Genome sequences of two Synechococcus ecotypes inhabiting the Octopus Spring microbial mat in Yellowstone National Park revealed the presence of all genes required for nitrogenase biosynthesis. We demonstrate that nif genes of the Synechococcus ecotypes are expressed in situ in a region of the mat that varies in temperature from 53.5°C to 63.4°C (average 60°C); transcripts are only detected at the end of the day when the mat becomes anoxic. Nitrogenase activity in mat samples was also detected in the evening. Hitherto, N 2 fixation in hot spring mats was attributed either to filamentous cyanobacteria (not present at >50°C in these mats) or to heterotrophic bacteria. To explore how energy-generating processes of the Synechococcus ecotypes track natural light and O 2 conditions, we evaluated accumulation of transcripts encoding proteins involved in photosynthesis, respiration, and fermentation. Transcripts from photosynthesis (cpcF, cpcE, psaB, and psbB) and respiration (coxA and cydA) genes declined in the evening. In contrast, transcripts encoding enzymes that may participate in fermentation fell into two categories; some (ldh, pdhB, ald, and ackA) decreased in the evening, whereas others (pflB, pflA, adhE, and acs) increased at the end of the day and remained high into the night. Energy required for N 2 fixation during the night may be derived from fermentation pathways that become prominent as the mat becomes anoxic. In a broader context, our data suggest that there are critical regulatory switches in situ that are linked to the diel cycle and that these switches alter many metabolic processes within the microbial mat.Synechococcus ͉ diel regulation ͉ fermentation ͉ nitrogenase ͉ Yellowstone National Park
Nitrogen fixation, a prokaryotic, O 2 -inhibited process that reduces N 2 gas to biomass, is of paramount importance in biogeochemical cycling of nitrogen. We analyzed the levels of nif transcripts of Synechococcus ecotypes, NifH subunit and nitrogenase activity over the diel cycle in the microbial mat of an alkaline hot spring in Yellowstone National Park. The results showed a rise in nif transcripts in the evening, with a subsequent decline over the course of the night. In contrast, immunological data demonstrated that the level of the NifH polypeptide remained stable during the night, and only declined when the mat became oxic in the morning. Nitrogenase activity was low throughout the night; however, it exhibited two peaks, a small one in the evening and a large one in the early morning, when light began to stimulate cyanobacterial photosynthetic activity, but O 2 consumption by respiration still exceeded the rate of O 2 evolution. Once the irradiance increased to the point at which the mat became oxic, the nitrogenase activity was strongly inhibited. Transcripts for proteins associated with energy-producing metabolisms in the cell also followed diel patterns, with fermentation-related transcripts accumulating at night, photosynthesis-and respiration-related transcripts accumulating during the day and late afternoon, respectively. These results are discussed with respect to the energetics and regulation of N 2 fixation in hot spring mats and factors that can markedly influence the extent of N 2 fixation over the diel cycle.
We have studied by neutron diffraction the magnetic ordering in Al-free crystals of PrBa 2 Cu 3 O 61x (x 0.35 and 0.92) that do not display the AFII Cu magnetic phase. We find that the Pr ordering below 20 K is accompanied by a counterrotation of the Cu antiferromagnetism on each plane of the bilayer. The maximum turn angle between the two planes is 60 ± 6 9 ± for the x 0.92 crystal, and 40 ± 6 11 ± for the x 0.35 crystal. This is the first observation of a noncollinear ordering of Cu moments in the bilayer, and is evidence for significant magnetic coupling between the Cu and Pr sublattices. [S0031-9007(96)01945-X]
X-ray absorption spectroscopic measurements and density functional calculations suggest that the hydrogenase H-cluster is best described as an electronically inseparable 6Fe-cluster due to extensive delocalization of frontier molecular orbitals of the iron centres, sulfide and the non-innocent dithiolate ligands.
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