Effect of Japanese Paramecium bursaria extract on photosynthetic carbon fixation of symbiotic algae

To investigate the relationship between the Japanese Paramecium bursaria host and its symbiont, we studied the effect of a host cell-free extract on carbon fixation and photosynthate release of the symbiont. The host extract enhanced symbiotic algal carbon fixation about 3-fold at an increased concentration; however, release of photosynthate hardly changed. Since the enhancing effect was not affected by elimination of carbon dioxide from the host extract, the existence of a host factor that stimulates algal carbon fixation was made clear. The host factor is a heat-stable, low molecular weight substance. In relation to the pH dependence, the extract improved carbon fixation at acidic and neutral pH and showed almost no effect at pH 9.0. Therefore, the stimulation of carbon fixation by the host factor is unlikely to be caused by intracellular pH change. The extract also improved carbon fixation of several Chlorella species, symbiotic and free-living, and apparently exhibited no species specificity. Therefore, the host seems to regulate the photosynthesis of the symbiont via a specific compound.     

Coordination of photosynthetic and respiratory metabolism in Chlorella vulgaris UAM 101 in the light

In Chlorella vulgaris UAM 101, the presence of glucose altered the photosynthetic and respiratory metabolism in the light. When glucose was added to the growth medium, an increase in the cellular level of enzymes involved in glucose oxidation, namely glucose-6-P dehydrogenase (EC 1.1.1.49) and NAD⁺-glyceraldehyde-3-P dehydrogenase (EC 1.2.1.12), was observed. Glucose also enhanced respiratory O₂ consumption. In addition, CO₂ released by glucose oxidation was refixed in photosynthesis. The presence of glucose also affected photosynthesis. Phosphoribulokinase (EC 2.7.1.19) and NADP⁺-dependent glyceraldehyde-3-P dehydrogenase (EC 1.2.1.13), two regulatory enzymes of the reductive pentose phosphate cycle, were increased by glucose. However, Rubisco (EC 4.1.1.39) activity of these cells was lower than that of autotrophic cells. Despite these alterations, the photosynthetic O₂ evolution was not significantly inhibited by glucose. On the other hand, an increase in the cytosolic NADP⁺-glyceraldehyde-3-P dehydrogenase (EC 1.2.1.9) that is involved in obtaining reducing power for anabolic processes was observed. The CO₂ levels in the growth medium did not significantly affect the cellular level of enzymes measured in this work, except those involved in biosynthetic pathways. These data suggest that the effect of glucose on photosynthesis and respiration can be explained by alteration of the cellular level of photosynthetic enzymes and respiratory substrates, respectively.     

Characteristics of photosynthetic carbon metabolism of spikelets in rice

In lemmas and paleae of rice, the amount of pyruvate, Pi dikinase (PPDK) protein increased dramatically 6 d after anthesis and this change was consistent with that in the activity of PPDK. Since lemmas and paleae at this stage also showed high activities of the other marker enzymes of C4 pathway including phosphot enolpyruvate carboxylase (Imaizumi et al. (1990) Plant Cell Physiol 31: 835–843), photosynthetic carbon metabolism with lemmas at this stage were characterized. In a 14C pulse-12C chase study by photosynthetic CO2 fixation, about 35% and 25% of 14C fixed in lemmas were incorporated initially into 3-phosphoglycerate (3-PGA) and C4 acids, respectively. This suggests that lemmas participate mainly in C3-type photosynthetic metabolism, but that lemmas may also participate in the metabolism of C4 acids to some extent. To clarify this possibility, large amounts of 14C-labeled C4 acids were synthesized in vivo by a light-enhanced dark CO2 fixation (LED) method and the fate of 14C in C4 acids in the light was investigated. The percentage distribution of 14C in C-4 position of malate was about 90% and 83% after 10 s of photosynthetic 14CO2 fixation and 110 s of LED, respectively. Some of the 14C incorporated into C4 acids was transferred into 3-PGA and sugar phosphates. The possibility of direct fixation of CO2 by phosphot enolpyruvate carboxylase and metabolic pathway of CO2 released by decarboxylation of malate produced were discussed.     

Photoacclimation and the effect of the symbiotic environment on the photosynthetic response of symbiotic dinoflagellates in the tropical marine hydroid Myrionema amboinense

Symbiotic dinoflagellates of the genus Symbiodinium and residing in the tropical hydroid Myrionema amboinense acclimate to low photon flux associated with low light 'shade' environments by increasing the amount of photosynthetic pigments per algal cell. The photosynthetic light intensity (PI) curves suggested that the low-light pigment response involved an increase in the number of photosynthetic units (PSU) in the chloroplast in addition to any increases in PSU size. Comparisons of light-dependent portion of the P-I curves of freshly isolated zooxanthellae (FIZ) with those from symbionts within the intact animal suggest that the host cell environment reduced average light levels reaching the symbiotic algae by more than half. This phenomenon may protect the algae from photobleaching of pigments and/or photoinhibition of photosynthesis at high light intensities present in shallow water habitats. In addition, maximum photosynthesis (P(max)) of symbionts removed from the host cell was higher than that recorded from dinoflagellates in the intact association, suggesting that the availability of carbon dioxide for photosynthesis may be limited in the intact hydroid. Shaded polyps contained fewer zooxanthellae and had less tissue biomass (measured as protein) than unshaded polyps. However symbionts from shaded polyps acclimated to the low light intensities by increasing chlorophyll levels and photosynthetic rates. The higher photosynthetic rates may have resulted from increased availability of carbon dioxide associated with lower symbiont density. Calculations of the contribution of zooxanthellae carbon to the host animal's respiratory demand (CZAR) showed that zooxanthellae from shaded polyps living in the field potentially provide about the same amount of carbon to their host as zooxanthellae from polyps living in the field in unshaded high light intensities.     

Effects of free amino acids on the isolated symbiotic algae of the coral Plesiastrea versipora (Lamarck): absence of a host release factor response

Symbiotic algae incubated in host tissue homogenate of the coral Plesiastrea versipora for 2 h in the light released at least four and a half times as much photosynthetically fixed carbon (range 13.8±3.1 to 158±9.5 nmol C/10⁶ algae) as algae incubated in seawater (range 1.4±0.3 to 10.8±0.6 nmol C/10⁶ algae) indicating the presence of ‘host release factor’. When algae were incubated in a low molecular weight fraction of homogenate containing partially purified ‘host release factor’ they also released more carbon (range 62.2±3.7 to 279±11.4 nmol C/10⁶ algae) than algae incubated in seawater. This low molecular weight fraction contained free amino acids. We tested the hypothesis that the free amino acids in this fraction were responsible for ‘host release factor’ activity. Algae incubated in a mixture of free amino acids equivalent to those found in this fraction, released more fixed carbon (range 2.4±0.3 to 25.2±0.2 nmol C/10⁶ algae) than algae incubated in seawater but in each experiment, release was much lower than when algae were incubated in host tissue homogenate. These data indicate that the stimulation of release of photosynthetically fixed carbon from the symbiotic algae of Plesiastrea versipora incubated in partially purified host release factor is not primarily due to the presence of free amino acids. We are continuing further studies to determine the exact nature of the active compound.     

The distribution of mycosporine-like amino acids (MAAs) and the phylogenetic identity of symbiotic dinoflagellates in cnidarian hosts from the Mexican Caribbean

A survey of 54 species of symbiotic cnidarians that included hydrozoan corals, anemones, gorgonians and scleractinian corals was conducted in the Mexican Caribbean for the presence of mycosporine-like amino acids (MAAs) in the host as well as the Symbiodinium fractions. The host fractions contained relatively simple MAA profiles, all harbouring between one and three MAAs, principally mycosporine-glycine followed by shinorine and porphyra-334 in smaller amounts. Symbiodinium populations were identified to sub-generic levels using PCR-DGGE analysis of the Internal Transcribed Spacer 2 (ITS2) region. Regardless of clade identity, all Symbiodinium extracts contained MAAs, in contrast to the pattern that has been found in cultures of Symbiodinium, where clade A symbionts produced MAAs whereas clade B, C, D, and E symbionts did not. Under natural conditions between one and four MAAs were identified in the symbiont fractions, mycosporine-glycine (λ_max = 310 nm), shinorine (λ_max = 334 nm), porphyra-334 (λ_max = 334 nm) and palythine (λ_max = 320 nm). One sample also contained mycosporine-2-glycine (λ_max = 331 nm). These data suggest that Symbiodinium is restricted to producing five MAAs and there also appears to be a defined order of appearance of these MAAs: mycosporine-glycine followed by shinorine (in one case mycosporine-2-glycine), then porphyra-334 and palythine. Overall, mycosporine-glycine was found in highest concentrations in the host and symbiont extracts. This MAA, unlike many other MAAs, absorbs within the ultraviolet-B range (UVB, 280-320 nm) and is also known for moderate antioxidant properties thus potentially providing protection against the direct and indirect effects of UVR. No depth-dependent changes could be identified due to a high variability of MAA concentrations when all species were included in the analysis. The presence of at least one MAA in all symbiont and host fractions analyzed serves to highlight the importance of MAAs, and in particular the role of mycosporine-glycine, as photoprotectants in the coral reef environment.     

Effects of temperature on the regulation of photosynthetic carbon assimilation in leaves of maize and barley

The aim of this work was to examine the effect of temperature in the range 5 to 30 ° C upon the regulation of photosynthetic carbon assimilation in leaves of the C₄ plant maize (Zea mays L.) and the C₃ plant barley (Hordeum vulgare L.). Measurements of the CO₂-assimilation rate in relation to the temperature were made at high (735 bar) and low (143 bar) intercellular CO₂ pressure in barley and in air in maize. The results show that, as the temperature was decreased, (i) in barley, pools of phosphorylated metabolites, particularly hexose-phosphate, ribulose 1,5-bisphosphate and fructose 1,6-bisphosphate, increased in high and low CO₂; (ii) in maize, pools of glycerate 3-phosphate, triose-phosphate, pyruvate and phosphoenolpyruvate decreased, reflecting their role in, and dependence on, intercellular transport processes, while pools of hexose-phosphate, ribulose 1,5-bis phosphate and fructose 1,6-bisphosphate remained approximately constant; (iii) the redox state of the primary electron acceptor of photosystem II (Q_A) increased slightly in barley, but rose abruptly below 12° C in maize. Non-photochemical quenching of chlorophyll fluorescence increased slightly in barley and increased to high values below 20 ° C in maize. The data from barley are consistent with the development of a limitation by phosphate status at low temperatures in high CO₂, and indicate an increasing regulatory importance for regeneration of ribulose 1,5-bisphosphate within the Calvin cycle at low temperatures in low CO₂. The data from maize do not show that any steps of the C₄ cycle are particularly cold-sensitive, but do indicate that a restriction in electron transport occurs at low temperature. In both plants the data indicate that regulation of product synthesis results in the maintenance of pools of Calvin-cycle intermediates at low temperatures.Abbreviations Glc6P glucose-6-phosphate - Fru6P fructase-6-phosphate - Frul,6bisP fructose-1,6-bisphosphate - PGA glycerate-3-phosphate - p _i intercellular partial pressure of CO₂ - RuBP ribulose-1,5-bisphosphate - triose-P sum of glyceraldehyde-3-phosphate and dihydroxyacetone phosphate We thank the Agricultural and Food Research Council, UK (Research grant PG50/67) and the Science and Engineering Research Council, UK for financial support. C.A.L. was supported by the British Council, by the Conselho Nacional de Desenvolvimento Cientiflco e Tecnologico (CNPq), Brazil and by an Overseas Research Student Award. We also thank Mark Stitt (Bayreuth, FRG) and Debbie Rees for helpful discussions.     

Effects of climatic variability on the annual carbon sequestration by a boreal aspen forest

To evaluate the carbon budget of a boreal deciduous forest, we measured CO₂ fluxes using the eddy covariance technique above an old aspen (OA) forest in Prince Albert National Park, Saskatchewan, Canada, in 1994 and 1996 as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). We found that the OA forest is a strong carbon sink sequestering 200 ± 30 and 130 ± 30 g C m^–2 y^–1 in 1994 and 1996, respectively. These measurements were 16–45% lower than an inventory result that the mean carbon increment was about 240 g C m^–2 y^–1 between 1919 and 1994, mainly due to the advanced age of the stand at the time of eddy covariance measurements. Assuming these rates to be representative of Canadian boreal deciduous forests (area ≈ 3 × 10⁵ km²), it is likely they can sequester 40–60 Tg C y^–1, which is 2–3% of the missing global carbon sink. The difference in carbon sequestration by the OA forest between 1994 and 1996 was mainly caused by the difference in leaf emergence date. The monthly mean air temperature during March–May 1994, was 4.8 °C higher than in 1996, resulting in leaf emergence being 18–24 days earlier in 1994 than 1996. The warm spring and early leaf emergence in 1994 enabled the aspen forest to exploit the long days and high solar irradiance of mid-to-late spring. In contrast, the 1996 OA growing season included only 32 days before the summer solstice. The earlier leaf emergence in 1994 resulted 16% more absorbed photosynthetically active radiation and a 90 g C m^–2 y^–1 increase in photosynthesis than 1996. The concomitant increase in respiration in the warmer year (1994) was only 20 g C m^–2 y^–1. These results show that an important control on carbon sequestration by boreal deciduous forests is spring temperature, via the influence of air temperature on the timing of leaf emergence.     

The effect of water stress on photosynthetic carbon metabolism in four species grown under field conditions

Abstract. The effect of gradually-developing water-stress has been studied in Lupinus albus L., Helianthus annuus L., Vitis vinifera cv. Rosaki and Eucalyptus globulus Labill. Water was withheld and diurnal rhythms were investigated 4–8d later, when the predawn water deficit was more negative than in watered plants, and the stomata closed almost completely early during the photoperiod. The contribution of ‘stomatal’ and ‘non-stomatal’ components to the decrease of photosynthetic rate was investigated by (1) comparing the changes of the rate of photosynthesis in air with the changes of stomatal conductance and (2) measuring photosynthetic capacity in saturating irradiance and 15% CO₂. Three species (lupin, eucalyptus and sunflower) showed larger changes of stomatal conductance than photosynthesis in air, and showed little or no decrease of photosynthetic capacity in saturating CO₂. Photosynthesis in air also recovered fully overnight after watering the plants in the evening. In grapevines, stomatal conductance and photosynthesis in air changed in parallel, there was a marked decrease of photosynthetic capacity, and photosynthesis and stomatal conductance did not recover overnight after watering water-stressed plants. Relative water content remained above 90% in grapevine. We conclude that non-stomatal components do not play a significant role in lupins, sunflower or eucalyptus, but could in grapevine. The effect of water-stress on partitioning of photosynthate was investigated by measuring the amounts of sucrose and starch in leaves during a diurnal rhythm, and by measuring the partitioning of ¹⁴C-carbon dioxide between sucrose and starch. In all four species, starch was depleted in water-stressed leaves but sucrose was maintained at amounts similar to, or higher than, those in watered plants. Partitioning into sucrose was increased in lupins and eucalyptus, and remained unchanged in grapevine and sunflower. It is concluded that water-stressed leaves in all four species maintain high levels of soluble sugars in their leaves, despite having lower rates of field photosynthesis, decreased rates of export, and low amounts of starch in their leaves.     

Effects of carbon dioxide and oxygen on the regulation of photosynthetic carbon metabolism by ammonia in spinach mesophyll cells

Photosynthetic carbon metabolism of isolated spinach mesophyll cells was characterized under conditions favoring photorespiratory (PR; 0.04% CO₂ and 20% O₂) and nonphotorespiratory (NPR; 0.2% CO₂ and 2% O₂) metabolism, as well as intermediate conditions. Comparisons were made between the metabolic effects of extracellularly supplied NH₄⁺ and intracellular NH₄⁺, produced primarily via PR metabolism. The metabolic effects of ¹⁴CO₂ fixation under PR conditions were similar to perturbations of photosynthetic metabolism brought about by externally supplied NH₄⁺; both increased labeling and intracellular concentrations of glutamine at the expense of glutamate and increased anaplerotic synthesis through α-ketoglutarate. The metabolic effects of added NH₄⁺ during NPR fixation were greater than those during PR fixation, presumably due to lower initial NH₄⁺ levels during NPR fixation. During PR fixation, addition of ammonia caused decreased pools and labeling of glutamate and serine and increased glycolate, glyoxylate, and glycine labeling. The glycolate pathway was thus affected by increased rates of carbon flow and decreased glutamate availability for glyoxylate transamination, resulting in increased usage of serine for transamination. Sucrose labeling decreased with NH₄⁺ addition only during PR fixation, suggesting that higher photosynthetic rates under NPR conditions can accommodate the increased drain of carbon toward amino acid synthesis while maintaining sucrose synthesis.     

Effects of drought on non-mycorrhizal and mycorrhizal maize: changes in the pools of non-structural carbohydrates, in the activities of invertase and trehalase, and in the pools of amino acids and imino acids

To study the response of non-mycorrhizal and mycorrhizal maize plants to drought, the changes in the pools of non-structural carbohydrates and amino acids were analysed in leaves and roots of two maize cvs. Plants well colonized by the arbuscular mycorrhizal fungus Glomus mosseae (Nicol. & Gerd.) (60% of root length infected) and comparable non-mycorrhizal plants were subjected to moderate drought stress by reducing the water supply. This stress induced a conspicuous increase in the trehalose pool in the mycorrhizal roots, probably because it was accumulated by the fungal symbiont. Furthermore, glucose and fructose were accumulated in leaves and roots of non-mycorrhizal plants but not in the mycorrhizal ones. Starch disappeared completely from the leaves of both mycorrhizal and non-mycorrhizal plants in response to drought. Activities of soluble acid invertase and trehalase were also measured. Acid invertase activity increased during drought in the leaves of both non-mycorrhizal and mycorrhizal plants whilst in the roots it was unaffected in non-mycorrhizal plants and decreased in the mycorrhizal ones. Without drought stress, trehalase activity was considerably higher in the leaves and roots of mycorrhizal plants than in those of non-mycorrhizal plants. It increased conspicuously during drought, primarily in the leaves of non-mycorrhizal plants. A drought-induced accumulation of amino acids as well as imino acids was found in roots and leaves of both mycorrhizal and non-mycorrhizal plants; leaves of mycorrhizal plants accumulated more imino acids than those of non-mycorrhizal ones. Our results show that drought stress and the presence of a mycorrhizal fungus have a considerable effect on carbon partitioning, imino acid and amino acid accumulation in maize plants.