Photosynthetic Oxygen Release Research Articles

Evidence of a key role for photosynthetic oxygen release in oil storage in developing soybean seeds

Based on the topographical analysis of photosynthesis and oil storage, we propose in a companion paper that photosynthetic oxygen release plays a key role in the local energy state, storage metabolism and flux toward lipid biosynthesis in developing soybean seeds. To test this hypothesis, we combined topographical analysis of ATP gradients across tissues, microsensor quantifications of internal O2 levels, assays of energy balance, metabolite profiles and isotope-labelling studies. Seeds show a marked degree of oxygen starvation in vivo (minimum O2 levels 0.1 kPa, approximately 1.3 microm), affecting ATP gradients, overall energy state, metabolite pools and storage activity. Despite the low light availability, photosynthesis supplies significant amounts of oxygen to the hypoxic seed tissue. This is followed by an increase in local ATP levels, most prominently within the lipid-synthesizing (inner) regions of the embryo. Concomitantly, partitioning of 14C-sucrose to lipids is increased, suggesting higher rates of lipid biosynthesis. It is concluded that both respiratory and biosynthetic fluxes are dynamically adjusted to photosynthetic oxygen supply.

Gradients of lipid storage, photosynthesis and plastid differentiation in developing soybean seeds

This study establishes a topographical framework for functional investigations on the regulation of lipid biosynthesis and its interaction with embryo photosynthesis in developing soybean seed. Structural observations, combined with molecular and functional parameters, revealed the gradual transformation of chloroplasts into storage organelles, starting from inner regions going outwards. This is evidenced by electron microscopy, confocal laser scanning microscopy, in situ hybridization and histochemical/biochemical data. As a consequence of plastid differentiation, photosynthesis becomes distributed along a gradient within the developing embryo. Electron transport rate, effective quantum yield and O2 production rate are maximal in the embryo periphery, as documented by imaging pulse-amplitude-modulated fluorescence and O2 release via microsensors. The gradual loss of photosynthetic capacity was accompanied by a similarly gradual accumulation of starch and lipids. Noninvasive nuclear magnetic resonance spectroscopy of mature seeds revealed steep gradients in lipid deposition, with the highest concentrations in inner regions. The inverse relationship between photosynthesis and lipid biosynthesis argues against a direct metabolic involvement of photosynthesis in lipid biosynthesis during the late storage stage, but points to a role for photosynthetic oxygen release. This hypothesis is verified in a companion paper.

Meristematic oxygen variability in eelgrass (Zostera marina)

We examined the variability in oxygen content of meristematic tissues in eelgrass in order to evaluate its potential role in events of sudden mass mortality within eelgrass beds. Oxygen content within intact eelgrass plants could be described by use of microelectrode techniques at high temporal and spatial resolution in the laboratory and in the field. Under both situations, the meristematic oxygen content was highly variable, ranging from 0 to 200% of air saturation depending on environmental conditions. Changes from steady‐state maximum oxygen content to steady‐state minimum content occurred within 30 min following experimental manipulation. The internal oxygen content exceeded water column oxygen concentration in the light and was intimately coupled to changes in irradiance because of the photosynthetic oxygen release within the leaves. The photosynthetically produced pool of oxygen could, however, not function as an efficient storage to support nighttime respiration. In the dark, oxygen was primarily supplied from the water column via diffusion into leaves, and the meristem quickly turned anoxic if the water column was anoxic. Experimental reduction of oxygen conditions immediately around the basal plant meristem had no major effect on internal oxygen content. High temperatures had a dramatic effect on the internal oxygen balance of eelgrass. Increasing temperature stimulated plant respiration more than photosynthesis, and the meristem went anoxic, even in the light, at water temperatures above 30°C. We hypothesize that low meristematic oxygen content is a key factor in events of seagrass die‐off.

In-situ measurement of photosynthetic oxygen production in the water column

A novel hardware device is described to determine photosynthetic and respiratory oxygen uptake and release, respectively, in organisms in their natural habitat even under the rough conditions of their marine environment. Both macroalgae and phytoplankton can be utilized and oxygen exchange can be determined in solar radiation. The chamber can be used at or above the water surface or can be lowered into the water column. The data of oxygen concentration, irradiance and temperature are constantly monitored by a laptop computer and stored in disk files. The experimental data measured in some macroalgae as well as in phytoplankton indicate that the irradiance window for positive net photosynthetic oxygen production is fairly limited under natural conditions; at too low irradiances respiration exceeds photosynthesis and at too high irradiances photosynthesis is shut down by photoinhibition, at least in species not adapted to unattenuated solar radiation.

Determination of the photosynthetic oxygen release time by amperometry

Flash-induced amperometric oxygen pulses from Chlorella or chloroplast samples were studied using a special electrode setup (so-called ‘centrifuged electrode’). With this setup, the active particles were brough into close contact with a plane platinum cathode by (ultra)centrifugation, resulting in a fast response to any photosynthetic oxygen emission. The situation could be simply modelled and the experimental curves analyzed in terms of two independently adjustable parameters, the diffusion time and the oxygen release time. It was consistently found that the pulse kinetics is nearly diffusion-limited, and confirmed that the photosynthetic oxygen release time was in the millisecond or sub-millisecond range.

EFFECT OF TEMPERATURE, LIGHT AND pH ON GROWTH, PHOTOSYNTHESIS AND RESPIRATION OF THE DINOFLAGELLATE PERIDINIUM CINCTUM FA. WESTII IN LABORATORY CULTURES1

ABSTRACTOptimum light, temperature, and pH conditions for growth, photosynthetic, and respiratory activities of Peridinium cinctum fa. westii (Lemm.) Lef were investigated by using axenic clones in batch cultures. The results are discussed and compared with data from Lake Kinneret (Israel) where it produces heavy blooms in spring. Highest biomass development and growth rates occurred at ca. 23° C and ≥50 μE· m−2·s1 of fluorescent light with energy peaks at 440–575 and 665 nm. Photosynthetic oxygen release was more efficient in filtered light of blue (BG 12) and red (RG 2) than in green (VG 9) qualities. Photosynthetic oxygen production occurred at temperatures ranging from 5° to 32° C in white fluorescent light from 10 to 105 μE·m−2·s−1 with a gross maximum value of 1500 × 10−12 g·cell−1·h−1 at the highest irradiance. The average respiration amounted to ca. 12% of the gross production and reached a maximum value of ca. 270·10−12 g·cell−1·h−1 at 31° C. A comparison of photosynthetic and respiratory Q10‐values showed that in the upper temperature range the increase in gross production was only a third of the corresponding increase in respiration, although the gross production was at maximum. Short intermittent periods of dark (>7 min) before high light exposures from a halogen lamp greatly increased oxygen production. Depending on the physiological status of the alga, light saturation values were reached at 500–1000 μE·m−2·s−1 of halogen light with compensation points at 20–40 μE·m−2·s−1 and Ik‐values at 100–200 μE·m−2·s−1. The corresponding values in fluorescent light in which it was cultured and adapted, were 25 to 75% lower indicating the ability of the alga to efficiently utilize varying light conditions, if the adaptation time is sufficient. Carbon fixation was most efficient at ca. pH 7, but the growth rates and biomass development were highest at pH 8.3.

Photosynthesis—light relationships of a mixed culture of phytoplankton in fluctuating light

Rates of oxygen production and inorganic carbon uptake by a mixed culture of phytoplankton were determined in a highly variable light regime. Scatter diagrams of the results revealed a smoothly varying relationship between irradiance and photosynthetic oxygen release except when the rate of oxygen release was depressed by CO2 depletion. There was no indication that the instantaneous rate of oxygen production depended on light intensities received in the recent (5–30 min) past. However, the net rate of inorganic carbon uptake exhibited large, transient fluctuations in response to rapid changes in light as CO2 became depleted (<0.001 mg·liter−1). Spectral analysis showed that the transient fluctuations in inorganic carbon exchange were related to the rate of change in irradiance. Enhanced transport of inorganic carbon into the cells at high pH may account for the highest rates of carbon uptake; diffusional loss of intracellular CO2, photorespiration, or both may account for the highest rates of carbon release. Models derived from incubations at constant light intensities cannot account for the observed fluctuations in inorganic carbon uptake, indicating the importance of light variability, especially in systems that become depleted of CO2.

Chemical composition and physiological properties of fucoids under conditions of reduced salinity

Thallus segments of Fucus serratus L. and F. vesiculosus L. (Phaeophyceae, Fucales) were transferred into seawater media with a salinity range from 32.65 to 2.25‰ and maintained for at least 2 weeks. Several parameters of chemical composition as well as rates of photosynthetic and respiratory oxygen exchange, 14C-assimilate patterns, and release of 14C-assimilates into the culture medium have been investigated. Compared to controls, in both species dry weight, ash, chloride, and mannitol contents distinctly decline proportionally to reduction of salinity in the incubation media, whereas content of total N (in terms of protein content) remarkably increase. Respiratory O2-consumption is markedly increased at lower salinities, whereas rate of photosynthetic O2-evolution shows some depression. Relatively little effects of salinity changes are observed in distribution of photosynthetically assimilated 14C among the major groups of photosynthates. Release of 14C-assimilates into the incubation medium never exceeds 2% of total 14C-uptake, but is stimulated in media of reduced salt content. The results are discussed with emphasis on phenomena of long-term adaptation and osmoregulation in the marine fucoid species.