Range Of Photon Flux Densities Research Articles

A microfluidic photobioreactor for simultaneous observation and cultivation of single microalgal cells or cell aggregates

Microalgae are an ubiquitous and powerful driver of geochemical cycles which have formed Earth’s biosphere since early in the evolution. Lately, microalgal research has been strongly stimulated by economic potential expected in biofuels, wastewater treatment, and high-value products. Similar to bacteria and other microorganisms, most work so far has been performed on the level of suspensions which typically contain millions of algal cells per millilitre. The thus obtained macroscopic parameters average cells, which may be in various phases of their cell cycle or even, in the case of microbial consortia, cells of different species. This averaging may obscure essential features which may be needed for the correct understanding and interpretation of investigated processes. In contrast to these conventional macroscopic cultivation and measuring tools, microfluidic single-cell cultivation systems represent an excellent alternative to study individual cells or a small number of mutually interacting cells in a well-defined environment. A novel microfluidic photobioreactor was developed and successfully tested by the photoautotrophic cultivation of Chlorella sorokiniana. The reported microbioreactor facilitates automated long-term cultivation of algae with controlled temperature and with an illumination adjustable over a wide range of photon flux densities. Chemical composition of the medium in the microbioreactor can be stabilised or modulated rapidly to study the response of individual cells. Furthermore, the algae are cultivated in one focal plane and separate chambers, enabling single-cell level investigation of over 100 microcolonies in parallel. The developed platform can be used for systematic growth studies, medium screening, species interaction studies, and the thorough investigation of light-dependent growth kinetics.

Effects of light and temperature on the attachment and development of Gloiopeltis tenax and Gloiopeltis furcata tetraspores

Two 60-day experiments were conducted to study the influence of photon flux density (PFD) and temperature on the attachment and development of Gloiopeltis tenax and Gloiopeltis furcata tetraspores. In the first experiment, tetraspores of the two Gloiopeltis species were incubated at five temperature ranges (8°C, 12°C, 16°C, 20°C, 24°C) under a constant PFD of 80 μmol photons m−2 s−1 with a photoperiod of 12:12. In a second experiment, tetraspores were incubated under five PFD gradients (30, 55, 80, 105, 130 μmol photons m−2 s−1) at a constant temperature of 16°C with a photoperiod of 12:12. Maximum density of attached tetraspores was observed at 16°C for both species. Maximum per cent of spore germinating into disc was recorded at 12–16°C for G. tenax and 8–12°C for G. furcata. Maximum per cent of discs producing erect axes for G. tenax and G. furcata were recorded at 24°C and 20°C, respectively. Light had no significant effect on tetraspore attachment and developing into disc, but it affected the growth, sprouting and survival of its discs. Under 30–55 μmol photons m−2 s−1, the discs of the two species of Gloiopeltis did not form thallus until the end of the experiment. Optimum PFD range for G. tenax discs was 80–105 μmol photons m−2 s−1, whilst it was 80–130 μmol photons m−2 s−1 for G. furcata. Results presented in this study are expected to assist the progress of artificial seeding of Gloiopeltis.

ACCLIMATION OFEMILIANIA HUXLEYI(PRYMNESIOPHYCEAE) TO PHOTON FLUX DENSITY1

Growth rate, pigment composition, and noninvasive chlafluorescence parameters were assessed for a noncalcifying strain of the prymnesiophyteEmiliania huxleyiLohman grown at 50, 100, 200, and 800 μmol photons·m−2·s−1.Emiliania huxleyigrown at high photon flux density (PFD) was characterized by increased specific growth rates, 0.82 d−1for high PFD grown cells compared with 0.38 d−1for low PFD grown cells, and higherin vivochlaspecific attenuation coefficients that were most likely due to a decreased pigment package, consistent with the observed decrease in cellular photosynthetic pigment content. High PFD growth conditions also induced a 2.5‐fold increase in the pool of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin responsible for dissipation of excess energy. Dark‐adapted maximal photochemical efficiency (Fv/Fm) remained constant at around 0.58 for cells grown over the range of PFDs, and therefore the observed decline, from 0.57 to 0.33, in the PSII maximum efficiency in the light‐adapted state, (Fv′/Fm′), with increasing growth PFD was due to increased dissipation of excess energy, most likely via the xanthophyll cycle and not due to photoinhibition. The PSII operating efficiency (Fq′/Fm′) decreased from 0.48 to 0.21 with increasing growth PFD due to both saturation of photochemistry and an increase in nonphotochemical quenching. The changes in the physiological parameters with growth PFD enableE. huxleyito maximize rates of photosynthesis under subsaturating conditions and protect the photosynthetic apparatus from excess energy while supporting higher saturating rates of photosynthesis under saturating PFDs.

Inter‐specific differences in photosynthetic carbon uptake, photosynthate partitioning and extracellular organic carbon release by deep‐water characean algae

1. The effect of light intensity on photosynthesis and the fate of newly fixed organic carbon was compared for three characean algae collected at the same depth (10 m) but differing in their depth distributions. For each species we determined photosynthesis–irradiance (P–E) responses, the partitioning of newly fixed carbon into four intracellular pools (low molecular‐weight compounds, polysaccharides, lipids and proteins) and the extracellular organic carbon (EOC) release at a range of photon flux densities (PFD) 0–60 μmol m–2 s–1.2. The P–E responses differed between the three species, with the light compensation point (Ec) and dark respiration rate highest in the shallowest species (Chara fibrosa), intermediate in the mid‐range species (C. globularis) and lowest in the deepest species (C. corallina). Photosynthetic efficiency (α) and photosynthesis: respiration ratios were lowest in C. fibrosa and highest in C. corallina.3. In all three species, the low molecular weight pool was the principal photosynthetic product (>60% of fixed C) at 3 μmol m–2 s–1 PFD, but its proportional contribution decreased rapidly with increasing irradiance. Polysaccharide rose to become the major product (>35% of fixed C) at saturating PFD (35 μmol m–2 s–1).4. Protein synthesis was saturated at 5 μmol m–2 s–1 in all species and was consistently a lower proportion of the fixed carbon in C. corallina than the other species. The fraction incorporated in the lipid pool increased slightly with irradiance but was always less than 10% of fixed C, while the proportion lost as EOC was unaffected by light, being significantly higher in C. fibrosa than the other species.5. A kinetic experiment with C. fibrosa at 35 μmol m–2 s–1 PFD revealed a continued increase in net polysaccharide, protein and lipid synthesis during a 22.5‐h light period, whereas the net size of the low molecular weight pool remained constant. In a subsequent dark period, protein and lipid synthesis continued at the expense of the polysaccharide and low‐molecular‐weight pools. The EOC release rose to a constant low release in the light, then peaked slightly immediately after the dark–light transition before returning to the same rate as in the light. Extrapolating these data over 24 h suggests that the proportion of fixed carbon lost as EOC may be as high as 10% in this species.6. The interspecific differences in carbon acquisition between the three species reflected their depth distributions, with the deeper species having more efficient photosynthetic metabolism, lower P:R ratios and less EOC release, although no apparent differences in internal partitioning of photosynthate.

Analysis of photoluminescence efficiency and surface recombination velocity of MBE-grown AlGaAs layers

Rigorous computer analysis of the band-edge PL efficiency (IPL/Φ) and the effective surface recombination velocity (Seff) was performed for n-Al0.33Ga0.67As surfaces versus the surface state density, NSS0, in a wide range of photon flux density, Φ (from 1012 to 1025 cm−2 s−1). The behavior of electron (EFn) and hole (EFp) quasi-Fermi levels was also studied. In addition, the simulated IPL/Φ–Φ dependencies have been compared with experimental data for MBE-grown AlGaAs wafers passivated by ultrathin Si interface control layer.

Photoabatement by Anthocyanin Shields Photosynthetic Systems from Light Stress

Leaves and other chlorophyllous tissues of plants often show transient or permanent anthocyanin coloration. The question of whether anthocyanins can function as effective light screens to modulate photosynthesis in plants was addressed by comparing photosynthetic responses in reddish-purple pods with those in green pods of the ornamental leguminous tree Bauhinia variegata. For these comparisons the actinic radiation employed was either red radiation (RR) which was poorly absorbed by anthocyanin or blue-green radiation (BGR) which was strongly absorbed by anthocyanin. Photon yields of photosystem 2 (PS2) photochemistry and photochemical chlorophyll fluorescence quenching coefficients (qp), measured over a range of photon flux densities (PFD) up to 1200 µmol m-2 s-1 at 23 °C and at five temperatures from 8 to 28 °C at a PFD of 260 µmol m-2 s-1, were almost identical in green pods irradiated with either RR or BGR and in purple pods irradiated with RR. However, qp values remained much higher in purple pods irradiated with BGR, e.g., 0.80 in BGR versus 0.29 in RR at a PFD of 1200 µmol m-2 s-1 at 23 °C, and 0.67 in BGR versus 0.28 in RR at a PFD of 260 µmol m-2 s-1 at 8 °C. The higher values of qp in BGR compared to RR indicated that photoabatement by anthocyanin allowed the first stable acceptor of PS2, QA, to be kept in a more oxidized state, thus decreasing the likelihood of photoinhibition. This was confirmed by demonstrating a lower susceptibility to photoinhibition in purple pods than in green pods in the sunlight, either naturally in pods on trees or in detached pods exposed to photoinhibitory conditions. We conclude that photoabatement by anthocyanin is a mechanism for allowing maintenance of higher oxidative levels of PS2 acceptor during episodes of high radiation stress, thereby minimizing photodamage to photosynthetic tissues.

Some characteristics of reduced leaf photosynthesis at midday in maize growing in the field

Maize was grown in the high-radiation arid summer environment of Davis, California, and its leaf photosynthetic rate was measured over diurnal courses on cloudless days with the leaf held perpendicular to the sunlight. On days of high atmospheric vapor pressure deficit (VPD), leaf photosynthesis reached a maximum in the late morning and then decreased gradually as the day progressed, though the soil was well irrigated. When CO 2 concentration in the measurement chamber was raised to about 1000 μmol mol −1, photosynthesis was enhanced, but more in the afternoon than in the morning. As a result, rates measured at high CO 2 in the morning and afternoon were essentially the same. There was also no difference in the curves of photosynthetic rate ( A) versus intercellular CO 2 concentration ( C i) for the morning and afternoon. Hence, photosynthetic capacity was similar for the two periods and there was no evidence of photoinhibition by the high photosynthetic photon flux density at noon. Further, C i and photosynthetic rates A measured over a range of photon flux density were lower in the afternoon than in the morning. These results indicate that A at noon and early afternoon was more limited than in the morning by epidermal conductance (mostly stomatal). On a day of low VPD, however, midday depression in A and epidermal conductance were not evident for the well-irrigated plants. Without irrigation and with leaves at a lower midday water potential, midday reduction in conductance and A was much more marked, beginning late in the morning. Epidermal conductance of maize grown in the field in Davis is are not sensitive to VPD. Therefore, the midday reduction in conductance and A was more likely the result of low leaf water potential caused by high transpiration rates.

Photosynthetic responses of New Zealand Sphagnum species

Samples of New Zealand Sphagnum moss (S. cristatum and S. australe) were collected from different sites in both the North and South Islands of New Zealand. Optimal conditions for photosynthesis and carbon assimilation for each species were determined from gas exchange and photosystem II (PSII) chlorophyll a fluorescence measurements under a range of photon flux densities (PFD), temperatures and water contents (WC). All samples displayed a typical light saturation response, saturating at 111–266 μmol m‐2 s‐1, which is generally lower than reported for Northern Hemisphere species. Little difference was found in the photosynthetic response to light between the different New Zealand moss species or between samples from different sites. Greater differences were found within S. cristatum samples of different colour. Brown samples of S. cristatum reached light saturated photosynthesis at a higher PFD, and had lower photosynthetic rates, lower quantum efficiencies, and higher light compensation points than green samples. This is attributed to the screening effect of the pigments in the brown moss. Green moss samples of S. cristatum had an optimum WC of 1200–2000% dry weight, whereas the brown samples of the same species had a wider range of optimum WC of 1400–3000% dry weight. Photosynthesis declined rapidly at WC below the optimum and more gradually at higher WC. Optimal temperatures for net photosynthesis were found to be 20–25°C in two species studied from two South Island sites, with similar rates measured in all samples over the temperature range investigated. Although broadly similar to Sphagnum photosynthetic responses reported elsewhere, specific differences were noted and possible reasons for this are discussed.

Influence of light on DNA content of Helianthus annuus Linnaeus.

Mean nuclear 2C DNA content (C equaling haploid DNA per nucleus) of the first leaf of the sunflower, Helianthus annuus L., is influenced by the quality and the quantity of light. Seedlings of two inbred lines, RHA 299 and RHA 271 were germinated and grown in controlled environmental conditions. Lighting was adjusted to provide different combinations of photon flux densities and red to far red (R:FR) ratios. At R:FR = 5.8 and photon flux densities of 170 mumol.m-2.s-1, 200 mumol.m-2.s-1, and 230 mumol.m-2.s-1, DNA content remained high and relatively constant (x = 6.97 pg for RHA 271 and x = 7.32 pg for RHA 299). When the photon flux density range (R:FR = 5.8) was elevated to 350 mumol.m-2.s-1, 410 mumol.m-2.s-1, and 470 mumol.m-2.s-1, mean DNA content was reduced to 6.23 pg (RHA 271) and 6.46 pg (RHA 299). At R:FR = 1.5, mean DNA content was consistently high (7.2-7.9 pg) only at the lowest photon flux density of 170 mumol.m-2.s-1. Significant decreases in DNA content (< or = 12%) were observed at photon flux densities of 200 mumol.m-2.s-1 and 230 mumol.m-2.s-1. At the higher photon flux densities (350 mumol.m-2.s-1, 410 mumol.m-2.s-1, and 470 mumol.m-2.s-1) and R:RF = 1.5, the plants had extremely low DNA contents (mean x = 3.36 pg for RHA 271 and 3.41 pg for RHA 299) and high between-plant variance. The instability of DNA content, particularly for plants grown under light that is far red rich, suggests that phytochromes may be involved in regulating DNA content of the sunflower.

Regulation of Photosynthetic Induction State in High- and Low-Light-Grown Soybean and Alocasia macrorrhiza (L.) G. Don.

Alocasia (Alocasia macrorrhiza [L.] G. Don) and soybean (Glycine max [L.]) were grown under high or low photon flux density (PFD) conditions to achieve a range of photosynthetic capacities and light-adaptation modes. The CO2 assimilation rate and in vivo linear electron transport rate (Jf) were determined over a range of PFDs and under saturating 1-s-duration lightflecks applied at a range of frequencies. At the same mean PFD, the assimilation rate and the Jf were lower under the lightfleck regimes than under constant light. The activation state of two, key enzymes of the photosynthetic carbon reduction cycle pathway, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and fructose-1,6-bisphosphatase, and the photosynthetic induction states (ISs) were also found to be lower under flashing as compared to continuous PFD. Under all conditions, the IS measured 120 s after an increase in PFD to constant and saturating values was highly correlated with the Rubisco activation state and stomatal conductances established in the light regime before the increase. Both the fructose-1,6-bisphosphatase and Rubisco activities established in a particular light regime were highly correlated with the mean Jf in that regime. The relationships between enzyme activation state and Jf and between IS and enzyme activation state were similar in soybean and Alocasia and were not affected either by growth-light regime, and hence photosynthetic capacity, or by flashing versus constant PFD. The common relationship between the linear Jf and the activation state of key enzymes suggests that electron transport may be the determinant of the signal regulating IS, at least to the extent that the IS is controlled by the activation state of key stromal enzymes.

Consequences of LHC II deficiency for photosynthetic regulation in chlorina mutants of barley

The light-harvesting chlorophyll a/b proteins associated with PS II (LHC II) are often considered to have a regulatory role in photosynthesis. The photosynthetic responses of four chlorina mutants of barley, which are deficient in LHC II to varying degrees, are examined to evaluate whether LHC II plays a regulatory role in photosynthesis. The efficiencies of light use for PS I and PS II photochemistry and for CO2 assimilation in leaves of the mutants were monitored simultaneously over a wide range of photon flux densities of white light in the presence and absence of supplementary red light. It is demonstrated that the depletions of LHC II in these mutants results in a severe imbalance in the relative rates of excitation of PS I and PS II in favour of PS I, which cannot be alleviated by preferential excitation of PS II. Analyses of xanthophyll cycle pigments and fluorescence quenching in leaves of the mutants indicated that the major LHC II components are not required to facilitate the light-induced quenching associated with zeaxanthin formation. It is concluded that LHC II is important to balance the distribution of excitation energy between PS I and PS II populations over a wide range of photon flux densities. It appears that LHC II may also be important in determining the quantum efficiency of PS II photochemistry by reducing the rate of quenching of excitation energy in the PS II primary antennae.

The relationship between photoacclimation and phagotrophy with respect to chlorophyll a, carbon and nitrogen content, and cell size of Chrysochromulina brevifilum (Prymnesiophyceae)

Abstract The chlorophyll content of photosynthetic algae varies as a response to changes in photon flux density. Mixotrophic algae that are predominantly heterotrophic have also been shown to alter their chlorophyll a content as a response to changes in prey density. The marine flagellate Chrysochromulina brevifilum Parke et Manton (Prymnesiophyceae) was subjected to a range of photon flux densities, both with and without Marsupiomonas pelliculata Jones (Pedinophyceae) as a prey organism, and the ingestion of prey, chlorophyll a content, carbon and nitrogen content and cell size were monitored. Results demonstrated that the chlorophyll a content per cell of C. brevifilum changed with photon flux density, but prey density had no effect. Under a relatively low photon flux density (25 μmol photons m−2 s−1) both the ingestion of prey and the cellular concentration of chlorophyll a were greater than under a higher photon flux density (100 μmol photons m−2 s−1). However, the recent history of cultures of C. bre...

CELL AND GROWTH CHARACTERISTICS OF TYPES A AND B OF EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) AS DETERMINED BY FLOW CYTOMETRY AND CHEMICAL ANALYSES1

ABSTRACTTwo morphotypes of Emiliania huxleyi (Lohmann 1902) Hay et al. 1967, types A and B, known to be unequally distributed in the oceans, were grown in dilution cultures at a range of photon flux densities (PFDs) (1.5–155 μmol photons·m−2·s−1) and two temperatures (10° and 15° C). Calcite carbon and organic carbon content of the cells as well as instantaneous growth rate, cell size, chlorophyll fluorescence, and light‐scatter properties clearly depended on growth conditions and differed considerably for the two morphotypes. The ratio between calcite carbon and organic carbon production showed an optimum of 0.65 in E. huxleyi type A cells at PFD = 17.5. The ratio increased slightly with a temperature increase from 10° to 15°C but remained &lt; 1.0 at both temperatures in light‐limited cells. In contrast, calcite carbon production exceeded organic carbon production (ratio: 1.4–2.2) in phosphate‐deprived cultures. Emiliania huxleyi type B generally showed a higher calcite carbon/organic carbon ratio than E. huxleyi type A, but the relation with PFD was similar.The content of calcite carbon and organic carbon as well as the instantaneous growth rate, cell size, chlorophyll fluorescence, and light‐scatter properties showed large diel variations that were closely related to the division cycle. Our results show the importance of mapping the structure of any sampled cell population with respect to the phase in the cell division cycle, as this largely determines the outcome of not only “per cell” measurements but also short time (less than 24 h) flux measurements. For instance, dark production of calcite by E. huxleyi was negatively affected by cell division. Slowly growing (phosphate‐stressed) cultures produced calcite in the light and in the dark. In contrast, rapidly growing cultures at 10°C produced calcite only in the light, whereas in the dark there was a significant loss of calcite due to dissolution.

On the relationship between the quantum yield of Photosystem II electron transport, as determined by chlorophyll fluorescence and the quantum yield of CO2-dependent O2 evolution

We tested the two empirical models of the relationship between chlorophyll fluorescence and photosynthesis, previously published by Weis E and Berry JA 1987 (Biochim Biophys Acta 894: 198-208) and Genty B et al. 1989 (Biochim Biophys Acta 990: 87-92). These were applied to data from different species representing different states of light acclimation, to species with C3 or C4 photosynthesis, and to wild-type and a chlorophyll b-less chlorina mutant of barley. Photosynthesis measured as CO2-saturated O2 evolution and modulated fluorescence were simultaneously monitored over a range of photon flux densities. The quantum yields of O2 evolution (ØO2) were based on absorbed photons, and the fluorescence parameters for photochemical (qp) and non-photochemical (qN) quenching, as well as the ratio of variable fluorescence to maximum fluorescence during steady-state illumination (F'v/F'm), were determined. In accordance with the Weis and Berry model, most plants studied exhibited an approximately linear relationship between ØO2/qp (i.e., the yield of O2 evolution by open Photosystem II reaction centres) and qN, except for wild-type barley that showed a non-linear relationship. In contrast to the linear relationship reported by Genty et al. for qp×F'v/F'm (i.e., the quantum yield of Photosystem II electron transport) and ØCO2, we found a non-linear relationship between qp×F'v/F'm and ØO2 for all plants, except for the chlorina mutant of barley, which showed a largely linear relationship. The curvilinearity of wild-type barley deviated somewhat from that of other species tested. The non-linear part of the relationship was confined to low, limiting photon flux densities, whereas at higher light levels the relationship was linear. Photoinhibition did not change the overall shape of the relationship between qp×F'v/F'm and ØO2 except that the maximum values of the quantum yields of Photosystem II electron transport and photosynthetic O2 evolution decreased in proportion to the degree of photoinhibition. This implies that the quantum yield of Photosystem II electron transport under high light conditions may be similar for photoinhibited and non-inhibited plants. Based on our experimental results and theoretical analyses of photochemical and non-photochemical fluoresce quenching processes, we conclude that both models, although not universal for all plants, provide useful means for the prediction of photosynthesis from fluorescence parameters. However, we also discuss that conditions which alter one or more of the rate constants that determine the various fluorescence parameters, as well as differential light penetration in assays for oxygen evolution and fluorescence emission, may have direct effect on the relationships of the two models.

Photosynthesis-irradiance relationships and carbon metabolism of different ice algal assemblages collected from Weddell Sea pack ice during austral spring (EPOS 1)

Photosynthesis-irradiance relationships and the carbon metabolism of different ice algal assemblages collected from Weddell Sea pack ice were investigated during the EPOS 1 cruise. Infiltration- and interstitial assemblages exhibited the photosynthetic characteristics of high-light adapted ice algae with a mean assimilation number of 1.81±0.93 mg C (mg Chl a)−1 h−1. A higher light harvesting efficiency under light limited conditions (alphaB-value), as well as a lower light intensity for light saturation (IK-value) was determined for the interstitial assemblage. An increase in light intensity from 3.5 to 106 μmol m−2s−1 resulted in increased synthesis of polymeric carbohydrates (presumably reserve material) in a band assemblage. However, the absolute incorporation of radiolabel into lipid- and amino acid fractions remained essentially constant over this range of photon flux densities. Light-saturated rates of photosynthesis of three infiltration assemblages under hypersaline conditions (approx. 50 and 110%) decreased by 13–55% (controls: approx. 32–34%). The adverse effect of salinity treatment was much less pronounced under hyposaline conditions (approx. 20‰), where maximal photosynthetic rates were only slightly decreased (-9%) or even stimulated (14–22%). These observations suggest that sea ice microalgae in the ice edge region of the Weddell Sea during spring, being in a metabolically active stage, may have the potential to initiate or contribute to phytoplankton blooms upon release into the water column.

Photoinhibition and recovery of photosynthesis in intact barley leaves at 5 and 20°C

Photoinhibition of photosynthesis and its recovery were studied in intact barley (Hordeum vuigare L. cv. Gunilla) leaves grown in a controlled environment by exposing them to two temperatures, 5 and 20°C, and a range of photon flux densities in excess of that during growth. Additionally, photoinhibtion was examined in the presence of chloramphenicol (CAP, an inhibitor of chloroplast protein synthesis) and of 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU). Susceptibility to photoinhibition was much higher at 5 than at 20°C. Furthermore, at 20°C. CAP exacerbated photoinhibition strongly, whereas CAP had little additional effect (10%) at 5°C. These results support the model that net photoinhibition is the difference between the inactivation and repair of photosystem II (PSII); i.e. the degradation and synthesis of the reaction centre protein, Dl. Furthermore, the steady‐state extent of photoinhibition was strongly dependent on temperature and the results indicated this was manifested through the effects of temperature on the repair process of PSII. We propose that the continuous repair of PS II at 20°C conferred at least some protection from photoinhibition. At 5°C the repair process was largely inhibited, with increased photoinhibition as a consequence. However, we suggest where repair is inhibited by low temperature, some protection is alternatively conferred by the photoinhibited reaction centres. Providing they are not degraded, such centres could still dissipate excitation energy non‐radiatively, thereby conferring protection of remaining photochemically active centres under steady‐state conditions.A fraction of PS II centres were capable of resisting photoinhibition when the repair process was inhibited by CAP. This is discussed in relation to PS II heterogeneity. Furthermore, the repair process was not apparently activated within 3 h when barley leaves were transferred to photoinhibitory light conditions at 20°C.

Predictions of Mn and Fe use efficiencies of phototrophic growth as a function of light availability for growth and of C assimilation pathway

SUMMARYIron is involved in many photosynthetic, respiratory and nitrogen assimilation reactions of plants as Fe bound tightly to polypeptides catalysing redox reactions. Manganese is involved as tightly bound Mn in photoreaction II of photosynthesis and in certain superoxide dismutases, while loosely bound Mn2+ is the unique activator of some enzymes, and is an alternative to Mg2+ in activating many enzymes. This paper uses data on the quantitative role of Fe and Mn in catalysts to predict the efficiency with which Fe and Mn are used in C assimilation [mol C assimilated (mol catalytic metal in enzyme)−1 s−1] and the metal cost of C assimilation [mol catalytic metal in enzyme (mol C assimilated)−1 s−1] in photolithotrophic growth in relation to genetic and environmental variables. The genetic variables were the relative content of thylakoid proteins in major taxa (cyanobacteria and red algae, chlorophytes and chromophytes) and smaller‐scale taxonomic differences (various subtypes of C4 metabolism, and C3 metabolism, in terrestrial vascular plants). The environmental variables were the range of photon flux densities in which photolithotrophic growth of O2 evolvers can occur, and the inorganic C supply conditions controlling the repression/de‐repression of the inorganic C concentrating mechanism in cyanobacteria and microalgae.The results of the computations yield the following conclusions. The largest predicted difference in Fe and Mn costs of photolithotrophic growth is related to changes in the photon flux density for growth. The predicted Fe cost increased 50‐fold, and the Mn cost increased 80‐fold, at the lowest extreme of photon flux density compared to the highest found naturally. The increase is partly countered by the larger ratio of light‐harvesting pigments to thylakoid protein complexes assumed for the cells grown at low photon flux densities, although the extent of the increase in photosynthetic unit size is limited by considerations of efficiency of excitation energy transfer. However, the major influences are the higher pigment content in biomass enabling a larger fraction of incident light to the absorbed, and the sub‐maximal specific reaction rates of redox catalysts (whose content is constrained via excitation energy transfer considerations) at very low photon flux densities. A smaller difference, four‐fold or less, in Fe and Mn costs of photolithotrophic growth, is predicted by comparing major taxa (cyanobacteria plus red algae; chlorophytes plus chromophytes) with contrasting ratios of thylakoid redox catalysts. Differences in Fe and Mn costs of growth of less than two‐fold are predicted when the Fe‐ and Mn‐efficient organisms with CO2− concentrating mechanisms (C4 land plants; algae with active inorganic C influx) and high requirements for ATP relative to NADPH, are compared with organisms relying on CO2 diffusion from air or air‐equilibrated solutions and C3 biochemistry.These predictions of variations in Fe and Mn costs of photolithotrophy have implications for the ecology of phototrophs.

Growth, photosynthesis and agar in wild-type strains of Gracilaria verrucosa and G. conferta (Gracilariales, Rhodophyta), as a strain selection experiment

The local species Gracilaria conferta and the foreign G. verrucosa were grown together under a wide range of photon flux density and temperature conditions. Gracilaria verrucosa showed a higher growth rate, especially under low temperatures, and higher photosynthetic performances as well as higher ribulose-1,5-bisphosphate carboxylase activity as compared with G. conferta. Gracilaria verrucosa also showed a better quality and yield of agar, suggesting that this species could be more suitable than G. conferta for outdoor cultivation in Israel and may improve winter growth in ponds. Growth rate and agar quality (gel strength) were rated as the most suitable characteristics influencing the preference of strains for outdoor cultivation.

INFLUENCE OF IRRADIANCE ON THE FATTY ACID COMPOSITION OF PHYTOPLANKTON1

ABSTRACTEight species of marine phytoplankton commonly used in aquaculture were grown under a range of photon flux densities (PEDs) and analyzed for their fatty acid (FA) composition. Fatty and composition changed considerably at different PFDs although no consistent correlation between the relative proportion of a single FA and μ or chl a · cell−1 was apparent. Within an individual species the percentage of certain fatty acids covaried with PFDs, growth rate and/or chl a · cell−1. The light conditions which produced the greatest proportion of the essential fatty acids was species specific. Eicosapentaenoic acid. 20:5ω3 increased from 6.1% to 15.5% of the total fatty acids of Chaetoceros simplex Ostenfield grown at PFDs which decreased from 225 μE · m−2· s−1 to 6 μE · m−2· s−1, respectively. Most species had their greatest proportion of 20: 5ω3 at low levels of irradiance. Conversely, docosahexaenoic acid, 22:6ω3, decreased from 9.7% to 3.6% of the total fatty acids in Pavlova lutheri Droop as PFD decreased. The percentage of 22:6ω3 generally decreased with decreasing irradiances. In all diatoms the percentage of 16:0 was significantly correlated with PFD, and in three of five diatoms, with growth rate (μ). Results suggest that fatty acid composition is a highly dynamic component of cellular physiology, which responds significantly to variation in PFD.

Quenching and enhancement of photoconductivity in semi-insulating GaAs

The time evolution of low temperature photoconductivity (PC) in Liquid Encapsulated Czochralski Semi-insulating GaAs for below-the- gap energy photons is studied. For a certain range of photon flux densities both the quenching and enhancement part of the PC curve can be observed. It is shown that the transformation of EL2 defects into their metastable states cannot be responsible for the quenching part of PC curve. The good correlation of the PC with deep trap occupancy is established during both quenching and enhancement part of PC curve, leading to the conclusion that filling/emptying of deep traps plays the key role in peculiar time evolution of PC.