of Johannesburg

About: University of Johannesburg is a education organization based out in Johannesburg, South Africa. It is known for research contribution in the topics: Population & Context (language use). The organization has 8070 authors who have published 22749 publications receiving 329408 citations. The organization is also known as: UJ.

Isotopic evidence for biological nitrogen fixation by molybdenum-nitrogenase from 3.2 Gyr

Abstract: Nitrogen isotope ratios from rocks between 3.2 and 2.75 billion years old are most readily explained by biological nitrogen fixation, most probably using the metal molybdenum as a cofactor, showing that nitrogen fixation is at least 3.2 billion years old and suggesting that molybdenum was available to organisms long before the Great Oxidation Event. The evolution of the enzyme nitrogenase, through which organisms can fix atmospheric nitrogen, was a clearly major step in the history of life. What is less certain is the timing. Eva Stueken et al. have determined nitrogen isotope ratios in marine and fluvial sedimentary rocks between 3.2 and 2.75 billion years old. The ratios are most readily explained by biological nitrogen fixation, probably with molybdenum as a cofactor. This suggests that nitrogen fixation is at least 3.2 billion years old, and contradicts previous suggestions that marine molybdenum was scarce before the Great Oxidation Event. Nitrogen is an essential nutrient for all organisms that must have been available since the origin of life. Abiotic processes including hydrothermal reduction1, photochemical reactions2, or lightning discharge3 could have converted atmospheric N2 into assimilable NH4+, HCN, or NOx species, collectively termed fixed nitrogen. But these sources may have been small on the early Earth, severely limiting the size of the primordial biosphere4. The evolution of the nitrogen-fixing enzyme nitrogenase, which reduces atmospheric N2 to organic NH4+, thus represented a major breakthrough in the radiation of life, but its timing is uncertain5,6. Here we present nitrogen isotope ratios with a mean of 0.0 ± 1.2‰ from marine and fluvial sedimentary rocks of prehnite–pumpellyite to greenschist metamorphic grade between 3.2 and 2.75 billion years ago. These data cannot readily be explained by abiotic processes and therefore suggest biological nitrogen fixation, most probably using molybdenum-based nitrogenase as opposed to other variants that impart significant negative fractionations7. Our data place a minimum age constraint of 3.2 billion years on the origin of biological nitrogen fixation and suggest that molybdenum was bioavailable in the mid-Archaean ocean long before the Great Oxidation Event.

Measurements of the nuclear modification factor for jets in Pb+Pb Collisions at √sNN = 2.76 TeV with the ATLAS detector

Abstract: Measurements of inclusive jet production are performed in pp and Pb+Pb collisions at √sNN = 2.76 TeV with the ATLAS detector at the LHC, corresponding to integrated luminosities of 4.0 pb−1 and 0.14 nb−1, respectively. The jets are identified with the anti-kt algorithm with R = 0.4, and the spectra are measured over the kinematic range of jet transverse momentum 32 < pT < 500 GeV, and absolute rapidity |y| < 2.1 and as a function of collision centrality. The nuclear modification factor, RAA, is evaluated and jets are found to be suppressed by approximately a factor of two in central collisions compared to pp collisions. The RAA shows a slight increase with pT and no significant variation with rapidity.