Decrease In Absorption Cross Section Research Articles

Growth and fatty acid distribution over lipid classes in Nannochloropsis oceanica acclimated to different temperatures.

After light, temperature is the most relevant environmental parameter in outdoors cultivation of microalgae. Suboptimal and supraoptimal temperatures negatively impact growth and photosynthetic performance with a subsequent effect on lipid accumulation. It is generally recognised that lower temperatures trigger an increase in fatty acid desaturation while higher temperatures trigger the opposite reaction. The effect of temperature on lipid classes has been less studied in microalgae and in certain cases, the effect of light cannot be completely excluded. In this research, the effect of temperature on growth, photosynthesis, and lipid class accumulation in Nannochloropsis oceanica was studied at a fixed light gradient with a constant incident light intensity (670 μmol m-2 s-1). A turbidostat approach was used to achieve temperature acclimated cultures of Nannochloropsis oceanica. Optimal growth was found at 25-29°C, while growth was completely arrested at temperatures higher than 31°C and lower than 9°C. Acclimation to low temperatures triggered a decrease in absorption cross section and photosynthesis rates with a tipping point at 17°C. Reduced light absorption was correlated with a decrease in content of the plastid lipids monogalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol. The increase of diacylglyceryltrimethylhomo-serine content at lower temperatures indicated a relevant role of this lipid class in temperature tolerance. Triacylglycerol content increased at 17°C and decreased at 9°C emphasising a metabolic switch in stress response. Total and polar eicosapentaenoic acid content remained constant at 3.5 and 2.4% w/w, despite the fluctuating lipid contents. Results show an extensive mobilisation of eicosapentaenoic acid between polar lipids classes at 9°C to ensure cell survival under critical conditions.

Influence of the Ga addition on optical properties of Pr in Ge–Sb–Se glasses

Absorption and emission spectra for the 3H 4↔( 3F 2, 3H 6) transition of Pr 3+ ions embedded in Ge–Sb–Se glasses turned out to change systematically upon the introduction of a small amout of Ga. Clear blueshift of the absorption peak wavelengths together with the decrease of absorption cross-section was evident in these glasses containing Ga. We believe that the Ga addition into the conventional covalent selenide glasses makes chemical bonds between rare earth atoms and Se atoms more ionic due to preferential location of the GaSe 4 tetrahedra at the second coordination shell of a rare earth atom. Taking into consideration the hypersensitive nature of the Pr 3+: 3H 4↔ 3F 2 transition, the observed blueshift may manifest the enhanced ionicity of the chemical bonds between Pr and Se in the current Ga-containing glasses.

Relation between the mode of binding of nitrite and the energy distribution between the two photosystems

The reversibility of nitrite‐induced inhibition in relation to energy distribution between the two photosystems was studied in spinach thylakoid membranes. Measurements of electron transfer rate catalyzed by photosystem I (PS I) and photosystem II (PS II), chlorophyll a (Chl a) fluorescence induction kinetics, S2 state multiline spectra, and room temperature electron paramagnetic resonance (EPR) signals indicated that nitrite anions bind PS II in two ways: dissociable (loose) and non‐dissociable (tight). The inhibition caused by the dissociable binding was reversible in washed (nitrite‐treated samples washed with nitrite‐free medium) samples, while the inhibition caused by the non‐dissociable binding was irreversible. At 77 K, an increase in absorption cross section of PS I (as inferred from the excitation spectra of Chl a fluorescence) and a decrease in absorption cross section of PS II in nitrite‐treated sample when compared with sample washed with nitrite‐free medium and control sample suggested that nitrite plays a role in regulating the distribution of absorbed excitation energy between the two photosystems. We propose, for the first time, that the removal of loosely bound nitrite leads to migration of light‐harvesting complex II back to the PS II, and thus the mode of binding of nitrite regulates the extent of migration of antenna molecules between the two photosystems.

Characterization of the non-photochemical quenching of chlorophyll fluorescence that occurs during the active accumulation of inorganic carbon in the cyanobacterium Synechococcus PCC 7942

Previous work has shown that the maximum fluorescence yield from PS 2 of Synechococcus PCC 7942 occurs when the cells are at the CO2 compensation point. The addition of inorganic carbon (Ci), as CO2 or HCO3 (-), causes a lowering of the fluorescence yield due to both photochemical (qp) and non-photochemical (qN) quenching. In this paper, we characterize the qN that is induced by Ci addition to cells grown at high light intensities (500 μmol photons m(-2) s(-1)). The Ci-induced qN was considerably greater in these cells than in cells grown at low light intensities (50 μmol photons m(-2) s(-1)), when assayed at a white light (WL) intensity of 250 μmol photons m(-2) s(-1). In high-light grown cells we measured qN values as high as 70%, while in low-light grown cells the qN was about 16%. The qN was relieved when cells regained the CO2 compensation point, when cells were illuminated by supplemental far-red light (FRL) absorbed mainly by PS 1, or when cells were illuminated with increased WL intensities. These characteristics indicate that the qN was not a form of energy quenching (qE). Supplemental FRL illumination caused significant enhancement of photosynthetic O2 evolution that could be correlated with the changes in qp and qN. The increases in qp induced by Ci addition represent increases in the effective quantum yield of PS 2 due to increased levels of oxidized QA. The increase in qN induced by Ci represents a decrease in PS 2 activity related to decreases in the potential quantum yield. The lack of diagnostic changes in the 77 K fluorescence emission spectrum argue against qN being related to classical state transitions, in which the decrease in potential quantum yield of PS 2 is due either to a decrease in absorption cross-section or by increased 'spill-over' of excitation energy to PS 1. Both the Ci-induced qp (t 0.5<0.5 s) and qN (t 0.5≃1.6 s) were rapidly relieved by the addition of DCMU. The two time constants give further support for two separate quenching mechanisms. We have thus characterized a novel form of qN in cyanobacteria, not related to state transitions or energy quenching, which is induced by the addition of Ci to cells at the CO2-compensation point.

Neutron Absorption Cross Sections ofU235andPu239in the 5- to 50-kev Energy Range

The absorption cross sections of ${\mathrm{U}}^{235}$ and ${\mathrm{Pu}}^{239}$ have been measured at six energies in the 5- to 50-kev energy range. Neutrons from the reaction ${\mathrm{V}}^{51}(p, n){\mathrm{Cr}}^{51}$ were used in spherical-shell transmission measurements. The results show, in general, a decrease in absorption cross section with increasing energy up to about 30 kev and then a nearly constant value (up to 50 kev) of about 2.8 barns for ${\mathrm{U}}^{235}$ and 2.2 barns for ${\mathrm{Pu}}^{239}$.