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Photorespiration, or C2 photosynthesis, is generally considered a futile cycle that potentially decreases photosynthetic carbon fixation by more than 25%. Nonetheless, many essential processes, such as nitrogen assimilation, C1 metabolism, and sulfur assimilation, depend on photorespiration. Most studies of photosynthetic and photorespiratory reactions are conducted with magnesium as the sole metal cofactor despite many of the enzymes involved in these reactions readily associating with manganese. Indeed, when manganese is present, the energy efficiency of these reactions may improve. This review summarizes some commonly used methods to quantify photorespiration, outlines the influence of metal cofactors on photorespiratory enzymes, and discusses why photorespiration may not be as wasteful as previously believed.

Starch is a product of photosynthetic activities in leaves. Wheat yields largely depend on photosynthetic carbon fixation and carbohydrate metabolism in flag leaves. The mapping of quantitative trait loci (QTLs) associated with flag leaf starch content (FLSC) in wheat (Triticum aestivum L.) was completed using unconditional and conditional QTL analyses. The FLSC of this population during the early grain-filling stage was measured at six stages in six environments. Combining unconditional and conditional QTL mapping methods, eight unconditional QTLs and nine conditional QTLs were detected, with five QTLs identified as unconditional and conditional QTLs. Four unconditional QTLs (i.e. qFLS-1B, qFLS-1D-1, qFLS-4A, and qFLS-7D-1) and one conditional QTL (i.e. qFLS-3A-1) were identified in two of six environments. Two QTLs (qFLS-1D-2 and qFLS-7D-1), which significantly affected the FLSC, were identified using the unconditional QTL mapping method, while three QTLs (i.e. qFLS-1A, qFLS-3A-1, and qFLS-7D-1) were detected using the conditional QTL mapping method. Our findings provide new insights into the genetic mechanism and regulatory network underlying the diurnal FLSC in wheat.

The photosynthetic electron transport chain (PETC) provides energy and redox equivalents for carbon fixation by the Calvin-Benson-Bassham (CBB) cycle. Both of these processes have been thoroughly investigated and the underlying molecular mechanisms are well known. However, it is far from understood by which mechanisms it is ensured that energy and redox supply by photosynthesis matches the demand of the downstream processes. Here, we deliver a theoretical analysis to quantitatively study the supply-demand regulation in photosynthesis. For this, we connect two previously developed models, one describing the PETC, originally developed to study non-photochemical quenching, and one providing a dynamic description of the photosynthetic carbon fixation in C3 plants, the CBB Cycle. The merged model explains how a tight regulation of supply and demand reactions leads to efficient carbon fixation. The model further illustrates that a stand-by mode is necessary in the dark to ensure that the carbon fixation cycle can be restarted after dark-light transitions, and it supports hypotheses, which reactions are responsible to generate such mode in vivo.

The authors thank the Australian Research Council for its support through a Future Fellowship grant, under contract FT140100645.

Photosynthetic carbon fixation regulates air-sea CO 2 fluxes in the waters of coral reefs. However, little has been docu mented on the effects of solar UV radiation (UVR, 280-400 nm) upon photosynthetic behaviors of phytoplankton dwelling in these ecosystems. In order to evaluate the aforesaid, surface dwelling tropical coral reef phytoplankton assemblages collected from the South China Sea were exposed to solar radiation (i.e., photosynthetically active radiation [PAR] + UV radiation A [UVA] + UV radiation B [UVB], 280-700 nm; PAR + UVA, 320-700 nm; and PAR, 400-700 nm) under static or simulated-mixing conditions. Under the static condition, UVA and UVB significantly reduced the carbon fixation with the maximum of 22.4 and 15.3%, respectively; while lower UVR-related photosynthetic inhibition was observed in case of phytoplankton samples being subjected to mixing. At a moderate level of mixing (i.e., circulation time 80 min), the UVA and UVB caused inhibition were lowered by 52.1 and 79.6%, respectively. Based on this it could be stated that vertical mixing induced by winds and/or tides in the natural environments could reduce the inhibitory effect of solar UVR on phytoplankton productivity in the coral reefs water.

Changes in calcification of coccolithophores may affect their photosynthetic responses to both, ultraviolet radiation (UVR, 280–400 nm) and temperature. We operated semi-continuous cultures of Emiliania huxleyi (strain CS-369) at reduced (0.1 mM, LCa) and ambient (10 mM, HCa) Ca2+ concentrations and, after 148 generations, we exposed cells to six radiation treatments (>280, >295, >305, >320, >350 and >395 nm by using Schott filters) and two temperatures (20 and 25 °C) to examine photosynthesis and calcification responses. Overall, our study demonstrated that: (1) decreased calcification resulted in a down regulation of photoprotective mechanisms (i.e., as estimated via non-photochemical quenching, NPQ), pigments contents and photosynthetic carbon fixation; (2) calcification (C) and photosynthesis (P) (as well as their ratio) have different responses related to UVR with cells grown under the high Ca2+ concentration being more resistant to UVR than those grown under the low Ca2+ level; (3) elevated temperature increased photosynthesis and calcification of E. huxleyi grown at high Ca2+ concentrations whereas decreased both processes in low Ca2+ grown cells. Therefore, a decrease in calcification rates in E. huxleyi is expected to decrease photosynthesis rates, resulting in a negative feedback that further reduces calcification.

The influence of vertical mixing on phytoplankton sensitivity to UV light has been assessed over an annual cycle. Photosynthesis rates of natural assemblages were compared in samples that were incubated at fixed position in a light gradient and with duplicate samples that simulated vertical mixing by movement in the same gradient with a periodicity of 4 h. This is the typical time-scale of vertical mixing in coastal waters in the English Channel. There were clear seasonal differences in the short-term response of phytoplankton to enhanced UVA+UVB. For most of the year, there was no detectable effect of UV on photosynthetic carbon fixation. But natural assemblages in late winter/early spring, when high UV light may sporadically occur at this latitude, were sensitive to UVA+UVB. In some samples, primary production was 40% of that measured in the absence of UV light. At the time of maximum sensitivity to UV, the phytoplankton assemblage was dominated by diatoms. Simulated vertical mixing resulted in more inhibition of photosynthesis by UVA+UVB light than when samples were at constant light with the same time-integrated irradiance. Transient increases in UVA+UVB due to ozone depletion, such as have been observed over Northern Europe, could have a serious impact on coastal phytoplankton production in late winter / early spring.

The Western Photosynthesis Conference is a regional conference that is held on an annual basis to bring together researchers primarily from the Western United States to share their newest research advances on photosynthetic processes. The 23rd conference was focused on both fundamental and more applied research on the biological conversion of solar energy to various energy storage forms. Several particular areas of solar energy conversion were emphasized in this conference (see below). Some of these topics, such as carbon limitations on photosynthesis, biomimicry and phenotyping, have traditionally not been incorporated extensively in the Western Photosynthesis Conference. We found that these topics have substantially broadened of the scope of this meeting.

The control of gaseous exchange between the leaf and bulk atmosphere by stomata governs CO2 uptake for photosynthesis and transpiration, determining plant productivity and water use efficiency. The balance between these two processes depends on stomatal responses to environmental and internal cues and the synchrony of stomatal behavior relative to mesophyll demands for CO2. Here we examine the rapidity of stomatal responses with attention to their relationship to photosynthetic CO2 uptake and the consequences for water use. We discuss the influence of anatomical characteristics on the velocity of changes in stomatal conductance and explore the potential for manipulating the physical as well as physiological characteristics of stomatal guard cells in order to accelerate stomatal movements in synchrony with mesophyll CO2 demand and to improve water use efficiency without substantial cost to photosynthetic carbon fixation. We conclude that manipulating guard cell transport and metabolism is just as, if not more likely to yield useful benefits as manipulations of their physical and anatomical characteristics. Achieving these benefits should be greatly facilitated by quantitative systems analysis that connects directly the molecular properties of the guard cells to their function in the field.

Microscopic stomatal pores in the epidermes of aerial plant organs allow the loss of water vapor to the atmosphere in a process known as transpiration and the entry of CO2 into the plant for photosynthetic carbon fixation. Stomatal apertures are rapidly and reversibly regulated by pairs of guard

To test the possibility of inorganic carbon limitation of the marine unicellular alga Emiliania huxleyi (Lohmann) Hay and Mohler, its carbon acquisition was measured as a function of the different chemical species of inorganic carbon present in the medium. Because these different species are interdependent and covary in any experiment in which the speciation is changed, a set of experiments was performed to produce a multidimensional carbon uptake scheme for photosynthesis and calcification. This scheme shows that CO2 that is used for photosynthesis comes from two sources. The CO2 in seawater supports a modest rate of photosynthesis. The HCO3- is the major substrate for photosynthesis by intracellular production of CO2 (HCO3- + H+ → CO2 + H2O → CH2O + O2). This use of HCO3- is possible because of the simultaneous calcification using a second HCO3- , which provides the required proton (HCO3- + Ca2+ → CaCO3 + H+). The HCO3- is the only substrate for calcification. By distinguishing the two sources of CO2 used in photosynthesis, it was shown that E. huxleyi has a K½ for external CO2 of ‘‘only’’ 1.9 ± 0.5 µM (and a Vmax of 2.4 ± 0.1 pmol·cell-1·d-1). Thus, in seawater that is in equilibrium with the atmosphere ([CO2] = 14 µM, [HCO3-] = 1920 µM, at fCO2 = 360 µatm, pH = 8, T = 15° C), photosynthesis is 90% saturated with external CO2. Under the same conditions, the rate of photosynthesis is doubled by the calcification route of CO2 supply (from 2.1 to 4.5 pmol·cell-1·d-1). However, photosynthesis is not fully saturated, as calcification has a K½ for HCO3- of 3256 ± 1402 µM and a Vmax of 6.4 ± 1.8 pmol·cell-1·d-1. The H+ that is produced during calcification is used with an efficiency of 0.97 ± 0.08, leading to the conclusion that it is used intracellularly. A maximum efficiency of 0.88 can be expected, as NO3- uptake generates a H+ sink (OH- source) for the cell. The success of E. huxleyi as a coccolithophorid may be related to the efficient coupling between H+ generation in calcification and CO2 fixation in photosynthesis.

Plants, restricted by their environment, need to integrate a wide variety of stimuli with their metabolic activity, growth and development. Sugars, generated by photosynthetic carbon fixation, are central in coordinating metabolic fluxes in response to the changing environment and in providing cells and tissues with the necessary energy for continued growth and survival. A complex network of metabolic and hormone signaling pathways are intimately linked to diverse sugar responses. A combination of genetic, cellular and systems analyses have uncovered nuclear HXK1 (hexokinase1) as a pivotal and conserved glucose sensor, directly mediating transcription regulation, while the KIN10/11 energy sensor protein kinases function as master regulators of transcription networks under sugar and energy deprivation conditions. The involvement of disaccharide signals in the regulation of specific cellular processes and the potential role of cell surface receptors in mediating sugar signals add to the complexity. This chapter gives an overview of our current insight in the sugar sensing and signaling network and describes some of the molecular mechanisms involved.

[1] During this decade, the Amazon region has suffered two severe droughts in the short span of five years – 2005 and 2010. Studies on the 2005 drought present a complex, and sometimes contradictory, picture of how these forests have responded to the drought. Now, on the heels of the 2005 drought, comes an even stronger drought in 2010, as indicated by record low river levels in the 109 years of bookkeeping. How has the vegetation in this region responded to this record-breaking drought? Here we report widespread, severe and persistent declines in vegetation greenness, a proxy for photosynthetic carbon fixation, in the Amazon region during the 2010 drought based on analysis of satellite measurements. The 2010 drought, as measured by rainfall deficit, affected an area 1.65 times larger than the 2005 drought – nearly 5 million km2 of vegetated area in Amazonia. The decline in greenness during the 2010 drought spanned an area that was four times greater (2.4 million km2) and more severe than in 2005. Notably, 51% of all drought-stricken forests showed greenness declines in 2010 (1.68 million km2) compared to only 14% in 2005 (0.32 million km2). These declines in 2010 persisted following the end of the dry season drought and return of rainfall to normal levels, unlike in 2005. Overall, the widespread loss of photosynthetic capacity of Amazonian vegetation due to the 2010 drought may represent a significant perturbation to the global carbon cycle.

The study of regulation has previously involved indirect, and largely correlative, approaches. In the following contribution I illustrate the uses and limitations of these traditional approaches, and then discuss how molecular genetics provide a new tool to test directly ideas about regulation in vivo in the intact organism.

Limitations of carbon fixation within spinach leaves due to light and CO2 were investigated. Under equivalent photon fluxes, carbon fixation was higher when leaves were irradiated adaxially compared to abaxially. Maximal carbon fixation occurred in the middle of the palisade mesophyll under adaxial illumination, whereas, maximal carbon fixation occurred in the spongy mesophyll under abaxial illumination. Total carbon fixation and the pattern of carbon fixation across leaves were similar, when leaves were irradiated with 800 micromol quanta m(-2) s(-1) either adaxially alone or adaxially plus abaxially (1,600 micromol quanta m(-2) s(-1)). In contrast, when both leaf surfaces were irradiated simultaneously with 200 micromol quanta m(-2) s(-1), total carbon fixation increased and carbon fixation in the middle of the leaf was higher compared to leaves irradiated unilaterally with the low light. Feeding 14CO2 through either the adaxial or abaxial leaf surface did not change the pattern of carbon fixation across the leaf. Increasing 14CO2 pulse-feeding times from 5 s to 60 s allowed more 14CO2 to be fixed but did not change the pattern of 14CO2 fixation across the leaf. We concluded that in spinach, the distribution of both light and Rubisco activity within leaves has significant effects on the patterns of carbon fixation across leaves; whereas CO2 diffusion does not appear to affect the carbon fixation pattern within spinach leaves.

The goals of our DOE OMP project are to (1) understand regulation of ribulose bisphosphate carboxylase (RubisCO) in phytoplankton cultures in response to light regime; (2) determine regulation of RubisCO in response to light during nutrient limitation in these cultures; (3) to determine mechanisms of RubisCO regulation in natural populations of phytoplankton on the ocean margins in the Gulf of Mexico and (4) to measure regulation of RubisCO in phytoplankton of the Hatteras System. Two goals are laboratory-based, and two are ship-based.

This article analyses the evolution of a major photorespiratory enzyme, glycolate oxidase (GOX), finding that plant GOX was phylogenetically derived from cyanobacterial lactate oxidase. Cyanobacterial GOX-like proteins are today found only in N2-fixing strains, in which they play an important role in the protection of nitrogenase.

Guard cells are located in the leaf epidermis and pairs of guard cells surround and form stomatal pores, which regulate CO(2) influx from the atmosphere into leaves for photosynthetic carbon fixation. Stomatal guard cells also regulate water loss of plants via transpiration to the atmosphere. Signal transduction mechanisms in guard cells integrate a multitude of different stimuli to modulate stomatal apertures. Stomata open in response to light. Stomata close in response to drought stress, elevated CO(2), ozone and low humidity. In response to drought, plants synthesize the hormone abscisic acid (ABA) that triggers closing of stomatal pores. Guard cells have become a highly developed model system for dissecting signal transduction mechanisms in plants and for elucidating how individual signaling mechanisms can interact within a network in a single cell. Many new findings have been made in the last few years. This chapter is an update of an electronic interactive chapter in the previous edition of The Arabidopsis Book (Mäser et al. 2003). Here we focus on mechanisms for which genes and mutations have been characterized, including signaling components for which there is substantial signaling, biochemical and genetic evidence. Ion channels have been shown to represent targets of early signal transduction mechanisms and provide functional signaling and quantitative analysis points to determine where and how mutations affect branches within the guard cell signaling network. Although a substantial number of genes and proteins that function in guard cell signaling have been identified in recent years, there are many more left to be identified and the protein-protein interactions within this network will be an important subject of future research. A fully interactive clickable electronic version of this publication can be accessed at the following web site: http://www-biology.ucsd.edu/labs/schroeder/clickablegc2/. The interactive clickable version includes the following features: Figure 1. Model for the roles of ion channels in ABA signaling.Figure 2. Blue light signaling pathways in guard cells.Figure 3. ABA signaling pathways in guard cells.Figure 1 is linked to explanations that appear upon mouse-over. Figure 2 and Figure 3 are clickable and linked to info boxes, which in turn are linked to TAIR, to relevant abstracts in PubMed, and to updated background explanations from Schroeder et al (2001), used with permission of Annual Reviews of Plant Biology.

Germanic acid (/sup 68/Ge) was used as a tracer to study the uptake and incorporation of silicic acid by natural phytoplankton populations in the Gulf of California. The technique was useful in measuring silicic acid incorporation rates as low as 0.5 nM h/sup -1/. The incorporation followed saturation kinetics. K/sub s/ values at two stations were 1.59 and 2.53 ..mu..M. The incorporation was light dependent, maximum rates being achieved at 0.28 x 10/sup 21/ photons cm/sup -2/ d/sup -1/ (0.044 ly min/sup -1/; 9 percent of surface irradiance) and half-maximum rate at 0.02 x 10/sup 21/ photons cm/sup -2/ d/sup -1/ (0.003 ly min/sup -1/; 0.6 percent of surface irradiance); rate of incorporation in the dark was 44 percent of that at light saturation. Enrichment with silicic acid caused stimulation of photosynthetic carbon fixation at two stations.

Photosynthetic fixation of 14CO2 in the bean Phaseolus vulgaris, cv. Pencil Pod Black Wax, resulted in the appearance of labeled compounds that were characterized as phosphoserine and phosphohydroxypyruvate by chromatographic separation and by the synthesis of chemical derivatives. In 14CO2/12CO2 pulse-chase experiments these metabolites demonstrated the rapid pool saturation and depletion of 14C characteristic of early intermediates in photosynthetic carbon fixation. They were present in sufficient amounts to account for about 35% of total carbon fixed in 1 minute.

Organelles morphologically similar to microbodies have been found in several tissues of atmospheric species of Tillandsia from different habitats. The presence of catalase was demonstrated by the DAB reaction thus confirming the microbody nature of these organelles. They are a feature of the Tillandsia species with normal photosynthetic carbon fixation and with CAM. Their size is consistently small. The nucleoid observed in the microbodies shows a characteristic morphology which has not been reported before within other plant microbodies. This nucleoid is composed of minute tubular structures, for which the authors here propose a three-dimensional arrangement.

Excised apical portions of green wheat leaf sections were treated with aminotriazole to prevent formation of new chloroplasts. Illumination retarded the decline in chlorophyll content per leaf section, the disintegration of chloroplast ultrastructure, and the loss of capacity for photosynthetic carbon fixation. We interpret these 3 effects of illumination as facets of a single light effect in retarding chloroplast senescence. This light effect in retarding chloroplast senescence has features differing from characteristics of photosynthetic carbon fixation. For example, A) application of the photosynthetic inhibitor 3-(3,4-dichlorophenyl)-1, 1-dimethylurea did not decrease, and may have even slightly increased, the effectiveness of light; B) although the action spectrum contains peaks in the blue and red regions, it differs from the action spectrum for photosynthetic CO(2) assimilation in wheat; C) in nonphotosynthesizing tissue, application of sugars did not retard chloroplast senescence; D) light saturation was achieved by only a few hundred microwatts/cm(2). Considered together with the well-known light requirement for chloroplast formation, our results indicate that light has a dual, photomorphogenetic control in maintaining the green status of the plant by also exerting a second effect: retarding of senescence of chloroplasts already present.

1. The effect of varying light intensities on the rates of calcification and CO2 fixation in the coralMontastrea annularis is described. 2. High light intensities, above the saturation point of symbiont photosynthesis, are shown to inhibit the rate of calcification in both shallow and deep specimens. 3. Light enhanced calcification in reef-building corals appears to be an energy-requiring process involving the active transport of calcium and carbonate ions, and the translocation of symbiont photosynthate.

We propose that plants possessing the C4-dicarboxylic acid pathway of photosynthetic carbon fixation are generally inferior food sources for herbivores, and are often avoided by them, relative to plants possessing only the C3-Calvin cycle pathway. As initial support of this hypothesis, we present data from the literature, dealing primarily with insects, that suggest that C4 species are ingested in proportions lower than their availability in a number of natural situations, that they tend to be avoided in laboratory preference tests, and that they result in lower survival and fecundity in herbivores feeding on them. We suggest a number of physiological, anatomical, and nutritional differences between C3 and C4 species that may be involved in generating the observed pattern of herbivore preference. The ecological relevance of the differences between C3 and C4 species has been discussed to date only in relation to hypothetical effects on interspecific competition. If our hypothesis is true, selective herbivo...

Seven species of marine microalgae including chlorophytes, diatoms, and a chrysomonad, of tropical, Antarctic, and temperate origin, were used to measure photosynthetic carbon fixation as affected by concentration of dissolved O2 at two levels of CO2. An apparent O2 inhibition of carbon fixation was observed in some species, but others were affected little or not at all by concentration of dissolved O2. In several cases, O2 appeared necessary for maximal carbon fixation.

Colonies of the alga Phaeocystispouchetj~ (average colony diameter ca 4 mm) were subjected to light/dark periods in culture, and production and consumption of macromolecules of the colony matrix were measured. In the light, up to ca 32 % of the total photosynthetic carbon fixation, consisting mostly of large-molecular-weight products (MW > 1800), were accumulated in the colonial matrix. In the dark, these macromolecular compounds disappeared, whereas monomeric and oligomeric compounds were found, suggesting reassimilation of colonial substances by P. pouchetii cells. Stepw~se filtration allowed separate analysis of cells, colonial matrix and surrounding medium. The latter 2 compartments differed widely in size spectra of organic compounds (as determined by gel-permeation) and in fluctuations over a light/dark cycle, suggesting that the very high rates of carbon excretion during P. pouchetii blooms in previous reports should be reconsidered.

Coccolithophores are single-celled photosynthesizing marine algae, responsible for half of the calcification in the surface ocean, and exert a strong influence on the distribution of carbon among global reservoirs, and thus Earth’s climate. Calcification in the surface ocean decreases the buffering capacity of seawater for CO2, whilst photosynthetic carbon fixation has the opposite effect. Experiments in culture have suggested that coccolithophore calcification decreases under high CO2 concentrations ([CO2(aq)]) constituting a negative feedback. However, the extent to which these results are representative of natural populations, and of the response over more than a few hundred generations is unclear. Here we describe and apply a novel rationale for size-normalizing the mass of the calcite plates produced by the most abundant family of coccolithophores, the Noëlaerhabdaceae. On average, ancient populations subjected to coupled gradual increases in [CO2(aq)] and temperature over a few million generations in a natural environment become relatively more highly calcified, implying a positive climatic feedback. We hypothesize that this is the result of selection manifest in natural populations over millennial timescales, so has necessarily eluded laboratory experiments.

Article d'introduction a ce numero special de la revue consacre au role des microorganismes dans les ecosystemes aquatiques et faisant suite au seminaire tenu en aout 1986 sur ce theme, dans le cadre du 4eme congres INTECOL (Syracuse, Etats Unis)