Photosynthetic Light Absorption Research Articles

Green/red light-sensing mechanism in the chromatic acclimation photosensor.

Certain cyanobacteria alter their photosynthetic light absorption between green and red, a phenomenon called complementary chromatic acclimation. The acclimation is regulated by a cyanobacteriochrome-class photosensor that reversibly photoconverts between green-absorbing (Pg) and red-absorbing (Pr) states. Here, we elucidated the structural basis of the green/red photocycle. In the Pg state, the bilin chromophore adopted the extended C15-Z,anti structure within a hydrophobic pocket. Upon photoconversion to the Pr state, the bilin is isomerized to the cyclic C15-E,syn structure, forming a water channel in the pocket. The solvation/desolvation of the bilin causes changes in the protonation state and the stability of π-conjugation at the B ring, leading to a large absorption shift. These results advance our understanding of the enormous spectral diversity of the phytochrome superfamily.

Plant biostimulating effects of the cyanobacterium Nostoc piscinale on maize (Zea mays L.) in field experiments

Biostimulants, when applied to plants in small amounts, increase crop yield and plant tolerance to abiotic and biotic stress. They play an important role in the development of new environmentally sustainable technologies. The aim of the current experiment was to investigate the potential of a cyanobacterium strain (Nostoc piscinale) to improve the growth, grain yield and stress tolerance of maize (Zea mays SY Zephir hybrid). Field trials were established at two sites. Freeze-dried biomass of N. piscinale resuspended in tap water (1g/L DW) was applied as a single foliar treatment (400 L/ha) at the V6-V7 phenological stage. Number of leaves, chlorophyll content, relative water content (RWC%) and free proline content were measured weekly. Grain yield, yield components and grain protein content were measured at harvest. N. piscinale treated maize had significantly earlier development in the vegetative growth stages with a higher number of leaves. Chlorophyll content (SPAD value) was significantly higher in the treated plants during the reproductive stages. There was little difference in the RWC and proline content compared to control plants. Faster vegetative growth and higher chlorophyll content in the cyanobacterium treated plants meant great photosynthetic light absorption over a longer period of time, resulting in significantly higher grain yield (6.5% and 11.5% at the two production sites) and increased grain protein content. Grain yield was significantly influenced by cob length and thousand grain weight. In conclusion, it was proved in field trials conducted in two different regions in Hungary that a single foliar application of a cyanobacterium-based biostimulant can contribute to crop production in a sustainable and environmentally friendly manner.

Effect of high temperature on the balance between photosynthetic light absorption and energy utilization in Chlorella pyrenoidosa (Chlorophyceae)

Chlorella pyrenoidosa (Chlorophyceae) is widely cultured for production of health food and animal feed. In outdoors, mass cultivation of C. pyrenoidosa often suffers from high temperature. A better understanding of the effects of high temperature on photosynthesis and photoprotection can help optimize the productivity of C. pyrenoidosa cultures. In this study, we investigated effect of high temperature (35, 38, or 41°C) on the balance between photosynthetic light absorption and energy utilization of C. pyrenoidosa. In contrast to 30°C, higher temperature of 35 or 38°C did not inhibit the growth of C. pyrenoidosa. Treatment in 35°C maintained the balance. Moreover, the PSI acceptor side in 38°C was over-reduced and PSII reaction centers were over-excited under strong light, which destroyed the balance and generated active oxygen species (AOS). However, the activated antioxidant enzymes might remove completely the over-production of AOS, thereby protect C. pyrenoidosa cells from photodamage. It shows that this C. pyrenoidosa strain could tolerate as high as 38°C. Furthermore, treatment in 41°C resulted in more lack of the balance than that in 38°C. However, the activities of antioxidant enzymes stopped increasing in 41°C, and were not strong enough to remove the excess AOS. Therefore, treatment in 41°C could decrease the growth of C. pyrenoidosa. In addition, strong and longtime light exposure would cause serious photodamage to C. pyrenoidosa cells.

Diurnal regulation of photosynthetic light absorption, electron transport and carbon fixation in two contrasting oceanic environments

Abstract. Understanding the dynamics of marine phytoplankton productivity requires mechanistic insight into the non-linear coupling of light absorption, photosynthetic electron transport and carbon fixation in response to environmental variability. In the present study, we examined the variability of phytoplankton light absorption characteristics, light-dependent electron transport and 14C-uptake rates over a 48 h period in the coastal subarctic north-east (NE) Pacific. We observed an intricately coordinated response of the different components of the photosynthetic process to diurnal irradiance cycles, which acted to maximize carbon fixation, while simultaneously preventing damage by excess absorbed light energy. In particular, we found diurnal adjustments in pigment ratios, excitation energy transfer to reaction centre II (RCII), the capacity for non-photochemical quenching (NPQ), and the light efficiency (α) and maximum rates (Pmax) of RCII electron transport (ETRRCII) and 14C uptake. Comparison of these results from coastal waters to previous observations in offshore waters of the subarctic NE Pacific provides insight into the effects of iron limitation on the optimization of photosynthesis. Under iron-limited, low-biomass conditions, there was a significant reduction of iron-rich photosynthetic units per chlorophyll a, which was partly offset by higher light absorption and electron transport per photosystem II (PSII). Iron deficiency limited the capacity of phytoplankton to utilize peak midday irradiance for carbon fixation and caused an upregulation of photoprotective mechanisms, including NPQ, and the decoupling of light absorption, electron transport and carbon fixation. Such decoupling resulted in an increased electron requirement (Φe,C) and decreased quantum efficiency (ΦC) of carbon fixation at the iron-limited station. In both coastal and offshore waters, Φe,C and ΦC correlated strongly to NPQ, albeit with a significantly different slope. We discuss the implications of our results for the interpretation of bio-optical data and the parameterization of numerical productivity models, both of which are vital tools in monitoring marine photosynthesis over large temporal and spatial scales.

LHCSR3 is a nonphotochemical quencher of both photosystems in Chlamydomonas reinhardtii.

Photosynthetic organisms prevent oxidative stress from light energy absorbed in excess through several photoprotective mechanisms. A major component is thermal dissipation of chlorophyll singlet excited states and is called nonphotochemical quenching (NPQ). NPQ is catalyzed in green algae by protein subunits called LHCSRs (Light Harvesting Complex Stress Related), homologous to the Light Harvesting Complexes (LHC), constituting the antenna system of both photosystem I (PSI) and PSII. We investigated the role of LHCSR1 and LHCSR3 in NPQ activation to verify whether these proteins are involved in thermal dissipation of PSI excitation energy, in addition to their well-known effect on PSII. To this aim, we measured the fluorescence emitted at 77 K by whole cells in a quenched or unquenched state, using green fluorescence protein as the internal standard. We show that NPQ activation by high light treatment in Chlamydomonas reinhardtii leads to energy quenching in both PSI and PSII antenna systems. By analyzing quenching properties of mutants affected on the expression of LHCSR1 or LHCSR3 gene products and/or state 1-state 2 transitions or zeaxanthin accumulation, namely, npq4, stt7, stt7 npq4, npq4 lhcsr1, lhcsr3-complemented npq4 lhcsr1 and npq1, we showed that PSI undergoes NPQ through quenching of the associated LHCII antenna. This quenching event is fast-reversible on switching the light off, is mainly related to LHCSR3 activity, and is dependent on thylakoid luminal pH. Moreover, PSI quenching could also be observed in the absence of zeaxanthin or STT7 kinase activity.

Evolution of light-independent protochlorophyllide oxidoreductase.

The nonhomologous enzymes, the light-independent protochlorophyllide reductase (DPOR) and the light-dependent protochlorophyllide oxidoreductase (LPOR), catalyze the reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide) in the penultimate step of biosynthesis of chlorophyll (Chl) required for photosynthetic light absorption and energy conversion. The two enzymes differ with respect to the requirement of light for catalysis and oxygen sensitivity. DPOR and LPOR initially evolved in the ancestral prokaryotic genome perhaps at different times. DPOR originated in the anoxygenic environment of the Earth from nitrogenase-like enzyme of methanogenic archaea. Due to the transition from anoxygenic to oxygenic photosynthesis in the prokaryote, the DPOR was mostly inactivated in the daytime by photosynthetic O2 leading to the evolution of oxygen-insensitive LPOR that could function in the light. The primary endosymbiotic event transferred the DPOR and LPOR genes to the eukaryotic phototroph; the DPOR remained in the genome of the ancestor that turned into the plastid, whereas LPOR was transferred to the host nuclear genome. From an evolutionary point of view, several compelling theories that explain the disappearance of DPOR from several species cutting across different phyla are as follows: (i) pressure of the oxygenic environment; (ii) change in the light conditions and temperature; and (iii) lineage-specific gene losses, RNA editing, and nonsynonymous substitution. Certain primary amino acid sequence and the physiochemical properties of the ChlL subunit of DPOR have similarity with that of LPOR suggesting a convergence of these two enzymes in certain evolutionary event. The newly obtained sequence data from different phototrophs will further enhance the width of the phylogenetic information on DPOR.

Effects of modified clay on the physiological and photosynthetic activities of Amphidinium carterae Hulburt

Among the strategies for treating harmful algal blooms, flocculation using modified clay (MC) has been widely applied in the field. This paper studied the mitigation of MC on Amphidinium carterae Hulburt, finding that MC could not only effectively remove A. carterae, but also affect the physiological activities of the residual algae and inhibit their normal growth. The superoxide dismutase (SOD) activity, catalase (CAT) activity and malondialdehyde (MDA) content of the residual algae significantly increased compared with the control, indicating that MC stimulated the accumulation of reactive oxygen species (ROS) in algal cells. In addition, the cell density was significantly correlated with the SOD activity, CAT activity and MDA content in the experiment groups, suggesting that intracellular ROS might be the main internal factor inhibiting cell growth. To reveal the mechanism of ROS generation, this paper further evaluated the effect of MC on photosynthesis in the residual microalgae, and found that compared with the control the absorption flux per photosystem II (PSII) reaction center (ABS/RC), the trapping flux per RC (TR0/RC) and the electron transport flux per RC (ET0/RC) increased, while the TR0/ABS and ET0/ABS decreased after adding 0.10g/L and 0.25g/L MC. These findings indicate that the MC led to an imbalance between photosynthetic light absorption and energy utilization and that the partial RCs became non-primary quinone electron acceptor (QA)-RCs, further inducing the over-excitation of the active RCs. And MC caused the suppression of the electron transport chain (ETC): the ETC from the QA to the secondary quinone electron acceptor (QB) was blocked and the size of plastoquinone pool decreased, which could induce the over-reduction of PSII. The over-excitation of PSII and the damaged ETC likely induce the generation of ROS during photosynthesis. Thus, MC likely induced the accumulation of intracellular ROS due to photosynthesis inhibition, consequently hindering the growth of the residual algae.

Changes of Photosynthetic Behaviors and Photoprotection during Cell Transformation and Astaxanthin Accumulation in Haematococcus pluvialis Grown Outdoors in Tubular Photobioreactors.

The cell transformation from green motile cells to non-motile cells and astaxanthin accumulation can be induced in the green alga Haematococcus pluvialis cultured outdoors. In the initial 3 d of incubation (cell transformation phase), light absorption and photosynthetic electron transport became more efficient. After five days of incubation (astaxanthin accumulation phase), the light absorption per active reaction center (ABS/RC) increased, but the efficiency of electron transport (ψo) and the quantum yield of electron transport (φEo) decreased with increased time, indicating that the capacity of photosynthetic energy utilization decreased significantly during astaxanthin accumulation, leading to an imbalance between photosynthetic light absorption and energy utilization. It would inevitably aggravate photoinhibition under high light, e.g., at midday. However, the level of photoinhibition in H. pluvialis decreased as the incubation time increased, which is reflected by the fact that Fv/Fm determined at midday decreased significantly in the initial 3 d of incubation, but was affected very little after seven days of incubation, compared with that determined at predawn. This might be because the non-photochemical quenching, plastid terminal oxidase, photosystem I cyclic electron transport, defensive enzymes and the accumulated astaxanthin can protect cells against photoinhibition.

Differences in photosynthetic behaviour of Kappaphycus alvarezii and Kappaphycus striatus during dehydration and rehydration

The kappa-carrageenan-producing seaweed Kappaphycus striatus is different from K. alvarezii in that it grows well in the intertidal zone, where the thalli are exposed periodically to dehydration and rehydration. To investigate the specific tolerance mechanism of K. striatus, the photosynthetic behaviour of K. alvarezii and K. striatus exposed to osmotic variation was compared. K. striatus had a lower dehydration rate in the dark treatments and a higher rehydration rate in both the dark and light treatments as compared with K. alvarezii. The FV/FM of K. striatus was significantly higher than that of K. alvarezii during the process of dehydration and rehydration in both the light and dark treatments. The increase in amplitude of JIP-test fluorescence parameters VK, VJ and VI of K. striatus was smaller than that of K. alvarezii during the dehydration process in both the light and dark treatments. The increased amplitude of VK, VJ and VI in K. striatus was significantly lower than that in K. alvarezii when they were rehydrated in the dark treatments. Therefore, during the process of dehydration and rehydration, the tolerance of the oxygen evolving complex, the ability to balance photosynthetic light absorption and energy utilization, as well as the tolerance of PSII reaction centres were relatively higher in K. striatus compared with K. alvarezii. Such an increased tolerance of PSII may help to improve the tolerance of the whole plant to dehydration.

Measurement of solar spectra relating to photosynthesis and solar cells: An inquiry lab for secondary science

The process of photosynthesis is central to science curriculum at all levels. This article describes an inquiry-based laboratory investigation developed to explore the impact of light quality on photosynthesis and to connect this process to current research on harvesting solar energy, including bioenergy, artificial photosynthesis, and solar cells. This laboratory was used with high-school science teachers who then took this experience back to their classrooms. During this exercise, teachers used an economical spectroradiometer to measure the solar spectrum and relate this to photosynthetic light absorption by determining the quality of light beneath trees. Following this investigation, teachers learned about the plant-inspired dye-sensitized solar cells and constructed one. To connect their light quality investigation to the efficiency of photosynthesis and solar cells, teachers then collected data at locations with varying quality and intensity of light. In sum, this investigation provides a crucial connection between photosynthesis and cutting edge research on solar energy technologies. Our learning experience provides a new instructional model for understanding a little investigated aspect of photosynthesis and connects to authentic scientific research.

Mitochondrial alternative oxidase pathway protects plants against photoinhibition by alleviating inhibition of the repair of photodamaged PSII through preventing formation of reactive oxygen species in Rumex K‐1 leaves

The purpose of this study was to explore how the mitochondrial AOX (alternative oxidase) pathway alleviates photoinhibition in Rumex K-1 leaves. Inhibition of the AOX pathway decreased the initial activity of NADP-malate dehydrogenase (EC 1.1.1.82, NADP-MDH) and the pool size of photosynthetic end electron acceptors, resulting in an over-reduction of the photosystem I (PSI) acceptor side. The over-reduction of the PSI acceptor side further inhibited electron transport from the photosystem II (PSII) reaction centers to the PSII acceptor side as indicated by an increase in V(J) (the relative variable fluorescence at J-step), causing an imbalance between photosynthetic light absorption and energy utilization per active reaction center (RC) under high light, which led to the over-excitation of the PSII reaction centers. The over-reduction of the PSI acceptor side and the over-excitation of the PSII reaction centers enhanced the accumulation of reactive oxygen species (ROS), which inhibited the repair of the photodamaged PSII. However, the inhibition of the AOX pathway did not change the level of photoinhibition under high light in the presence of the chloroplast D1 protein synthesis inhibitor chloramphenicol, indicating that the inhibition of the AOX pathway did not accelerate the photodamage to PSII directly. All these results suggest that the AOX pathway plays an important role in the protection of plants against photoinhibition by minimizing the inhibition of the repair of the photodamaged PSII through preventing the over-production of ROS.

THRUMIN1 Is a Light-Regulated Actin-Bundling Protein Involved in Chloroplast Motility

Chloroplast movement in response to changing light conditions optimizes photosynthetic light absorption. This repositioning is stimulated by blue light perceived via the phototropin photoreceptors and is transduced to the actin cytoskeleton. Some actin-based motility systems use filament reorganizations rather than myosin-based translocations. Recent research favors the hypothesis that chloroplast movement is driven by actin reorganization at the plasma membrane, but no proteins affecting chloroplast movements have been shown to associate with both the plasma membrane and actin filaments in vivo. Here we identified THRUMIN1 as a critical link between phototropin photoreceptor activity at the plasma membrane and actin-dependent chloroplast movements. THRUMIN1 bundles filamentous actin in vitro, and it localizes to the plasma membrane and displays light- and phototropin-dependent localization to microfilaments in vivo. These results suggest that phototropin-induced actin bundling via THRUMIN1 is important for chloroplast movement. A mammalian homolog of THRUMIN1, GRXCR1, has been implicated in auditory responses and hair cell stereocilla development as a regulator of actin architecture. Studies of THRUMIN1 will help elucidate the function of this family of eukaryotic proteins.

Investigating the Dynamics of Organelle-Actin Assembly with Dielectrophoresis

Dielectrophoresis is a technique of using high-frequency electric fields to manipulate dielectric particles. This work applies this technique to study the changes of intracellular properties in response to environmental changes. In a number of plant cells, chloroplasts change their positions for optimizing photosynthetic light absorption. Only between 2 and 20% of the light energy absorbed by algae and higher plants is actually used in CO2 fixation. Comparing with other micro-manipulative methods, dielectrophoresis is a relative simple method for investigating the microscopic mechanisms involved in chloroplast repositioning. Leaves of Egeria densa were subjected to different lighting conditions before conducting dielectrophoresis. Under dim light, chloroplasts situate on the periclinal wall. Under high-intensity light, chloroplasts move from the periclinal wall to the anticlinal walls. Under electric fields, chloroplasts treated with different lighting condition exhibit different motions. It is expected that actin configuration as well as chloroplast-actin binding strength contributes to the repositioning of chloroplasts.

A CHLOROPHYLL FLUORESCENCE INDEX TO ESTIMATE SHORT‐TERM RATES OF PHOTOSYNTHESIS BY INTERTIDAL MICROPHYTOBENTHOS1

An index based on chl a fluorescence quenching analysis was tested as a predictor of photosynthetic rates of undisturbed intertidal microphytobenthic assemblages. The fluorescence index, Pfluo, was derived from the combination of different chl a fluorescence parameters chosen to represent the two main sources of short‐term variability in the community‐level microphytobenthic photosynthesis: 1) the quantum yield of photosynthesis of the microalgae present in the photic zone of the sediment, φP, and 2) the community‐level efficiency of photosynthetic light absorption, ɛ, determined by the microalgal concentration in the photic zone. Variations in φP were traced by the fluorescence index ΔF/Fm′ (the effective quantum yield of charge separation at PSII), whereas changes in ɛ were followed by the fluorescence parameter Fo (dark or minimum fluorescence level). Gross photosynthetic rate, P, and fluorescence parameters were measured nondestructively and simultaneously under in situ conditions, on the same samples, using oxygen microelectrodes and pulse amplitude modulation fluorometry, respectively. Despite the large and uncorrelated hourly variability in irradiance, photosynthetic rate, and fluorescence parameters included in Pfluo, highly significant correlations between Pfluo and P were found for all the sampling periods, encompassing hourly, biweekly, and seasonal time scales. The variability in P explained by Pfluo ranged from 84.3% to 91.4% when sampling periods were considered separately and reached 81.1% when all data were pooled. The results of the study show that despite its simplicity, the index Pfluo can be used to trace short‐term variations in the photosynthetic rate of undisturbed microphytobenthic assemblages undergoing rhythmic vertical migration.