The uptake of amino acids by the cyanobacterium Planktothrix rubescens is stimulated by light at low irradiances

The rates of uptake of five amino acids--alanine, glutamate, glycine, leucine and serine--by axenic cultures of the cyanobacterium Planktothrix rubescens were measured over a range of irradiances using the (14)C-labelled amino acids at the nanomolar concentrations observed in Lake Zürich. The rates in the light exceeded the dark rates by as much as two- to ninefold. The light-affinity constants for stimulation were similar, indicating a similar process for each of the five amino acids. The E(k) (light saturation irradiance) for light stimulation was only 1 micromol m(-2) s(-1), less than the compensation point for photosynthesis and autotrophic growth, and much lower than the E(k) for either process. The E(k) for amino acid uptake was also less than the irradiance at which filaments obtain neutral buoyancy, which determines the depth at which they stratify and the irradiance they receive. This indicates that stimulation of amino acid uptake by light of low irradiances provides a mechanism for supplementing growth of filaments stratifying deep in the metalimnion, which, while able to grow at low irradiances, are often left with insufficient light to sustain them. Acetate uptake was also stimulated by light, but the kinetics differed.     

Growth rates of phytoplankton under fluctuating light

• 1 The effect of light fluctuations on the growth rates of four species of freshwater phytoplankton was investigated. Experimental light regimes included constant irradiance and fluctuations of a step function form, with equal proportion of high (maximum of 240 µmol photons m^‐2 s^‐1) and low light (minimum of 5 µmol photons m^‐2 s^‐1) (or dark) in a period. Fluctuations of 1, 8 and 24‐h periods were imposed over several average irradiances (25, 50, 100 and 120 µmol photons m^‐2 s^‐1).
• 2 Growth rate responses to fluctuations were species‐specific and depended on both the average irradiance and the period of fluctuations. Fluctuations at low average irradiances slightly increased growth rate of the diatom Nitzschia sp. and depressed growth of the cyanobacterium Phormidium luridum and the green alga Sphaerocystis schroeteri compared to a constant irradiance.
• 3 Fluctuations at higher average irradiance did not have a significant effect on the growth rates of Nitzschia sp. and Sphaerocystis schroeteri (fluctuations around saturating irradiances) and slightly increased the growth rates of the cyanobacteria Anabaena flos‐aquae and Phormidium luridum (when irradiance fluctuated between limiting and inhibiting levels).
• 4 In general, the effect of fluctuations tended to be greater when irradiance fluctuated between limiting and saturating or inhibiting levels of a species growth‐irradiance curve compared to fluctuations within a single region of the curve.
• 5 The growth rates of species under fluctuating light could not always be predicted from their growth‐irradiance curves obtained under constant irradiance. When fluctuations occur between limiting and saturating or inhibiting irradiances for the alga and when the period of fluctuations is long (greater than 8 h), steady‐state growth‐irradiance curves may be insufficient to predict growth rates adequately. Consequently, additional data on physiological acclimation, such as changes in photosynthetic parameters, may be required for predictions under non‐constant light supply in comparison to constant conditions.

     

INTERACTING EFFECTS OF LIGHT AND NUTRIENT LIMITATION ON THE GROWTH RATE OF SYNECHOCOCCUS LINEARIS (CYANOPHYCEAE)1

The blue-green alga Synechococcus linearis (Naeg.) Kom. was grown in P- and N-limited chemostats over a range of potentially limiting irradiances in order to determine the combined effects of light and nutrient limitation on some aspects of the composition and metabolism of this alga. Over a narrow range of low irradiances, simultaneous limitation of growth rate by light and either N or P was shown. This simultaneous limitation of growth rate by a nutrient and a physical factor can be explained by the ability of an increased supply of one to compensate in part for a decreased supply of the other. At all irradiances, the internal concentration of the limiting nutrient increased with increasing dilution rate, and the results could be fitted to the Droop relationship. With decreasing irradiance, the internal concentration of the limiting nutrient increased. There appeared to be little or no effect of light on the minimum internal concentration of P but that of N increased with decreasing light. Both chlorophyll a and biliprotein per unit particulate C increased with increasing dilution rate and decreasing irradiance. The critical N/P ratio increased with decreasing light as the N requirement of N-limited cells increased faster than did the P requirement of P-limited cells. The composition of exponentially growing cells in complete medium varied much less with light. Neither dilution rate nor irradiance during growth had a great effect on saturated rates of P or N uptake or alkaline phosphatase activity. Calculated assimilation ratios increased with light and dilution rate. The role of the flexibility of nutrient composition in adaptation to adverse conditions and the implications of the results for the use of physiological indicators of nutrient status are discussed.     

RELATIONSHIP BETWEEN PHOTOSYNTHETIC OXYGEN CYCLING AND CARBON ASSIMILATION IN SYNECHOCOCCUS WH7803 (CYANOPHYTA)1

Mass spectrometric analysis of oxygen uptake and evolution in the light by marine Synechococcus WH7803 indicated that the respiration rate was near zero at low irradiance levels but increased significantly at high irradiances. The light intensity (I_r) at which oxygen uptake began to increase with increasing light intensity depended on the growth irradiance of the culture. In each case, I_r coincided with the minimum light intensity for saturation of carbon assimilation (I_k). At irradiances >I_r, net oxygen evolution rates paralleled carbon assimilation rates. Oxygen uptake at high light intensities was inhibited by DCMU, indicating that oxygen uptake was due to Mehler reaction activity. The onset of Mehler activity at I_k supports the idea that oxygen becomes an alternative sink for electrons from photosystem I when NADPH turnover is limited by the capacity of the dark reactions to utilize reductant.     

Physiological Responses of Mesodinium major to Irradiance,Prey Concentration and Prey Starvation

Ciliates within the Mesodinium rubrum/Mesodinium major species complex harbor chloroplasts and other cell organelles from specific cryptophyte species. Mesodinium major was recently described, and new studies indicate that blooms of M. major are just as common as blooms of M. rubrum. Despite this, the physiology of M. major has never been studied and compared to M. rubrum. In this study, growth, food uptake, chlorophyll a and photosynthesis were measured at six different irradiances, when fed the cryptophyte, Teleaulax amphioxeia. The results show that the light compensation point for growth of M. major was significantly higher than for M. rubrum. Inorganic carbon uptake via photosynthesis contributed by far most of total carbon uptake at most irradiances, similar to M. rubrum. Mesodinium major cells contain ~four times as many chloroplast as M. rubrum leading to up to ~four times higher rates of photosynthesis. The responses of M. major to prey starvation and refeeding were also studied. Mesodinium major was well adapted to prey starvation, and 51 d without prey did not lead to mortality. Mesodinium major quickly recovered from prey starvation when refed, due to high ingestion rates of > 150 prey/predator/d.     

Acquired phototrophy in Mesodinium and Dinophysis – A review of cellular organization,prey selectivity,nutrient uptake and bioenergetics

Acquired phototrophy, i.e. the use of chloroplasts from ingested prey, can be found among some species of dinoflagellates and ciliates. The best studied examples of this phenomenon in these groups are within the ciliate genus Mesodinium and the dinoflagellate genus Dinophysis, both ecologically important genera with a worldwide distribution. Mesodinium species differ considerably in their carbon metabolism. Some species rely almost exclusively on food uptake, while other species rely mostly on photosynthesis. In Mesodinium with acquired phototrophy, a number of prey organelles in addition to chloroplasts may be retained, and the host ciliate has considerable control over the acquired chloroplasts; Mesodinium rubrum is capable of dividing its acquired chloroplasts and can also photoacclimate. In Dinophysis spp., the contents of ciliate prey are sucked out, but only the chloroplasts are retained from the ingested prey. Some chloroplast house-keeping genes have been found in the nucleus of Dinophysis and some preliminary evidence suggests that Dinophysis may be capable for photoacclimation. Both genera have been claimed to take up inorganic nutrients, including NO₃⁻, indicating that processes beyond photosynthesis have been acquired. M. rubrum seems to depend upon prey species within the Teleaulax/Plagioselmis/Geminigera clade of marine cryptophytes. Up until now, Dinophysis species have only been maintained cultured on M. rubrum as food, but other ciliates may also be ingested. Dinophysis spp. and M. rubrum are obligate mixotrophs, depending upon both prey and light for sustained growth. However, while M. rubrum only needs to ingest 1–2% of its carbon demand per day to attain maximum growth, Dinophysis spp. need to obtain about half of their carbon demand from ingestion for maximum growth. Both Mesodinium and Dinophysis spp. can survive for months in the light without food. The potential role for modeling in exploring the complex balance of phototrophy and phago-heterotrophy, and its ecological implications for the mixotroph and their prey, is discussed.     

Is Karenia brevis really a low-light-adapted species?

Despite nearly annual blooms of the neurotoxic dinoflagellate Karenia brevis (Davis) G. Hansen and Moestrup in the Gulf of Mexico, defining the suite of biological traits that explain its proliferation has remained challenging. Studies have described K. brevis as a low-light-adapted species, incapable of sustaining growth under high light, which is at odds with observed surface aggregations sometimes within centimeters of the sea surface and also with short-term experiments showing photosynthetic machinery accommodating high irradiances. Here, growth and photophysiology of three K. brevis isolates were evaluated under a range of environmentally relevant irradiances (10–1500 μmol photons m⁻² s⁻¹) in the laboratory. No differences in growth–irradiance curves were observed among isolates; all sustained maximum growth rates at the highest irradiances examined, even in exposures as long as three weeks. The growth efficiency α of K. brevis under light-limiting conditions appeared mediocre among dinoflagellates, and poorer than that of other phytoplankton (e.g., diatoms, cyanobacteria), implying that K. brevis is not a low-light specialist. This finding substantially alters earlier parameterizations of K. brevis growth–irradiance curves. Therefore, a model was developed to contextualize how these new growth–irradiance curves might affect bottom growth rates. This model was subsequently applied to a case study comparing seasonal light forcing offshore of Pinellas County, FL, USA, with a single empirical value for light attenuation, and seasonal bottom water temperatures. Predictions suggested that light may limit bottom growth as close as 1 km from shore in winter, but would only begin limiting growth 20 km from shore in summer. Population maintenance (no net growth) was possible as far offshore as 90 km in summer and 68 km in winter. These ranges intercept areas thought to be important for bloom initiation.     

A PHYSIOLOGICAL TEST OF THE THEORY OF COMPLEMENTARY CHROMATIC ADAPTATION. II. BROWN,GREEN AND RED SEAWEEDS1

The effect of light quantity (irradiance) on the photosynthetic physiology of seven seaweed species was distinguished from the effect of light quality (color). Plants were grown in outdoor, continuous-flow tanks, at irradiances saturating and limiting to growth, and in spectral distributions that were either broadband (white) or narrowband (green). The green light field complemented the absorptance spectrum of phycoerythrin and approximated the spectral distribution of a submarine light field in turbid coastal water near the compensation depth. Physiological measurements, made after 6–15 days growth, included light-harvesting pigment densities, instantaneous photosynthesis-light relationships and growth rate. In all experiments, these photosynthetic properties were independent of spectral distribution (color) and in most experiments were dependent on irradiance. These data do not conform to the predictions of the theory of complementary chromatic adaptation for seaweeds.     

Production and excretion of okadaic acid,pectenotoxin-2 and a novel dinophysistoxin from the DSP-causing marine dinoflagellate Dinophysis acuta – Effects of light,food availability and growth phase

Diarrhetic shellfish poisoning (DSP) toxins constitute a severe economic threat to shellfish industries and a major food safety issue for shellfish consumers. The prime producers of the DSP toxins that end up in filter feeding shellfish are species of the marine mixotrophic dinoflagellate genus Dinophysis. Intraspecific toxin contents of Dinophysis spp. vary a lot, but the regulating factors of toxin content are still poorly understood. Dinophysis spp. have been shown to sequester and use chloroplasts from their ciliate prey, and with this rare mode of nutrition, irradiance and food availability could play a key role in the regulation of toxins contents and production. We investigated toxin contents, production and excretion of a Dinophysis acuta culture under different irradiances, food availabilities and growth phases. The newly isolated strain of D. acuta contained okadaic acid (OA), pectenotoxins-2 (PTX-2) and a novel dinophysistoxin (DTX) that we tentatively describe as DTX-1b isomer. We found that all three toxins were excreted to the surrounding seawater, and for OA and DTX-1b as much as 90% could be found in extracellular toxin pools. For PTX-2 somewhat less was excreted, but often >50% was found extracellularly. This was the case both in steady-state exponential growth and in food limited, stationary growth, and we emphasize the need to include extracellular toxins in future studies of DSP toxins. Cellular toxin contents were largely unaffected by irradiance, but toxins accumulated both intra- and extracellularly when starvation reduced growth rates of D. acuta. Toxin production rates were highest during exponential growth, but continued at decreased rates when cell division ceased, indicating that toxin production is not directly associated with ingestion of prey. Finally, we explore the potential of these new discoveries to shed light on the ecological role of DSP toxins.     

中甸角蒿光合作用对生长光强的响应

在原生地和引种地对高山花卉中甸角蒿（1ncarvillea zhongdianensis）光合作用和叶片性状对生长光强的响应进行研究。结果表明：在香格里拉,光合速率（Pn）、类胡萝卜素（Car）、色素比（Chla/b）均随光强的降低而下降;而比叶面积（SLA）、叶绿素b（Chlb）、叶氮含量（LNCa）随光强的降低而上升。中甸角蒿主要是通过叶片形态、生化效率和叶片氮分配来响应生长光强的变化,对生长光强的适应表现出较大可塑性,使得其相对比较容易引种驯化。中甸角蒿在香格里拉对光照具有较广的适应幅,从香格里拉移栽到昆明后,虽然Pn下降约10%,但RGR增加约30%,表明其可以在昆明较好生长。     

Light-limited growth of two strains of the marine diatom Phaeodactylum tricornutum Bohlin: Chemical composition,carbon partitioning and the diel periodicity of physiological processes

Two isolates of the marine pennate diatom Phaeodactylum tricornutum Bohlin were grown in semi-continuous, nutrient-sufficient culture at varying irradiances on a 12-h light, 12-h dark illumination cycle. The reponse of the isolates to varying degrees of light limitation differed with respect to all of the compositional parameters measured, including growth rates, elemental composition, chlorophyll content, and the partitioning of cellular carbon into four biochemical classes: proteins, lipids, polysaccharides, and low-molecular weight intermediates. The isolates also differed with respect to the relative contributions of light-period and dark-period uptake to the total uptake of ammonium and phosphate ions, although in all cases uptake took place at a reduced rate in the dark. They did not differ with respect to the diel periodicity of cell division, chlorophyll synthesis, and biochemical synthesis. Slightly more cell division took place during the dark period than during the light period. The specific rate of chlorophyll synthesis in the light period, when expressed as a function of irradiance, saturated rapidly; the rate was nearly constant for all irradiances > 100 βE · m^?2 · s^?1. Chlorophyll synthesis in the dark was positively correlated with irradiance over the entire range of irradiances, except where photoinhibition was involved. Protein was synthesized in both the light and dark periods, but at a reduced rate in the dark. Polysaccharides were synthesized during the light period and consumed during the dark period. Lipids and low molecular weight intermediates were synthesized during the light period, but showed little net change during the dark period.     

Synechococcus sp. (PTCC 6021) cultivation under different light irradiances—Modeling of growth rate-light response

Synechococcus sp. (PTCC 6021), a cyanobacterium species, was cultivated in an internally illuminated photobioreactor. The reactor was designed to achieve a monoseptic cultivation of the species. The goal was to study the growth–irradiance behavior of Synechococcus sp. (PTCC 6021). To accomplish this, different initial light irradiances were implemented inside the photobioreactor and the growth of the cells was monitored. It was observed that cell growth increased with higher light intensity until the photoinhibition occurrence at light irradiance higher than 250?μE?m^?2?s^?1. The maximum OD₆₀₀, maximum growth rate, and biomass productivity increased, and hence the extinction coefficient decreased, with the increase in light irradiance before photoinhibition. The maximum optical density (OD₆₀₀) of 5.91 was obtained with irradiance below 250?μE?m^?2?s^?1 during a growth period of 80 days. The modified Monod function could model the growth–irradiance of cells with satisfactory agreement with the experimental data. The comparison of growth–irradiance of the studied species with other photosynthetic organisms showed the same trend as for cyanobacteria with photoinhibition.     

Population and community responses of phytoplankton to fluctuating light

Light is a major resource in aquatic ecosystems and has a complex pattern of spatio-temporal variability, yet the effects of dynamic light regimes on communities of phytoplankton are largely unexplored. I examined whether and how fluctuating light supply affects the structure and dynamics of phytoplankton communities. The effect of light fluctuations was tested at two average irradiances: low, 25 μmol quanta m⁻² s⁻¹ and high, 100 μmol quanta m⁻² s⁻¹ in 2- and 18-species communities of freshwater phytoplankton. Species diversity, and abundances of individual species and higher taxa, depended significantly on both the absolute level and the degree of variability in light supply, while total density, total biomass, and species richness responded only to light level. In the two-species assemblage, fluctuations increased diversity at both low and high average irradiances and in the multispecies community fluctuations increased diversity at high irradiance but decreased diversity at low average irradiance. Species richness was higher under low average irradiance and was not affected by the presence or absence of fluctuations. Diatom abundance was increased by fluctuations, especially at low average irradiance, where they became the dominant group, while cyanobacteria and green algae dominated low constant light and all high light treatments. Within each taxonomic group, however, there was no uniform pattern in species responses to light fluctuations: both the magnitude and direction of response were species-specific. The temporal regime of light supply had a significant effect on the growth rates of individual species grown in monocultures. Species responses to the regime of light supply in monocultures qualitatively agreed with their abundances in the community experiments. The results indicate that the temporal regime of light supply may influence structure of phytoplankton communities by differentially affecting growth rates and mediating species competition. Received: 24 September 1997 / Accepted: 8 July 1998     

Acclimation of the photosynthetic response of Chromatium vinosum to light-limiting conditions

The photosynthetic response of the purple sulfur bacterium Chromatium vinosum DSM 185 to different degrees of illumination was analyzed. The microorganism was grown in continuous culture, and samples were taken from the effluent of the culture and incubated at different irradiances to determine the specific rate of sulfur oxidation as a measure of the photosynthetic activity of the organism. The activities obtained were plotted as a function of the specific rate of light uptake, and for each set of data a photosynthesis equation was fitted, which allowed the estimation of P_max (photosynthetic capacity), q_k (the threshold irradiance for light limitation), and m (maintenance coefficient). The results indicated that cells grown under light limitation are able to achieve higher photosynthetic activities than cells grown under light saturation. The photosynthetic capacity (P_max) remained constant under all the conditions of illumination tested, while the maintenance expenses (m) were higher under light limitation. The parameter q_k, on the contrary, decreased considerably at limiting irradiances. Received: 16 January 1998 / Accepted: 7 September 1998     

Does irradiance influence the tolerance of marine phytoplankton to high pH?

The effect of irradiance on the tolerance to high pH of the two marine protists Heterocapsa triquetra and Nitzschia navis-varingica were investigated by conducting a series of pH drift experiments. In order to select appropriate levels of irradiance for the pH drift experiments, the photosynthetic activity and the growth rates of the two species were investigated at irradiances from 5 to 250 µmol photons m^-2 s^-1. Irradiances of 20, 35, 80 and 250 µmol photons m^-2 s^-1 were subsequently selected for further studies, because they represented conditions from severely light limited to fully light saturated with respect to photosynthesis and growth. At all four light levels, the pH limits for growth were 9.29-9.42 for H. triquetra and 9.54-9.77 for N. navis-varingica. The pH limit for growth was not positively correlated to irradiance in either of the species. We therefore conclude that low light does not reduce the pH tolerance of the protists studied.     

Leaf anatomy as a constraint for photosynthetic acclimation: differential responses in leaf anatomy to increasing growth irradiance among three deciduous trees

Interspecific variation in the response to transfer from low to high growth irradiance with respect to anatomical and photosynthetic characteristics was studied in mature leaves of three tree species, Betula ermanii Cham., Acer rufinerve Sieb. et Zucc. and Fagus crenata Blume, which occur in different successional stages in temperate deciduous forests. Transfer from low to high irradiance increased the light-saturated rate of photosynthesis per unit leaf area ( P _max) significantly in B. ermanii and A. rufinerve , but not in F. crenata . Leaves of B. ermanii grown at low irradiance were relatively thick and had vacant spaces along the mesophyll cell surfaces which was not occupied by chloroplasts or other organelles. After transfer to high irradiance, chloroplasts enlarged to fill the space along with P _max without an increase in leaf thickness. Leaves of A. rufinerve were plastic in mesophyll cell surface area and in leaf thickness, both of which increased after the transfer to high irradiance, along with an increase in the amount of chloroplasts and in P _max. On the other hand, F. crenata had little mesophyll cell surface unoccupied by chloroplasts and leaf anatomy was not changed after the transfer. In all species, P _max was strongly correlated with chloroplast surface area adjacent to the exposed mesophyll surface across different growth irradiances. An increase in P _max was observed only when chloroplast volume also increased. We conclude that light acclimation potential is primarily determined by the availability of unoccupied cell surface into which chloroplasts expand, as well as by the plasticity of the mesophyll that allows an increase in its surface area.     

REGULATION OF GAS VESICLE CONTENT AND BUOYANCY IN LIGHT-OR PHOSPHATE-LIMITED CULTURES OF APHANIZOMENON FLOS-AQUAE (CYANOPHYTA)1

Measurements of the gas vesicle space in steady-state light or phosphate-limited cultures of Aphanizomenon flos-aquae Ralfs, strain 7905 showed that gas vesicle content decreased as energy-limited growth rate increased but was the same at several phosphate-limited growth rates. Upon a decrease in growth irradiance, gas vesicle content did increase in phosphate-limited cultures, but the cultures remained nonbuoyant as long as P was limiting. Buoyant, energy-limited cultures lost their buoyancy in less than 2 h when exposed to higher irradiances. The primary mechanism for buoyancy loss was the accumulation of polysaccharide as ballast. Collapse of gas vesicles by turgor pressure played a minor role in the loss of buoyancy. When cultures were exposed to higher irradiances, cells continued to synthesize gas vesicles at the same rate as before the shift for at least 1 generation time. The amount of ballast required to make individual filaments in the population sink varied 4-fold. This variation appears to be due to differences in gas vesicle content among individual filaments.     

ADAPTATION OF CERATIUM FURCA AND GONYAULAX POLYEDRA (DINOPHYCEACE) TO DIFFERENT TEMPERATURES AND IRRADIANCES: GROWTH RATES AND CELL VOLUMES1

Growth rates and cell volumes of Ceratium furca Ehrenberg and Gonyaulax polyedra Stein were determined during the log phase of growth in cultures which had been extensively adapted to one of three temperatures and five irradiances. At each temperature, curves for the growth rate vs. irradiance for both species had light-limited and light-saturated regions. Three properties of these curves characterized the response of each species to temperature: the light-saturated growth rate, the irradiance at which growth became light-saturated and the compensation irradiance for growth. For both species, the first two properties generally decreased with declining growth temperature, while the compensation irradiance declined for Ceratium but had a V-shaped response pattern for Gonyaulax. The light-saturated growth rates were generally higher for Ceratium than for Gonyaulax, while the irradiance at which growth became saturated and the compensation irradiance were lower for Ceratium. The changes in cell volume associated with the irradiance and temperature of growth were very different for Ceratium and Gonyaulax. The cell size of Gonyaulax increased as irradiance and temperature decreased, while cell volumes of Ceratium did not change with temperature but were smallest at the highest and lowest growth irradiances. In general, the growth rate patterns were similar for Ceratium and Gonyaulax, while those for cell size were different. The maximum growth rate, the irradiance at which growth became saturated, the compensation irradiance, and the cell volume all showed that Ceratium grew at the same rate or faster than Gonyaulax over the entire range of irradiances and temperatures examined.     

INTERACTIONS AMONG IRRADIANCE,OXYGEN EVOLUTION AND NITRITE UPTAKE BY CHLAMYDOMONAS (CHLOROPHYCEAE)1,2

Nitrite uptake and oxygenic photosynthesis by cultures of Chlamydomonas sp. isolated from Lake Superior were measured at different irradiances in order to compare predictive models of nitrite uptake and to assess the proportion of photoreductant (measured as oxygen evolution, mol × 4 eq. mol^?1) that is allocated to nitrite assimilation (measured as nitrite uptake, mol × 6 eq. mol^?1). These measurements are analogous to measurements of carbon fixation (CO₂ uptake) at different irradiances and photosynthetic activities. Nitrite uptake as a function of irradiance did not follow Michaelis-Menten kinetics as proposed for nitrate by MacIsaac and Dugdale (1972) because of inhibition at high irradiances. The Haldane equation described nitrite uptake better. Nitrite uptake as a function of oxygenic photosynthesis followed Michaelis-Menten kinetics. Irradiance-dependent (Haldane) and photosynthesis-dependent models described nitrite uptake equally well. We suggest that nitrite is taken up and assimilated in response to intracellular concentrations of photoreductant that are directly proportional to photosynthetic activity and are related indirectly to irradiance. This contention is supported by photosynthesis-dependent nitrite uptake (Michaelis-Menten) at both light-limited and photoinhibited photosynthetic activities. This is consistent conceptually with deactivation of light traps at high irradiance levels. The proportion of photoreductant allocated to nitrite uptake and assimilation increased markedly at low irradiance levels. This indicates that cells synthesize important N-containing biomolecules across a broader range of irradiance levels than fixation of carbon for synthesis of energy storage and structural products.     

Photosynthetic acclimation of Tradescantia albiflora to growth irradiance: morphological, ultrastructural and growth responses

Tradescantia albiflora (Kunth), a trailing ground species naturally occurring in deep shade in rainforests, has an unusual photosynthetic acclimation profile for growth irradiance. Although capable of increasing its capacity for electron transport, photophosphorylation and carbon fixation when grown in full sunlight, Tradescantia has constant chlorophyll alb ratios, photosystem reaction centre stoichiometry and pigment-protein composition at all growth irradiances (Chow et al. 1991. Physiol. Plant. 81: 175–182). To gain an insight into the compensatory strategies which allow Tradescantia to grow in both high and low lights, plants were grown under shade cloth (100 to 1.4% relative growth irradiance) and leaf and chloroplast attributes were compared. While shade Tradescantia chloroplasts had three times more chlorophyll per chloroplast and twice the length of thylakoid membranes compared to plants grown in full sunlight, the ratios of appressed to nonappressed thylakoid membranes were constant. The average net surface charge density of destacked thylakoids was the same for plants grown at moderate and low-irradiance, consistent with their similar stacking profiles. Tradescantia plants grown in direct sunlight had 10-times more fresh and dry weight per plant compared to plants grown in shade, despite a lower photosynthetic capacity on a leaf area basis with partial photoinhibition. We conclude that having a light-harvesting apparatus permanently locked into the "shade-plant mode " does not necessarily prevent a plant from thriving in high light. Analyses of leaf growth at different irradiances provide a partial explanation of the manner in which Tradescantia compensates for very low photosynthetic capacity per unit leaf in sunlight.