DIFFERENT RESPONSE MECHANISMS OF TWO PLANKTONIC DESMID SPECIES (CHLOROPHYCEAE) TO A SINGLE SATURATING ADDITION OF PHOSPHATE

Two planktonic algal species, Staurastrum chaetoceras (Schr.) G. M. Smith and Cosmarium abbreviatum Rac. var. planctonicum W. et G. S. West, from trophically different alkaline lakes, were compared in their response to a single saturating addition of phosphate (P) in a P-limited growth situation. Storage abilities were determined using the luxury coefficient R = Q_max/Q₀. Maximum cellular P quotas differed, depending on whether cells were harvested during exponential growth at μ_max (Q_max, R being 26.7 and 9.1 for C. abbreviatum and S. chaetoceras, respectively) or harvested after a saturating pulse at P-limited growth conditions (Q′_max, R being 53.5 and 20.2 for C. abbreviatum and S. chaetoceras, respectively). At stringent P-limited conditions, maximum initial uptake rates were higher in S. chaetoceras than in C. abbreviatum (0.094 and 0.073 pmol P·cell^?1·h^?1, respectively), but long-term (net) uptake rates (over ～20 min) were higher in C. abbreviatum than in S. chaetoceras (0.048 and 0.019 pmol P·cell^?1·h^?1, respectively). Before growth resumed after the onset of a large P addition (150 μmol·L^?1), a lag phase was observed for both species. This period lasted 2–3 days for S. chaetoceras and 3–4 days for C. abbreviatum, corresponding with the time to reach Q^′_max. Subsequent growth rates (over ～10 days) were 0.010 h^?1 and 0.006 h^?1 for S. chaetoceras and C. abbreviatum, respectively, being only 20%–30% of maximum growth rates. In conclusion, S. chaetoceras, with a relatively high initial P-uptake rate, short lag phase, and high initial growth rate, is well adapted to a P pulse of short duration. Conversely, C. abbreviatum, with a high long-term uptake rate and high storage capacity, appears competitively superior when exposed to an infrequent but lasting pulse. These characteristics provide information about possible strategies of algal species to profit from temporarily high P concentrations.     

COMPETITION FOR PHOSPHORUS AMONG PLANKTONIC DESMID SPECIES IN CONTINUOUS-FLOW CULTURE1

When grown under stringent P limitation, affinity for P uptake and growth in Staurastrum pingue Teil. and Staurastrum chaetoceras (Schr.) G. M. Smith (both originating from eutrophic lakes) were of the same magnitude, whereas these parameters for Cosmarium abbreviatum Rac. var. planctonicum W. & G. S. West (isolated from a meso-oligotrophic lake) were significantly higher in value. On the other hand, at all growth rates tested, maximum P uptake rates were lower in C. abbreviatum than in the two Staurastrum species. The outcome of competition between either Staurastrum species and C. abbreviatum in mixed chemostats under different levels of continuous P limitation was in agreement with what could be predicted from the species-specific affinity parameters: Staurastrum was outcompeted at dilution rates lower than 0.012 h^?1, calculated to correspond with external inorganic P concentrations lower than 0.02 μM P, but won out at higher dilution rates. When P was added in two pulses of 2.5 μmol · L^?1 a week instead of continuously, S. chaetoceras outcompeted C. abbreviatum at a slow rate. When P was supplied as a daily pulse of 0.7 μmol · L^?1, a stable coexistence of S. chaetoceras and C. abbreviatum was established, Staurastrum predominating the culture in cell numbers at ca. 85%. The results show that P uptake and growth characteristics of the three species can predict the outcome of competition under various P-limited conditions. Specific growth kinetic parameters as found in this study may also explain distribution patterns of the species observed in the field.     

Growth kinetic parameters of two planktonic desmid species under fluctuating phosphorus conditions in continuous-flow culture

Two planktonic desmid taxa, Staurastrum chaetoceras and Cosmariumabbreviatum var. planctonicum, were examined under pulsed phosphorus(P)-limited conditions in continuous culture. Two pulse regimeswere applied, i.e. 2.5 µM P twice a week and 10 µMP once every 2 weeks. Under both pulse regimes, S.chaetocerasshowed a higher maximum P uptake rate (V^max) and affinity forP uptake (V^max/affinity constant K^m) than C.abbreviatum. Affinityfor P uptake in S.chaetoceras just before pulsing was higherthan shortly after pulsing, whereas C.abbreviatum did not showany significant difference. Cell densities and cellular P quotain S.chaetoceras and C.abbreviatum fluctuated in a comparableway, but fluctuations in C.abbreviatum were less pronouncedthan in S.chaetoceras. The mean cell volume of both specieswas greater under the bimonthly than under the biweekly pulseregime, and the lowest under continuous P-limited conditions.Competition between S.chaetoceras and C.abbreviatum under thebimonthly pulse regime was won by S.chaetoceras, but displacementwas less fast than under more frequent pulsing (equalling thesame total P supply). The above-mentioned results are discussedin relation to possible ecological strategies.     

Negative Effects of P-Buffering and pH on Photosynthetic Activity of Planktonic Desmid Species

The photosynthetic activities of three planktonic desmid species (Staurastrum brachiatum, Staurodesmus cuspidatus var. curvatus, and Staurastrum chaetoceras) were compared after adaptation to medium enriched with either a 20 mM Na⁺-phosphate (P) or HEPES buffer. Incubations up to 2 d were carried out at pH 6 or 8 under normal air or air enriched with 5 % CO₂. Gross maximum photosynthetic rate (P _max) and growth rate were decreased in both S. brachiatum and Std. cuspidatus at higher pH when using the HEPES buffer and this effect was independent of CO₂ concentration, indicating that pH had an inhibitory effect on photosynthesis and growth in these species. The P-buffer at pH 8 caused a large decrease in P _max and quantum yield for charge separation in photosystem 2 (PS2), compared to HEPES-buffered algae. This effect was very large in both S. brachiatum and Std. cuspidatus, two species characteristic of soft water lakes, but also significant in S. chaetoceras, a species dominant in eutrophic, hard water lakes. The decreased P _max in P-buffer could not be related to a significant increase in cellular P content known to be responsible for inhibition in isolated chloroplasts. Experiments at pH 6 and 8 showed that two conditions, high pH and high Na⁺ concentration, both contributed to the decreased P _max and quantum yield in the desmids. Effects of a P-buffer were less pronounced by using K⁺-P buffer. The use of P-buffer at pH 8 possibly resulted in high irradiance stress in all species, indicated by damage in the PS2 core complex. In the soft water species pH 8 resulted in increased non-photochemical quenching together with a high de-epoxidation state of the xanthophyll cycle pigments.     

AUTOTROPHIC GROWTH AND PHOTOACCLIMATION IN KARLODINIUM MICRUM (DINOPHYCEAE) AND STOREATULA MAJOR (CRYPTOPHYCEAE)1

We compared autotrophic growth of the dinoflagellate Karlodinium micrum (Leadbeater et Dodge) and the cryptophyte Storeatula major (Butcher ex Hill) at a range of growth irradiances (E_g). Our goal was to determine the physiological bases for differences in growth–irradiance relationships between these species. Maximum autotrophic growth rates of K. micrum and S. major were 0.5 and 1.5 div.·d^?1, respectively. Growth rates were positively correlated with C‐specific photosynthetic performance (PP^C, g C·g C^?1·h^?1) (r²=0.72). Cultures were grouped as light‐limited (LL) and high‐light (HL) treatments to allow interspecific comparisons of physiological properties that underlie the growth–irradiance relationships. Interspecific differences in the C‐specific light absorption rate (E_a^C, mol photons·g C^?1·h^?1) were observed only among HL acclimated cultures, and the realized quantum yield of C fixation (φ_C(real.), mol C·mol photons^?1) did not differ significantly between species in either LL or HL treatments. The proportion of fixed C that was incorporated into new biomass was lower in K. micrum than S. major at each E_g, reflecting lower growth efficiency in K. micrum. Photoacclimation to HL in K. micrum involved a significant loss of cellular photosynthetic capacity (P_max^cell), whereas in S. major, P_max^cell was significantly higher in HL acclimated cells. We conclude that growth rate differences between K. micrum and S. major under LL conditions relate primarily to cell metabolism processes (i.e. growth efficiency) and that reduced chloroplast function, reflected in PP^C and photosynthesis–irradiance curve acclimation in K. micrum, is also important under HL conditions.     

Effects of temperature and pH on growth and photosynthesis of the thermophilic cyanobacterium Synechococcus lividus as measured by pulse-amplitude modulated fluorometry

In this study, the effects of five different temperatures and pH conditions on growth and photosynthetic performance of Synechococcus lividus Copeland from Taiwan were monitored in the field and the laboratory by using an underwater pulse‐amplitude modulated (Diving‐PAM) fluorometer. In the field, the optimal growth temperature of S. lividus was found to be 57°C. Such a finding was congruent with the growth rate in the laboratory culture, in which the optimal growth temperatures ranged from 45 to 60°C. In photosynthetic performance, the light‐saturated maximum relative electron transport rate (ETR_max) and the light‐limited slope (α^ETR) exhibited highest values at 50°C. At five different pH conditions, higher ETR_max and α^ETR were observed from pH 7 to 9. In addition, regression analysis demonstrated a significant positive relationship between the growth rate and the ETR_max values (R² = 0.9527), indicating that the growth of S. lividus was largely restricted to its photosynthetic performance. In conclusion, the photosynthetic performance and growth of the thermophilic cyanobacterium S. lividus were sensitive to fluctuations in temperature but not in pH. The present investigation offers a better understanding of the photosynthetic physiology.     

Growth and photosynthetic characteristics of several <Emphasis Type="Italic">Cosmarium</Emphasis> strains (Zygnematophyceae,Streptophyta) isolated from various geographic regions under a constant light-temperature regime

Numerous detailed studies have been made of climatically and environmentally influenced macroalgal geographic distribution patterns. However, so far, there have been only a few intrinsic investigations of the geographic distributions of microalgae. In order to investigate the physiological differences among geographically different microalgal strains, six Cosmarium strains were collected from various climate areas and studied. They were grown under a constant light-temperature regime (16°C and 30 μmol photons m⁻² s⁻¹) and nutrient supply. The arctic representative, C. crenatum var. boldtianum, and the typical tropical desmid, C. beatum, behaved like algae adapted to high light intensities, as judged from the distinctly high values of photosynthetic capacity and saturating irradiance measured, in accordance with the high solar radiation prevailing in their sampling areas. The arctic taxon appeared more optimally suited to the low cultivation temperature, as evidenced by the relatively high values of growth rates, maximum quantum yield and photosynthetic efficiency measured. The cosmopolitan taxa, C. meneghinii and C. punctulatum var. subpunctulatum, exhibited a high maximum quantum yield and photosynthetic efficiency concomitantly during growth, which explained their ubiquitous distribution. Nevertheless, two clones belonging to C. punctulatum var. subpunctulatum, collected from polar and mountainous tropical regions, differed significantly with regard to cell volume, growth rates, surface area to volume ratio and photosynthetic parameters. The physiological differences between the Cosmarium strains were in accordance with their geographic origin; they are discussed in detail in this study. Moreover, these differences were maintained despite the long-term cultivation under identical and constant laboratory conditions.     

Measures of ecological association

Summary The relationships of photosynthetic characteristics to the competitive interactions of a C₃ plant, Chenopodium album, and a C₄ plant, Amaranthis retroflexus, were investigated in different temperature and water supply regimes. Both species had similar photosynthetic rates at 25°C, but at higher temperatures, Amaranthus had substantially greater rates than Chenopodium. Conversely, at lower temperatures, Chenopodium had an advantage. The competitive abilities in mixtures exhibited a close parallel to the photosynthetic performances with Amaranthus having an advantage at high temperatures and Chenopodium an advantage at low temperatures. These competitive outcomes were determined primarily by differences in relative growth rates prior to canopy closure. In the respective, temperature regimes, the species having the highest photosynthetic rate, which resulted an more rapid growth, overtopped and shaded the other species at the time of canopy closure. These results demonstrate that differences in photosynthetic temperature response between C₄ and C₃ plants can be an important determinant in competitive interactions, but at least in this case, the influence is primarily through, events prior to the actual initiation of competition.In contrast to temperature, growth of the plants under limited water supply had no influence on the competitive interactions. Thus, the presence of the C₄ pathway alone was not sufficient to yield a competitive advantage over the C₃ species under water limited conditions.     

Effect of varying co2 and light levels on growth of hedyotis and sugarcane shoot cultures

Summary Shoot cultures of Hedyotis corymbosa, a C3 species, and sugarcane, a C4 species, were used to examine the effects of various CO₂ concentrations and two light intensities on growth and photosynthetic rates. The fresh and dry weights of new growth of Hedyotis shoots were higher when grown under the higher light intensity, while differences among shoots grown under different CO₂ levels were marginal. After 14 d of growth in various CO₂ concentrations, no significant differences could be observed in the newly produced leaves of Hedyotis with respect to stomatal distribution and number of mesophyll cell layers. Shoots grown under high light intensity did not show higher rates of photosynthesis than those grown under low light intensity. Also, sugarcane shoots grown in a CO₂-enriched environment did not have higher photosynthetic rates, perhaps because the C4 pathway is less sensitive to the ambient CO₂ concentration. The quantum yield of Hedyotis shoots grown on medium with 20 g l⁻¹ sucrose was lower than that of shoots on lower sucrose concentrations, supporting the view that photosynthesis is inhibited by high levels of sucrose. Our results suggest that Hedyotis shoots in culture exhibit some form of acclimation to high CO₂. so that there is no net gain in productivity by photosynthesis.     

Ecophysiological response of dune species to experimental burial under field and controlled conditions

Field, greenhouse, and growth chamber experiments were conducted to determine the effects of different burial treatments on photosynthesis (carbon dioxide exchange rate), chlorophyll-a fluorescence, leaf area, biomass, leaf thickness, total chlorophyll content and chlorophyll a/b ratio of ten sand dune species: Agropyron psammophilum, Cakile edentula, Cirsium pitcheri, Corispermum hyssopifolium, Elymus canadensis, Oenothera biennis, Panicum virgatum, Strophostyles helvola, Tusilago farfara, and Xanthium strumarium. Although there were significant differences between species, all of them exhibited stimulation in growth following burial in sand. Generally, buried plants of these species showed an increase in biomass, photosynthetic efficiency, and chlorophyll-a fluorescence because of higher energy content in their roots, rhizomes, and underground stems. The main reasons for the stimulation in growth were an increase in leaf area, leaf thickness, and root biomass. The total chlorophyll content of leaves of buried plants of A. psammophilum, E. canadensis, and P. virgatum was higher than controls, but there were no significant differences for Cirsium pitcheri, O. biennis, and S. helvola. The similarities and differences exhibited by the test species in their responses to burial would be ecologically adaptive to survive the harsh environmental conditions of foredunes. All species showed a clear compensatory response following recovery from the burial episode and surpassed control by enhancing the vital physiological, morphological and growth functions.     

Effects of CO2 concentrations on the freshwater microalgae, Chlamydomonas reinhardtii, Chlorella pyrenoidosa and Scenedesmus obliquus (Chlorophyta)

In order to investigate the possible impacts of increased atmospheric CO₂ levels on algal growth and photosynthesis, the influence of CO₂ concentration was tested on three planktonic algae (Chlamydomonas reinhardtii, Chlorella pyrenoidosa, and Scenedesmus obliquus). Increased CO₂ concentration enhanced significantly the growth rate of all three species. Specific growth rates reached maximal values at 30, 100, and 60 M CO₂ in C. reinhardtii, C. pyrenoidosa, and S. obliquus, respectively. Such significant enhancement of growth rate with enriched CO₂ was also confirmed at different levels of inorganic N and P, being more profound at limiting levels of N inC. pyrenoidosa and P in S. obliquus. The maximal rates of net photosynthesis, photosynthetic efficiency and light-saturating point increased significantly (p < 0.05) in high-CO₂-grown cells. Elevation of the CO₂ levels in cultures enhanced the photoinhibition of C. reinhardtii, but reduced that of C. pyrenoidosa and S. obliquus when exposed to high photon flux density. The photoinhibited cells recovered to some extent (from 71% to 99%) when placed under dim light or in darkness, with better recovery in high-CO₂-grownC. pyrenoidosa and S. obliquus. Although pH and pCO₂ effects cannot be distinguished from this study, it can be concluded that increased CO₂ concentrations with decreased pH could affect the growth rate and photosynthetic physiology of C. reinhardtii, C. pyrenoidosa, and S. obliquus.     

Comparative studies on photosynthesis and phosphate metabolism of Cylindrospermopsis raciborskii with Microcystis aeruginosa and Aphanizomenon flos-aquae

The physiological differences for three bloom-forming cyanobacteria (Cylindrospermopsis raciborskii, Microcystis aeruginosa, and Aphanizomenon flos-aquae) were investigated. In comparison with M. aeruginosa and A. flos-aquae, C. raciborskii exhibited a significantly higher concentration of carotenoids, higher values in maximum photosynthesis rate (P_m), apparent photosynthetic efficieny (a), and maximum electron transport rate (ETR_max) during the growth period. In addition, higher extracellular alkaline phosphatase activities and lower light compensation point (I_c) were also detected in C. raciborskii (p < 0.05, ANOVA). Therefore, it is suggested that the higher photosynthetic activities, more effective uptake and utilization to phosphate, and low light requirements might play important roles in the occurrence and invasive behavior of C. raciborskii.     

Effects of grazing on photosynthetic characteristics of major steppe species in the Xilin River Basin,Inner Mongolia,China

Species composition and photosynthetic characteristics of dominant species of ungrazed plot (UG), overgrazed plot (OG), and restored grazed plot (RG) were determined in the Xilin River Basin, Inner Mongolia, China. Both heavily grazing and restoration significantly affected the composition of different species and life forms. Leymus chinensis, Stipa grandis, and Cleistogenes polyphylla, three dominant perennial grasses in UG plot, contributed 58.9 % aboveground biomass to that of whole community, and showed higher net photosynthetic rate (P _N), transpiration rate (E), and intrinsic water-use efficiency (WUE). In OG plot, relative biomass of L. chinensis and S. grandis significantly decreased, while relative biomass of three shrubs/sub-shrubs, Caragana microphylla, Artemisia frigida, and Kochia prostrata, obviously increased. Heavy grazing significantly decreased P _N, E, and WUE of L. chinensis and S. grandis, while shrubs/sub-shrubs showed significantly higher photosynthetic activity and WUE than the grasses. After 18-year restoration, photosynthetic activities of L. chinensis and S. grandis were significantly higher than those in the OG plot. The proportion of L. chinensis, S. grandis, and C. microphylla significantly increased, and relative biomass of C. polyphylla, A. frigida, and K. prostrata markedly declined in RG plot. We found close relationships between physiological properties of species and their competitive advantage in different land use types. Higher photosynthetic capability means more contribution to total biomass. The variations in physiological characteristics of plants could partly explain the changes in species composition during degrading and restoring processes of Inner Mongolia typical steppes.     

Can Hydrodynamics Change Phosphorus Strategies of Diatoms?—Nutrient Levels and Diatom Blooms in Lotic and Lentic Ecosystems

Diatom blooms occur in many water bodies worldwide, causing significant ecological and social concerns. In order to understand the mechanisms of diatom blooms formation, the effects of varying phosphorus (P) concentration and hydrodynamics on the growth of diatoms were studied by combining results from field observations and laboratory experiments. The field investigation showed that spring diatom blooms (Cyclotella meneghiniana and Stephanodiscus hantzschii) occurred in Lake Taihu and Hanjiang River with similar environmental factors such as water temperature, pH, and dissolved oxygen in 2008. Concentrations of total phosphorus (TP), total nitrogen, and ammonia nitrogen (NH₄-N) in Lake Taihu were significantly higher than the concentrations in the Hanjiang River. Laboratory experiments were conducted to evaluate growth and physiological responses of four lotic diatoms (Cyclotella atomus, Fragilaria crotonensis, Nitzschia palea, and S. hantzschii, isolated from the Hanjiang River) and three lentic diatoms (C. meneghiniana, Melosira varians, and Stephanodiscus minutulus, isolated from Lake Taihu, Lake Donghu, and Guanqiao Pond, respectively) to various P concentrations under small-scale turbulent and standing conditions. Our results showed that, with turbulence, lotic diatoms C. atomus, F. crotonensis, N. palea, and S. hantzschii demonstrated a significant increase in affinity for P compared with lentic diatoms C. meneghiniana, M. varians, and S. minutulus. Moreover, lotic diatoms C. atomus, F. crotonensis, and N. palea had higher growth rates and photosynthetic efficiencies with small-scale turbulence than with standing conditions both in P-limited and P-replete conditions. Lotic species S. hantzschii and three lentic diatoms (C. meneghiniana, M. varians, and S. minutulus) grew well under standing conditions. Our results may explain our field observation that the occurrence of diatom blooms in lakes is often associated with higher TP concentrations whereas in rivers, diatom blooms occur at a wide range of TP concentrations under flows. Therefore, different hydrodynamics and nutrient concentrations determined the dominant diatom species, according to their habitat-dependent physiological characteristics.     

Increase of Copper Toxicity to Growth of Chlorella vulgaris with Increase of Light Intensity

Abstract In the absence of inhibitory concentrations of copper, the photoautotrophic growth of Chlorella vulgaris INETI58C at 27°C exhibited a higher specific growth rate and reached a higher maximal concentration of biomass, under irradiance of 150 W m⁻², compared with 100 W m⁻². However, when the mineral growth medium was supplemented with CuSO₄ (range 40–80 μM), algal growth was significantly affected at the higher light intensity. In the presence of Cu²⁺, the increase in dry biomass was uncoupled from the increase in cell concentration since more than 16 autospores gathered together, inside the enlarged mother cell, suggesting that copper arrested the normal bursting of the mother cell wall. At the higher irradiance, growth medium supplementation with 80 μM of CuSO₄ led to bleaching of photosynthetic pigments. No growth was observed, while, under the lower irradiance, growth was only slightly inhibited. Results clearly showed that copper toxicity to growth of C. vulgaris was strongly influenced by light intensity. Higher light intensity elicits lethal or sublethal Cu²⁺ damage at concentrations lower than the threshold level for damage at lower light intensities. Cu²⁺ may elicit lethal or sublethal light damage at irradiances lower than the threshold level for unpolluted aquatic systems. Received: 17 January 1997; Accepted 15 April 1997     

Interactive Effects of Abiotic Stress and Elevated CO2 on Physio-Chemical and Photosynthetic Responses in Suaeda Species

Suaeda fruticosa and S. monoica are important halophytes for ecological rehabilitation of saline lands. We report differential physio-chemical, photosynthetic, and chlorophyll fluorescence responses in these halophytes under 100 mM sodium chloride (NaCl), 50% strength (16.25 ppt) of seawater (SW)-imposed salinity, and 10% polyethylene glycol 6000 imposed osmotic stress at 380 (ambient) and 1200 (elevated) µmol mol^–1 CO₂ concentrations. SW salinity enhanced the growth in both species; however, compared with S. fruticosa, the S. monoica exhibited comparatively better growth and biomass accumulation under saline conditions at elevated CO₂. Results demonstrated better photosynthetic performances of S. monoica under stress conditions at both levels of CO₂, and this resulted in higher accumulation of carbon, nitrogen, sugar, and starch contents. S. monoica exhibited improved antenna size, electron transfer at PSII donor side, and efficient working of photosynthetic machinery at elevated CO₂, which might be due to efficient upstream utilization of reducing power to fix the CO₂. The δ¹³C results supported the operation of C₄ CO₂ fixation in S. monoica and C₃ or intermediate pathway of CO₂ fixation in S. fruticosa. Lower accumulation of reactive oxygen species, reduced membrane damage, lowered solute potential, and higher accumulation of proline and polyphenol contents indicated elevated CO₂-induced abiotic stress tolerance in Suaeda. Higher activity of antioxidant enzymes in both species at both levels of CO₂ help plants to combat the oxidative stress. Upregulation of NADP-dependent malic enzyme and NADP-dependent malate dehydrogenase genes indicated their role in abiotic stress tolerance as well as photosynthetic carbon (C) sequestration. Operation of C₄ type CO₂ fixation in S. monoica and an intermediate CO₂ fixation in S. fruticosa could be the possible reason for the superior photosynthetic efficiency of S. monoica under stress conditions at elevated CO₂.

     

Membrane lipid composition and restoration of photosynthesis during low temperature acclimation in Synechococcus sp. strain PCC 7942

We compared temperature acclimation of the cyanobacterium Synechococcus sp. strain PCC 7942 and two psbA inactivation mutants, R2K1 and R2S2C3, following shifts from 37 to 25°C. Wild-type cultures incubated in the dark at 25°C showed no chill-induction of lipid desaturation, probably because the lipid acclimation is dependent on photosynthesis. Incubation in the light at 25°C, however, induced considerable increases in membrane lipid desaturation, and within 24 h the monoenoic fatty acids increased from about 46 to about 57%. In parallel with this desaturation the ratio of monogalactosyldiacylglycerol to digalactosyldiacylglycerol (MGDG/DGDG) increased. Both of these lipid changes increase the repulsive forces of the hydrophobic chains of the membrane lipids and thereby alter the physical properties of the membrane. As expected, under irradiation this temperature shift also induced a reversible replacement of the constitutive photosystem II protein, D1:1, with an alternative stress form, D1:2. Photosynthesis decreased to 42% of the control level within the initial 2 h of cold incubation, but later recovered. The D1:2 protein accumulated to high levels between 2 and 4 h after the temperature shift, when desaturation of membrane lipids and MGDG/DGDG ratio had not yet increased significantly. Much of this accumulated D1:2 protein was in a higher molecular mass form, termed D1:2^*, which is probably an unprocessed precursor form of the protein. In contrast to the wild-type cells the psbA inactivation mutants, R2K1 and R2S2C3 did not accumulate any precursor form of D1 protein either at the optimal or low growth temperature. The R2S2C3 mutant strain expresses only the constitutive D1:1 protein and suffered severe photoinhibition following the temperature shift. Nevertheless, R2S2C3 eventually recovered some photosynthetic activity, induced lipid desaturation and slowly resumed growth at 25°C, thus demonstrating acclimation to the lower growth temperature. The R2K1 mutant synthesizes only the D1:2 stress form of D1 protein and maintained oxygen evolution at a high level (ca 70% of a control rate) after the low temperature shift. Chill-induced lipid desaturation and increase in MGDG/DGDG ratio did proceed but, for unknown reasons the strain did not resume growth at the lower temperature. The physical properties of the membrane lipids were not the limiting factor for growth resumption. Our results demonstrate that in the wild-type the chill-induced desaturation of membrane lipids follows after the exchange of the two forms of the D1 proteins, but the D1 exchange results in accumulation of unprocessed D1:2^* polypeptides until the lipid composition later acclimates. We also show that the lipid desaturation process in Synechococcus sp. strain PCC 7942 is dependent upon photosynthetic activity.     

<Emphasis Type="Italic">Rubribacterium polymorphum</Emphasis> gen. nov., sp. nov., a novel alkaliphilic nonsulfur purple bacterium from an Eastern Siberian soda lake

An alkaliphilic nonsulfur purple bacterial (NPB) strain “Green” was isolated from sediments of the littoral zone of the soda lake (mineralization 22 g/l, pH 9.5) in the Barguzin River valley (Eastern Siberia). The cells of the new isolate are ovoid or polymorphic at latter stages. The photosynthetic membrane structures are of vesicular type. Bacteriochlorophyll a and carotenoids of both spheroidene and spirilloxanthin type are the photosynthetic pigments. Two light-harvesting systems (LH1 and LH2) are present. The new isolate is a photoheterotroph and a facultative aerobe. It grows well in the dark on organic substrates; anaerobic phototrophic growth is poor. The isolate is alkaliphilic with pH optimum of 8.5–9.5. The most abundant cell growth occurred at 5–40 g/l NaCl (optimum at 10 g/l) and 30 °C. The DNA G+C base content was 69.9 mol %. Analysis of 16S rRNA gene sequences revealed a 10% difference with the most closely related NPB (Rhodobacter species). Rubrimonas cliftoensis, a bacteriochlorophyll a-containing bacterium, is the closest relative (93.3% similarity). It is proposed that strain “Green” should be placed in the new genus and new species Rubribacterium polymorphum gen. nov., sp. nov. GenBank accession number: 16S rRNA-EU857676.     

The influence of nitrogen on rain forest dipterocarp seedlings exposed to a large increase in irradiance

Dipterocarps dominate the canopy of lowland tropical rain forest in South‐east Asia. Seedlings of these species form diverse assemblages on the forest floor where low irradiance severely limits their growth. Further growth depends largely upon the increased irradiance that can occur with the creation of canopy gaps. However, the response of dipterocarp seedlings to increased irradiance and their subsequent establishment in the canopy may be influenced by the availability of other resources, such as nutrient availability. We investigated the influence of nitrogen supply on aspects of the photosynthetic physiology and growth of seedlings of four dipterocarp species (Shorea leprosula, Shorea johorensis, Shorea oleosa and Dryobalanops lanceolata) growing under low irradiance, during transfer from low to high irradiance, and during subsequent growth at high irradiance. All four species increased growth and photosynthetic capacity in response to N‐supply at high irradiances but not at low irradiance approximating that which can be expected to occur in the forest understorey. When seedlings grown at low irradiances and varying N‐supply were exposed to a large increase in irradiance, all species showed some degree of initial photodamage (measured through chlorophyll fluorescence), the extent of which was similar between species but differed markedly depending on the pre‐exposure growth irradiance and N‐supply. Greater photodamage occurred in seedlings grown at lower compared with higher N‐supply and irradiance. Despite these initial difference in the extent of this photodamage, all seedlings demonstrated a similar capacity to recover from damage. However, the alterations in the photosynthetic physiology of leaves during this recovery differed between species and depended on N‐supply. Under high N‐supply all species apart from S. oleosa increased photosynthetic capacity per unit chlorophyll following exposure to high irradiance by increasing photosynthetic capacity per unit leaf area while, under low N‐supply, an increase in photosynthetic capacity per unit leaf only occurred in D. lanceolata. Our results suggest that variations in N‐availability may have a much greater impact on the relative competitiveness of dipterocarp seedlings during the regenerative phase following canopy gap formation than physiological differences between seedlings. Our results demonstrate a potentially significant role for N‐availability in the regeneration dynamics and distribution of canopy‐dominating dipterocarp species.     

Temperature response and photoinhibition investigated by chlorophyll fluorescence measurements for four distinct species of dipterocarp trees

The effects of strong light in combination with elevated temperatures on the photosynthetic system were examined in 4 dipterocarp tree species with ecologically different habitats. The 4 dipterocarp tree species were: Shorea platyclados originated from upper dipterocarp forests, Shorea parvifolia– lowland and hill dipterocarp forests, Shorea assamica– lowland dipterocarp forests, and Dipterocarpus oblongifolius– riparian fringes. S. platyclados and D. oblongifolius have higher growth and survival rates in open sites than S. parvifolia and S. assamica. Tolerance of high temperature among the species was assessed by determining the critical temperatures (T_c) at which the minimal fluorescence (F_o) began to rise sharply. This was measured by exposing plants to an increasing temperature of about 1°C min^?1. The intrinsic thermotolerance of the thylakoid membrane appears to be the highest for D. oblongifolius (T_c=46.4°C), intermediate for S. platyclados (45.7°C), and lowest for S. parvifolia and S. assamica (45.2 and 45.3°C, respectively). The temperature‐dependent efficiency of PSII electron transport (ΔF/F′_m), photochemical quenching (q_P), and the efficiency of light capture of open PSII (F′_v/F′_m) were measured at the photosynthetic steady state at least 10 min after the light exposure (180 μmol m^?2 s^?1 PFD). Stable temperature responses of ΔF/F′_m and q_P were observed in S. platyclados and D. oblongifolius, while those in S. parvifolia and S. assamica were more temperature‐dependent and severely affected at 45°C. Little difference was observed in temperature‐dependent F′_v/F′_m among species. Photoinhibitory light exposure (1600 μmol m^?2 s^?1 PFD) for 2 h at 40°C had little effect on the recovery kinetics from photoinhibition of S. platyclados and D. oblongifolius compared with those at 35°C. In contrast, the recovery from photoinhibition was retarded in S. parvifolia and S. assamica. These findings suggest that even at 40°C, a temperature below T_c, an exposure to strong light exacerbated photoinhibition in S. parvifolia and S. assamica corresponding to the closure of PSII reaction centers, as indicated by the decrease in q_P at this temperature. Thus, S. platyclados and D. oblongifolius, which occur at uplands and riparian fringes with frequent disturbances, are suggested to have higher photosynthetic tolerance to elevated temperatures contributing to a circumvention of photoinhibition.