Light responses ofBeggiatoa

Studies on nativeBeggiatoa demonstrated diel vertical migration into, and out of, sediments at the bottom of warm spring pools. Laboratory experiments withBeggiatoa in natural sediments suggested that high light was the cause of the downward movement. The nature of this presumed photomotion was clarified by microscopic observation of individual filaments of nativeBeggiatoa at light/dark boundaries where the light was varied in intensity and quality. Using white light, a negative photo-response was demonstrated, and a dose-response curve was constructed which indicates an increasing response to light over three orders of magnitude of intensity. A coarse action spectrum implicated a pigment with a peak in the blue region as the receptor. Pure culture studies showed the negative response to be a step-up phobic one. The light intensity increase necessary to invoke reversals was a smaller percentage of the initial intensity for higher initial intensities. The light intensity levels and gradient strengths necessary to evoke reversals in single filaments were consistent with the hypothesis that the step-up response accounts for the disappearance in the field. This response has adaptive significance since full sunlight was completely inhibitory toBeggiatoa growth, even when filaments were aggregated in tufts. Dilute suspensions were also inhibited by as little as 5000 lux (fluorescent lamps).     

Experiments with Light

This article describes an unlikely collaboration between a high school chemistry teacher and a high school English teacher who attempted to teach scientific concepts through poetry. Inspired by poet John Updike's (1960) "Cosmic Gall," these two teachers crafted writing tasks aimed at teaching science content through literary devices. The result was an inspiring account and unlikely marriage of academic disciplines.     

Interaction between Light Quality and Light Quantity in the Photoregulation of Anthocyanin Production

The interaction between phytochrome photoequilibrium () and photon flux in the photoregulation of anthocyanin production under prolonged irradiation was studied in seedlings of Brassica oleracea L. and Lycopersicon esculentum Mill. In cabbage, anthocyanin production increases with decreasing , reaching a maximum at the lowest value ( = 0.13) used in this study; in tomato, the extent of the response is higher at intermediate values, reaching a maximum at = 0.46. In cabbage, the response increases with increasing photon flux at all values; however, the response to changes in photon flux is minimal at = 0.85, and, at = 0.13, minimal at photon fluxes higher than 5 micromolar per square meter per second. In tomato, the response increases with increasing photon flux at = 0.46, 0.65, and 0.85, the response to changes in photon fluxes being minimal at = 0.85; at = 0.13 and 0.29 the response first increases (significantly at = 0.29 and minimally at = 0.13) and then decreases with increasing photon fluxes, the transition occurring at about 1 micromolar per square meter per second at = 0.13, and at 5 micromolar per square meter per second at = 0.29. The patterns of light quality-quantity interaction in the photoregulation of anthocyanin production are significantly different in cabbage and tomato and are also significantly different than those observed for other photomorphogenic responses to prolonged irradiations.     

Light treatments of nail fungal infections

Nail fungal infections are notoriously persistent and difficult to treat which can lead to severe health impacts, particularly in the immunocompromized. Current antifungal treatments, including systemic and topical drugs, are prolonged and do not effectively provide a complete cure. Severe side effects are also associated with systemic antifungals, such as hepatotoxicity. Light treatments of onychomycosis are an emerging therapy that has localized photodynamic, photothermal or photoablative action. These treatments have shown to be an effective alternative to traditional antifungal remedies with comparable or better cure rates achieved in shorter times and without systemic side effects. This report reviews significant clinical and experimental studies in the field, highlighting mechanisms of action and major effects related to light therapy; in particular, the impact of light on fungal genetics.      

Light effects upon dry lettuce seeds

Germination of certain dry seeds (achenes) of Lactuca sativa L. cv. Grand Rapids was increased to ca. 75% after irradiation with 665 nm red light (R; 1x10³ J m^-2); this response was eliminated by far-red light (FR) following the R. The response of dry seeds required an order of magnitude more light than that of wetted seeds, and was not maximal until 48 h after irradiation. Other seeds, which could not be stimulated by R in dry state, showed a partial response after 10 min hydration. Irradiation of dry seeds (or seeds wetted 1 h) with FR (1x10³ J m^-2) reduced dark germination from 26% to 2%. Seeds dehydrated in an oven (60°C, 90 min) showed a decrease in germination if irradiated with R (1x10⁵ J m^-2) before wetting. The results show that phytochrome is present in dry lettuce seeds (and functional in some seed lots) prior to wetting; and that in other seed lots the molecule becomes functional within minutes after wetting the seeds. Transformation of the FR absorbing from of phytochrome (P_FR) to the inactive from (P_R) occurs at lower seed moisture content than the reverse reaction. It appears that dormancy in seeds ripened in sunlight might be assured during seed drying and maturation by the more effective transformation of P_FR to P_R than vice versa as phytochrome is dehydrated.Abbreviations FR far-red - R red - CAL seeds from California - NC seeds from North Carolina (see text)     

Light penetrance in lake kinneret

The characteristics of light penetrance in Lake Kinneret, Israel, were observed over the years 1970 to 1973. Light measurements were made concurrently with those of algal speciation and biomass, chlorophyll concentrations and primary production. Vertical extinction coefficients of green light (filter VG9), the most penetrating spectral component, ranged from 0.15 (August 1970) to 0.93 In units m^–1 (April 1970), reflecting the large differences between algal standing crops in non-bloom and bloom seasons. During the dinoflagellate bloom (Peridinium cinctum fa westii) from February through June, the increment of extinction coefficient per unit increase of chlorophyll concentration was 0.006 ln units mg^–1 m². The uneven vertical distribution of algae at this period caused irregularities in the depth curves of light penetrance. At other times, when the phytoplankton cells were more homogeneously dispersed with depth, regular light penetrance curves were observed; however, as previously noted (Rodhe, 1972), attenuation of algal photosynthetic activity often appeared to be regulated by the blue spectral component (filter BG 12). Ratios of absorbed to scattered light in the upper water column ranged from 85:15 to 75:25.     

生物组织光传输理论概论

组织光学是一个光学与生命科学相互交叉相互渗透的新兴研究领域,其根本任务是为医用光子技术的发展提供理论基础。本文集中介绍了现有的生物组织光传输理论,它是组织光学研究的基本内容：同时评述了现有光传输理论,如漫射近似,Monte Carlo模拟等的优缺点及适用条件;最后,探讨了处理实际光传输问题时近似理论的合理选择问题。     

Presynaptically Silent Synapses Studied with Light Microscopy

Synaptic plasticity likely underlies the nervous system''s ability to learn and remember and may also represent an adaptability that prevents otherwise damaging insults from becoming neurotoxic. We have been studying a form of presynaptic plasticity that is interesting in part because it is expressed as a digital switching on and off of a presynaptic terminal s ability to release vesicles containing the neurotransmitter glutamate. Here we demonstrate a protocol for visualizing the activity status of presynaptic terminals in dissociated cell cultures prepared from the rodent hippocampus. The method relies on detecting active synapses using staining with a fixable form of the styryl dye FM1-43, commonly used to label synaptic vesicles. This staining profile is compared with immunostaining of the same terminals with an antibody directed against the vesicular glutamate transporter 1 (vGluT-1), a stain designed to label all glutamate synapses regardless of activation status. We find that depolarizing stimuli induce presynaptic silencing. The population of synapses that is silent under baseline conditions can be activated by prolonged electrical silencing or by activation of cAMP signaling pathways.Open in a separate windowClick here to view.^{(61M, flv)}     

UV Light Inactivation of Bacterial Biothreat Agents

Seven species of bacterial biothreat agents were tested for susceptibility to UV light (254 nm). All gram-negative organisms tested required <12 mJ/cm² for a 4-log₁₀ reduction in viability (inactivation). Tailing off of the B. anthracis spore inactivation curves began close to the 2-log₁₀ inactivation point, with a fluence of approximately 40 mJ/cm², and 3-log₁₀ inactivation still was not achieved with a fluence of 120 mJ/cm².The security of our nation''s water supply is a concern for water providers and public health officials. Questions have been asked regarding the possibility of our drinking water becoming contaminated with biothreat agents and the efficacy of current disinfection practices for the reduction in viability (inactivation) of biothreat agents (5, 8, 14). The use of UV irradiation as a supplemental water disinfection practice is increasing for several reasons, among them improving control of protozoa, such as Cryptosporidium spp., and decreasing disinfection by-products created by chemical disinfectants (21). This study employed a bench-scale collimated beam test to determine the UV fluence (dose) required to inactivate seven representative bacterial biothreat agents.Seven species, two isolates each, from the Health and Human Services and U.S Dept. of Agriculture lists of select agents (http://www.selectagents.gov/resources/List%20of%20Select%20Agents%20and%20Toxins_111708.pdf) were used in this study: Bacillus anthracis Ames spores, B. anthracis 34F2 (Sterne) spores, Brucella melitensis ATCC 23456, B. melitensis IL195, Brucella suis KS528, B. suis MO562, Burkholderia mallei M9, B. mallei M13, Burkholderia pseudomallei ATCC 11688, B. pseudomallei CA650, Francisella tularensis LVS, F. tularensis NY98, Yersinia pestis A1122, and Y. pestis Harbin. B. anthracis was grown on soil extract-peptone-beef extract agar (SEA) (1) or in Schaeffer''s sporulation medium (SSM) (10) for 7 days, resulting in >99% spores as determined by phase-contrast microscopy. The cells and spores were then washed by centrifugation (8,000 × g), resuspended in ultrapure water, transferred to centrifuge tubes, treated with 50% ethanol for 1 h at room temperature, and washed five times with sterile ultrapure water before being stored in reverse-osmosis water at −70°C. F. tularensis isolates were grown on cysteine heart agar (1) and all other isolates on Trypticase soy agar with 5% sheep blood (TSA II; Becton Dickinson Microbiology Systems, Sparks, MD) for 24 h before testing. B. anthracis spores were adjusted to 10⁷ CFU/ml and other bacterial suspensions to 10⁸ CFU/ml in Butterfield buffer (3 mM KH₂PO₄, pH 7.2; Becton Dickinson Microbiology Systems), and then isolates were sonicated (40-Hz ultrasonic cleaner; VWR, Suwanee, GA) for 1 min to disperse aggregates. The suspensions were diluted 1:100 in Butterfield buffer for final test concentrations. Five milliliters of each suspension were placed into a small petri dish (50-mm diameter) along with a small sterile stir bar, and the petri dish was placed on a stir plate.UV irradiation was performed by using a collimated beam apparatus (Calgon Carbon, Pittsburgh, PA) equipped with a low-pressure lamp (254 nm) according to the standard method developed by Bolton and Linden (2). The surface of the suspension was placed 5 cm from the end of the collimating tube. The UV intensity was measured with a radiometer at 0.5-cm intervals across the test area and variability compensated for according to the UVCalc software directions (International UV association [http://www.iuva.org]). The fluences (UV doses) were determined using the UVCalc software, and the petri dishes were placed under the beam for at least five time periods to deliver a range of appropriate fluences to the organisms. Each irradiation test was conducted at room temperature (23 ± 2°C) in triplicate and in a random order of fluences. After exposure, 10-fold serial dilutions were performed, and the dilutions were spread plated in triplicate. Plates were placed in a dark incubator within 10 min of plating and incubated at the temperature appropriate for the organism, and the colonies were counted at 24 h and 48 h for B. anthracis and at 3 and 5 days for the remaining organisms. Colonies were counted, and the log₁₀ inactivation at each fluence was determined for each organism. A linear regression of the fluence response data determined the fluence required for 2-, 3-, and 4-log₁₀ inactivation.The UV fluences required for inactivation of each organism are reported in Table ​Table1.1. Little difference in UV susceptibility was seen between the gram-negative organisms. B. suis KS528 and B. melitensis ATCC 23456 required the greatest UV fluence of the gram-negative organisms for 4-log₁₀ inactivation (10.5 and 10.2 mJ/cm², respectively), while the two Y. pestis isolates required the lowest UV fluence (4.1 and 4.9 mJ/cm²) for the same 4-log₁₀ inactivation (Table ​(Table1).1). Generally, the two isolates of each species differed no more than 3 mJ/cm² in the UV fluence required for 4-log₁₀ inactivation.

TABLE 1.

UV fluence required for given log₁₀ inactivation of each organism

Major Components of the Light Microscope

The light microscope is a basic tool for the cell biologist, who should have a thorough understanding of how it works, how it should be aligned for different applications, and how it should be maintained as required to obtain maximum image-forming capacity and resolution. The components of the microscope are described in detail here.Download video file.^{(89M, mp4)}     

Effects of Light on Transport by Azolla pinnata

Effects of light flux density (LFD) during growth and uptakeassay on induction of transport system and kinetics of transport were studied using the Azolla pinnata-Anabaena azollae association (Azolla). Theinduction and uptake kinetics of the transport system were determined using an automated system that measuredthe NO₃ concentration in the growth medium as a function oftime, using an on-line high performance liquid chromatograph(HPLC) with a UV-VIS detector. Full induction of the transport system required about 1.5 to 2.0 h and occurred without any apparent lag phase,regardless of the LFD provided. The level of induction of transport of Azolla grown at 600 µmol m^–2s^–1 LFD was higher than for that grown at 100 µmolm^–2 s^–1. Similarly, 600 µmol m^–1 s^–1LFD during the assay resulted in a higher level of inductionthan did 100 umol m^–2 s^–1. An increase in the LFDeither during the growth or the assay period increased the uptake rate; however, an increase in LFD duringthe latter period had greater effect. Azolla grown and assayedat 600 umol m^–2 s^–1 had the highest uptake rate. The uptake rate at 50 cm³ m^–3ambient CO₂ concentration was initially higher than at 305 cm³m^–3, but the uptake rate decreased rapidly with time andeventually dropped below that at 305 cm³ m^–3 CO₂. Thesedata suggest that the energy required for transport in Azolla may bypass the photosynthetic CO₂ fixationand carbon-cycling. Key words: carbon dioxide, concentration dependence, light flux density, uptake     

Plants Actively Avoid State Transitions upon Changes in Light Intensity: Role of Light-Harvesting Complex II Protein Dephosphorylation in High Light

Photosystem II (PSII) core and light-harvesting complex II (LHCII) proteins in plant chloroplasts undergo reversible phosphorylation upon changes in light intensity (being under control of redox-regulated STN7 and STN8 kinases and TAP38/PPH1 and PSII core phosphatases). Shift of plants from growth light to high light results in an increase of PSII core phosphorylation, whereas LHCII phosphorylation concomitantly decreases. Exactly the opposite takes place when plants are shifted to lower light intensity. Despite distinct changes occurring in thylakoid protein phosphorylation upon light intensity changes, the excitation balance between PSII and photosystem I remains unchanged. This differs drastically from the canonical-state transition model induced by artificial states 1 and 2 lights that concomitantly either dephosphorylate or phosphorylate, respectively, both the PSII core and LHCII phosphoproteins. Analysis of the kinase and phosphatase mutants revealed that TAP38/PPH1 phosphatase is crucial in preventing state transition upon increase in light intensity. Indeed, tap38/pph1 mutant revealed strong concomitant phosphorylation of both the PSII core and LHCII proteins upon transfer to high light, thus resembling the wild type under state 2 light. Coordinated function of thylakoid protein kinases and phosphatases is shown to secure balanced excitation energy for both photosystems by preventing state transitions upon changes in light intensity. Moreover, PROTON GRADIENT REGULATION5 (PGR5) is required for proper regulation of thylakoid protein kinases and phosphatases, and the pgr5 mutant mimics phenotypes of tap38/pph1. This shows that there is a close cooperation between the redox- and proton gradient-dependent regulatory mechanisms for proper function of the photosynthetic machinery.Photosynthetic light reactions take place in the chloroplast thylakoid membrane. Primary energy conversion reactions are performed by synchronized function of the two light energy-driven enzymes PSII and PSI. PSII uses excitation energy to split water into electrons and protons. PSII feeds electrons to the intersystem electron transfer chain (ETC) consisting of plastoquinone, cytochrome b₆f, and plastocyanin. PSI oxidizes the ETC in a light-driven reduction of NADP to NADPH. Light energy is collected by the light-harvesting antenna systems in the thylakoid membrane composed of specific pigment-protein complexes (light-harvesting complex I [LHCI] and LHCII). The majority of the light-absorbing pigments are bound to LHCII trimers that can serve the light harvesting of both photosystems (Galka et al., 2012; Kouřil et al., 2013; Wientjes et al., 2013b). Energy distribution from LHCII is regulated by protein phosphorylation (Bennett, 1979; Bennett et al., 1980; Allen et al., 1981) under control of the STN7 and STN8 kinases (Depège et al., 2003; Bellafiore et al., 2005; Bonardi et al., 2005; Vainonen et al., 2005) and the TAP38/PPH1 and Photosystem II Core Phosphatase (PBCP) phosphatases (Pribil et al., 2010; Shapiguzov et al., 2010; Samol et al., 2012). LHCII trimers are composed of LHCB1, LHCB2, and LHCB3 proteins, and in addition to reversible phosphorylation of LHCB1 and LHCB2, the protein composition of the LHCII trimers also affects the energy distribution from the light-harvesting system to photosystems (Damkjaer et al., 2009; Pietrzykowska et al., 2014). Most of the LHCII trimers are located in the PSII-rich grana membranes and PSII- and PSI-rich grana margins of the thylakoid membrane, and only a minor fraction resides in PSI- and ATP synthase-rich stroma lamellae (Tikkanen et al., 2008b; Suorsa et al., 2014). Both photosystems bind a small amount of LHCII trimers in biochemically isolatable PSII-LHCII and PSI-LHCII complexes (Pesaresi et al., 2009; Järvi et al., 2011; Caffarri et al., 2014). The large portion of the LHCII, however, does not form isolatable complexes with PSII or PSI, and therefore, it separates as free LHCII trimers upon biochemical fractionation of the thylakoid membrane by Suc gradient centrifugation or in native gel analyses (Caffarri et al., 2009; Järvi et al., 2011), the amount being dependent on the thylakoid isolation method. Nonetheless, in vivo, this major LHCII antenna fraction serves the light-harvesting function. This is based on the fact that fluorescence from free LHCII, peaking at 680 nm in 77-K fluorescence emission spectra, can only be detected when the energy transfer properties of the thylakoid membrane are disturbed by detergents (Grieco et al., 2015).Regulation of excitation energy distribution from LHCII to PSII and PSI has, for decades, been linked to LHCII phosphorylation and state transitions (Bennett, 1979; Bennett et al., 1980; Allen et al., 1981). It has been explained that a fraction of LHCII gets phosphorylated and migrates from PSII to PSI, which can be evidenced as increase in PSI cross section and was assigned as transition to state 2 (for review, see Allen, 2003; Rochaix et al., 2012). The LHCII proteins are, however, phosphorylated all over the thylakoid membrane (i.e. in the PSII- and LHCII-rich grana core) in grana margins containing PSII, LHCII, and PSI as well as in PSI-rich stroma lamellae also harboring PSII-LHCII, LHCII, and PSI-LHCII complexes in minor amounts (Tikkanen et al., 2008b; Grieco et al., 2012; Leoni et al., 2013; Wientjes et al., 2013a)—making the canonical-state transition theory inadequate to explain the physiological role of reversible LHCII phosphorylation (Tikkanen and Aro, 2014). Moreover, the traditional-state transition model is based on lateral segregation of PSII-LHCII and PSI-LHCI to different thylakoid domains. It, however, seems likely that PSII and PSI are energetically connected through a shared light-harvesting system composed of LHCII trimers (Grieco et al., 2015), and there is efficient excitation energy transfer between the two photosystems (Yokono et al., 2015). Nevertheless, it is clear that LHCII phosphorylation is a prerequisite to form an isolatable PSI-LHCII complex called the state transition complex (Pesaresi et al., 2009; Järvi et al., 2011). Existence of a minor state transition complex, however, does not explain why LHCII is phosphorylated all over the thylakoid membrane and how the energy transfer is regulated from the majority of LHCII antenna that is shared between PSII and PSI but does not form isolatable complexes with them (Grieco et al., 2015).Plants grown under any steady-state white light condition show the following characteristics of the thylakoid membrane: PSII core and LHCII phosphoproteins are moderately phosphorylated, phosphorylation takes place all over the thylakoid membrane, and the PSI-LHCII state transition complex is present (Järvi et al., 2011; Grieco et al., 2012; Wientjes et al., 2013b). Upon changes in the light intensity, the relative phosphorylation level between PSII core and LHCII phosphoproteins drastically changes (Rintamäki et al., 1997, 2000) in the timescale of 5 to 30 min. When light intensity increases, the PSII core protein phosphorylation increases, whereas the level of LHCII phosphorylation decreases. On the contrary, a decrease in light intensity decreases the phosphorylation level of PSII core proteins but strongly increases the phosphorylation of the LHCII proteins (Rintamäki et al., 1997, 2000). The presence and absence of the PSI-LHCII state transition complex correlate with LHCII phosphorylation (similar to the state transitions; Pesaresi et al., 2009; Wientjes et al., 2013b). Despite all of these changes in thylakoid protein phosphorylation, the relative excitation of PSII and PSI (i.e. the absorption cross section of PSII and PSI measured by 77-K fluorescence) remains nearly unchanged upon changes in white-light intensity (i.e. no state transitions can be observed despite massive differences in LHCII protein phosphorylation; Tikkanen et al., 2010).The existence of the opposing behaviors of PSII core and LHCII protein phosphorylation, as described above, has been known for more than 15 years (Rintamäki et al., 1997, 2000), but the physiological significance of this phenomenon has remained elusive. It is known that PSII core protein phosphorylation in high light (HL) facilitates the unpacking of PSII-LHCII complexes required for proper processing of the damaged PSII centers and thus, prevents oxidative damage of the photosynthetic machinery (Tikkanen et al., 2008a; Fristedt et al., 2009; Goral et al., 2010; Kirchhoff et al., 2011). It is also known that the damaged PSII core protein D1 needs to be dephosphorylated before its proteolytic degradation upon PSII turnover (Koivuniemi et al., 1995). There is, however, no coherent understanding available to explain why LHCII proteins are dephosphorylated upon exposure of plants to HL and PSII core proteins are dephosphorylated upon exposure to low light (LL).The above-described light quantity-dependent control of thylakoid protein phosphorylation drastically differs from the light quality-dependent protein phosphorylation (Tikkanen et al., 2010). State transitions are generally investigated by using different light qualities, preferentially exciting either PSI or PSII. State 1 light favors PSI excitation, leading to oxidation of the ETC and dephosphorylation of both the PSII core and LHCII proteins. State 2 light, in turn, preferentially excites PSII, leading to reduction of ETC and strong concomitant phosphorylation of both the PSII core and LHCII proteins (Haldrup et al., 2001). Shifts between states 1 and 2 lights induce state transitions, mechanisms that change the excitation between PSII and PSI (Murata and Sugahara, 1969; Murata, 2009). Similar to shifts between state lights, the shifts between LL and HL intensity also change the phosphorylation of the PSII core and LHCII proteins (Rintamäki et al., 1997, 2000). Importantly, the white-light intensity-induced changes in thylakoid protein phosphorylation do not change the excitation energy distribution between the two photosystems (Tikkanen et al., 2010). Despite this fundamental difference between the light quantity- and light quality-induced thylakoid protein phosphorylations, a common feature for both mechanisms is a strict requirement of LHCII phosphorylation for formation of the PSI-LHCII complex. However, it is worth noting that LHCII phosphorylation under state 2 light is not enough to induce the state 2 transition but that the P-LHCII docking proteins in the PSI complex are required (Lunde et al., 2000; Jensen et al., 2004; Zhang and Scheller, 2004; Leoni et al., 2013).Thylakoid protein phosphorylation is a dynamic redox-regulated process dependent on the interplay between two kinases (STN7 and STN8; Depège et al., 2003; Bellafiore et al., 2005; Bonardi et al., 2005; Vainonen et al., 2005) and two phosphatases (TAP38/PPH1 and PBCP; Pribil et al., 2010; Shapiguzov et al., 2010; Samol et al., 2012). Concerning the redox regulation mechanisms in vivo, only the LHCII kinase (STN7) has so far been thoroughly studied (Vener et al., 1997; Rintamäki et al., 2000; Lemeille et al., 2009). The STN7 kinase is considered as the LHCII kinase, and indeed, it phosphorylates the LHCB1 and LHCB2 proteins (Bellafiore et al., 2005; Bonardi et al., 2005; Tikkanen et al., 2006). In addition to this, STN7 takes part in the phosphorylation of PSII core proteins (Vainonen et al., 2005), especially in LL (Tikkanen et al., 2008b, 2010). The STN8 kinase is required for phosphorylation of PSII core proteins in HL but does not significantly participate in phosphorylation of LHCII (Bellafiore et al., 2005; Bonardi et al., 2005; Vainonen et al., 2005; Tikkanen et al., 2010). It has been shown that, in traditional state 1 condition, which oxidizes the ETC, the dephosphorylation of LHCII is dependent on TAP38/PPH1 phosphatase (Pribil et al., 2010; Shapiguzov et al., 2010), whereas the PSII core protein dephosphorylation is dependent on the PBCP phosphatase (Samol et al., 2012). However, it remains unresolved whether and how the TAP38/PPH1 and PBCP phosphatases are involved in the light intensity-dependent regulation of thylakoid protein phosphorylation typical for natural environments.Here, we have used the two kinase (stn7 and stn8) and the two phosphatase (tap38/pph1and pbcp) mutants of Arabidopsis (Arabidopsis thaliana) to elucidate the individual roles of these enzymes in reversible thylakoid protein phosphorylation and distribution of excitation energy between PSII and PSI upon changes in light intensity. It is shown that the TAP38/PPH1-dependent, redox-regulated LHCII dephosphorylation is the key component to maintain excitation balance between PSII and PSI upon increase in light intensity, which at the same time, induces strong phosphorylation of the PSII core proteins. Collectively, reversible but opposite phosphorylation and dephosphorylation of the PSII core and LHCII proteins upon increase or decrease in light intensity are shown to be crucial for maintenance of even distribution of excitation energy to both photosystems, thus preventing state transitions. Moreover, evidence is provided indicating that the pH gradient across the thylakoid membrane is yet another important component in regulation of the distribution of excitation energy to PSII and PSI, possibly by affecting the regulation of thylakoid kinases and phosphatases.     

Supercritical Fluid Extraction of a Light PAH Contaminated Sand

This work was aimed at evaluating the feasibility of a remediation treatment performed by means of a supercritical carbon dioxide extraction on a sandy soil recently contaminated by light polycyclic aromatic hydrocarbons.

The soil utilized in this study was artificially contaminated by naphthalene and anthracene. The artificial contamination process was intended to simulate a recent accidental spillage of hydrocarbon fuels.

Several extractions, aimed at singling out the operating parameters (pressure, temperature, supercritical fluid mass flow rate) that are able to obtain the residual required concentration (50 mg/kg dry soil) in the shortest time, were carried out on a on-purpose made system.

The best extraction conditions were 120 bar and 40°C for a naphthalene contaminated soil and 200 bar and 80–100°C for an anthracene contaminated soil.

The results obtained in the experimental tests made it possible to build an analytical model able to correlate, for the given soil, the extraction length to the operating parameters such as supercritical fluid density, temperature and mass flow rate.

In order to evaluate the economic feasibility of the process, a unit treatment cost was evaluated for the case of an extraction carried out in a 10 m³ reactor in the presence of the best extraction conditions that were previously determined. The extraction unit cost was therefore equal to 35 000–65 000 €/t for a soil with a starting contaminant concentration equal to 1000 mg/kg of dry soil.     

Whole-cell Recordings of Light Evoked Excitatory Synaptic Currents in the Retinal Slice

We use the whole-cell patch clamp technique to study the synaptic circuitry that underlies visual information processing in the retina. In this video, we will guide you through the process of performing whole-cell recordings of light evoked currents of individual cells in the retinal slice preparation. We use the aquatic tiger salamander as an animal model. We begin by describing the dissection of the eye and show how slices are mounted for electrophysiological recordings. Once the slice is placed in the recording chamber, we demonstrate how to perform whole-cell voltage clamp recordings. We then project visual stimuli onto the photoreceptors in the slice to elicit light-evoked current responses. During the recording we perfuse the slice with pharmacological agents, whereby an 8-channel perfusion system allows us to quickly switch between different agents. The retinal slice preparation is widely used for patch clamp recordings in the retina, in particular to study amacrine or bipolar cells, which are not accessible in a whole-mount preparation.Download video file.^{(217M, mp4)}     

A Dual Strategy to Cope with High Light in Chlamydomonas reinhardtii

Absorption of light in excess of the capacity for photosynthetic electron transport is damaging to photosynthetic organisms. Several mechanisms exist to avoid photodamage, which are collectively referred to as nonphotochemical quenching. This term comprises at least two major processes. State transitions (qT) represent changes in the relative antenna sizes of photosystems II and I. High energy quenching (qE) is the increased thermal dissipation of light energy triggered by lumen acidification. To investigate the respective roles of qE and qT in photoprotection, a mutant (npq4 stt7-9) was generated in Chlamydomonas reinhardtii by crossing the state transition–deficient mutant (stt7-9) with a strain having a largely reduced qE capacity (npq4). The comparative phenotypic analysis of the wild type, single mutants, and double mutants reveals that both state transitions and qE are induced by high light. Moreover, the double mutant exhibits an increased photosensitivity with respect to the single mutants and the wild type. Therefore, we suggest that besides qE, state transitions also play a photoprotective role during high light acclimation of the cells, most likely by decreasing hydrogen peroxide production. These results are discussed in terms of the relative photoprotective benefit related to thermal dissipation of excess light and/or to the physical displacement of antennas from photosystem II.     

Light Harvesting and Utilization by Phytoplankton

In this study we use a model based on target theory to analyzesteady-state photosynthesis-irradiance relationships in continuouslight. From the average turnover time () of photosynthetic units(PSU_O2), numerical analyses of the model coefficients, and measurementsof the light field and cell absorptivity, apparent absorptioncrosssections of photosystem II (PSII) were determined for three species of marine unicellular algaegrown at different irradiance levels. These cross-sections generally,but not always, increased with decreased growth irradiance.Additionally, the ratios of photosystem I/photosystem II reactioncenters were calculated from measurements of oxygen flash yieldsand chlorophyll/P₇₀₀ ratios. From the ratios of the reactioncenters, cell absorptivity and the apparent absorption cross-sectionof photosystem II, the apparent absorption cross-sections ofphotosystem I (PSI) were also calculated. Finally, on the basis of our calculated absorptioncross-sections, we estimated the minimum quantum requirementsfor O₂ evolution. Our results suggest that the absorption cross-sectionsof PS I and PS II vary independently and the minimum quantumrequirements for O₂ vary by more than twofold, increasing from9.1 to 20.6 quanta/O₂, as growth irradiance increases. The increasein quantum requirement corresponds to larger apparent cross-sectionsfor photosystem I and higher carotenoid/chlorophyll ratios. (Received October 15, 1985; Accepted July 17, 1986)     

Arabidopsis Phytochrome B Promotes SPA1 Nuclear Accumulation to Repress Photomorphogenesis under Far-Red Light

Phytochrome A (phyA) is the primary photoreceptor mediating deetiolation under far-red (FR) light, whereas phyB predominantly regulates light responses in red light. SUPPRESSOR OF PHYA-105 (SPA1) forms an E3 ubiquitin ligase complex with CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1), which is responsible for the degradation of various photomorphogenesis-promoting factors, resulting in desensitization to light signaling. However, the role of phyB in FR light signaling and the regulatory pathway from light-activated phytochromes to the COP1-SPA1 complex are largely unknown. Here, we confirm that PHYB overexpression causes an etiolation response with reduced ELONGATED HYPOCOTYL5 (HY5) accumulation under FR light. Notably, phyB exerts its nuclear activities and promotes seedling etiolation in both the presence and absence of phyA in response to FR light. PhyB acts upstream of SPA1 and is functionally dependent on it in FR light signaling. PhyB interacts and forms a protein complex with SPA1, enhancing its nuclear accumulation under FR light. During the dark-to-FR transition, phyB is rapidly imported into the nucleus and facilitates nuclear SPA1 accumulation. These findings support the notion that phyB plays a role in repressing FR light signaling. Activity modulation of the COP1-SPA E3 complex by light-activated phytochromes is an effective and pivotal regulatory step in light signaling.     

Light maintains High Levels of Phytochrome Intermediates

THE plant photomorphogenetic pigment phytochrome exists in two forms, Pr and Pfr, interconvertible by light, which have peaks of absorption in the red and far-red regions of the spectrum respectively¹. Intermediates between Pr and Pfr have been demonstrated during photoconversion by Linschitz and his coworkers^2,3 using flash photolysis techniques. Low temperatures studies have also proved useful in identifying intermediates^4–8. Briggs and Fork^9,10 detected intermediates in vitro and in vivo in conditions of pigment cycling by mixed red and far-red light, but were restricted to studying the minor peaks of phytochrome absorption in the blue region of the spectrum because of the available instrumentation. In this type of measurement the problem is that actinic light has to be prevented from falling onto the photomultiplier. Briggs and Fork inserted a red cutoff filter, but this precluded measurement at the peaks of absorption of Pr and Pfr in the red and far-red regions of the spectrum. The design and construction of a sensitive quasi-continuous measuring spectrophotometer have now overcome this problem and made possible an investigation in vivo of phytochrome intermediates at any wavelength under conditions of pigment cycling, for example, in high intensity white incandescent light. The instrument can detect intermediates with a half life in excess of 0.2 ms. The longer lived intermediates between Pr and Pfr observed in the in vitro flash studies^2,3 should be readily detectable if they accumulate in conditions of cycling.