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Author(s): Charles R. Hipkin (corresponding author) [1, 2]; Deborah J. Simpson [1, 2, 3]; Stephen J. Wainwright [1]; Mansour A. Salem [1, 4] Nitrification is a key stage in the [...]

The Candida nitratophila assimilatory nitrate reductase (NR), which catalyzes the reduction of nitrate to nitrite, was purified and characterized. The results showed that each subunit of NR contains one molecule of heme and one atom of molybdenum. Spectral analysis showed that the prosthetic group of NR is cytochrome b557. A 40-kDa tryptic fragment was associated with the FAD- and cytochrome b557-dependent diaphorase activities. NR actitity was enhanced by an increase in ionic strength of assay mixtures and by the addition of bovine serum albumin. These effects were attributed to the stimulation of molybdenum-dependent terminal nitrate-reducing activity.

Altitudinally separated bristlecone pine populations in the White Mountains (California, USA) exhibit differential climate–growth responses as temperature and tree-water relations change with altitude. These populations provide a natural experiment to explore the ecophysiological adaptations of this unique tree species to the twentieth century climate variability. We developed absolutely dated annual ring-width chronologies, and cellulose stable carbon and oxygen isotope chronologies from bristlecone pine growing at the treeline (~3500 m) and ~200 m below for the period AD 1710–2010. These chronologies were interpreted in terms of ecophysiological adaptations to climate variability with a dual-isotope model and a leaf gas exchange model. Ring widths show positive tree growth anomalies at treeline and consistent slower growth below treeline in relation to the twentieth century warming and associated atmospheric drying until the 1980s. Growth rates of both populations declined during and after the 1980s when growing-season temperature and atmospheric vapour pressure deficit continued to increase. Our model-based interpretations of the cellulose stable isotopes indicate that positive treeline growth anomalies prior to the 1980s were related to increased stomatal conductance and leaf-level transpiration and photosynthesis. Reduced growth since the 1980s occurred with a shift to more conservative leaf gas exchange in both the treeline and below-treeline populations, whereas leaf-level photosynthesis continued to increase in response to rising atmospheric CO2 concentrations. Our results suggest that warming-induced atmospheric drying confounds positive growth responses of apparent temperature-limited bristlecone pine populations at treeline. In addition, the observed ecophysiological responses of attitudinally separated bristlecone pine populations illustrate the sensitivity of conifers to climate change. [ABSTRACT FROM AUTHOR]

In vivo labelling and in vitro translation studies were used to study the regulation of the synthesis of nitrate reductase in the yeast Candida nitratophila. These studies showed that synthesis of the enzyme subunit took place when ammonium-grown cells were nitrogen-starved and this was stimulated by subsequent addition of nitrate. Ammonium-grown cultures did not contain mRNA that could be translated into the nitrate reductase subunit in an in vitro system. Nitrate reductase mRNA could be extracted from nitrogen-starved and nitrate cultures. Synthesis of the enzyme is apparently controlled at the level of transcription in this yeast. [ABSTRACT FROM AUTHOR]

Changes in the maximum rates of ammonium and nitrate transport (Tmax; expressed as C-specific N transport) into Emiliana huxleyi and Heterosigma carterae were related to N status (cellular N:C). There was considerable variation in the magnitude of Tmax relative to N:C. For nitrate transport in both species and for ammonium in H.carterae, the curve linking N:C and Tmax was bell shaped; Tmax increased initially as N:C fell and then Tmax decreased. In contrast, Tmax for ammonium in E.huxleyi did not decrease to such low levels and N-specific N transport increased continually as N:C decreased. While Tmax for ammonium into E.huxleyi could be an order of magnitude higher than the N-transport rate required to support growth, for nitrate transport into both species Tmax was coupled more closely to growth rate. In H.carterae, Tmax for ammonium was up to four times that for nitrate. Relationships for ammonium and nitrate Tmax have been computed for simulations of the growth of E.huxleyi and H.carterae. The implications of these results for modelling ammonium-nitrate interactions are discussed. It is shown that attempts to obtain a relationship between Tmax and N:C from steady-state chemostat data may result in significant deviations from relationships determined using incubation techniques. Further, it is more important to obtain a good estimate for ammonium Tmax because of its dominant role in affecting the ammonium-nitrate interaction. [ABSTRACT FROM PUBLISHER]

Nitrite uptake in the nitrate-assimilating yeast, Candida nitratophila, was inhibited completely by addition of nitrate or ammonium to cultures at pH 8.0. Nitrite assimilation in these cultures was also completely dependent on the availability of a suitable carbon source (glucose). In cultures at pH 5.0, neither nitrate nor ammonium inhibited nitrite uptake completely. In these cultures, some nitrite was assimilated in the absence of a carbon source. Carbon-deficient cultures at pH 5.0 were able to reduce nitrite to ammonium but excreted this ammonium without further assimilation. Evidence for the uptake of nitrite by passive diffusion and by active transport via the nitrate uptake system is discussed. [ABSTRACT FROM AUTHOR]