Cell cycle propagation is driven by light–dark stimulation in a cultured symbiotic dinoflagellate isolated from corals

Endosymbiosis is an intriguing plant–animal interaction in the dinoflagellate–Cnidaria association. Throughout the life span of the majority of corals, the dinoflagellate Symbiodinium sp. is a common symbiont residing inside host gastrodermal cells. The mechanism of regulating the cell proliferation of host cells and their intracellular symbionts is critical for a stable endosymbiotic association. In the present study, the cell cycle of a cultured Symbiodinium sp. (clade B) isolated from the hermatypic coral Euphyllia glabrescens was investigated using flow cytometry. The results showed that the external light–dark (L:D) stimulation played a pivotal role in regulating the cell cycle process. The sequential light (40–100 μmol m⁻² s⁻¹ ~ 12 h) followed by dark (0 μmol m⁻² s⁻¹ ~ 12 h) treatment entrained a single cell cycle from the G₁ to the S phase, and then to the G₂/M phase, within 24 h. Blue light (~450 nm) alone mimicked regular white light, while lights of wavelengths in the red and infrared area of the spectrum had little or no effect in entraining the cell cycle. This diel pattern of the cell cycle was consistent with changes in cell motility, morphology, and photosynthetic efficiency (F _v /F _m ). Light treatment drove cells to enter the growing/DNA synthesis stage (i.e., G₁ to S to G₂/M), accompanied by increasing motility and photosynthetic efficiency. Inhibition of photosynthesis by 3-(3, 4-dichlorophenyl)-1, 1-dimethyl-urea (DCMU) treatment blocked the cell proliferation process. Dark treatment was required for the mitotic division stage, where cells return from G₂/M to G₁. Two different pools of adenylyl cyclase (AC) activities were shown to be involved in the growing/DNA synthesis and mitotic division states, respectively. Communicated by Biology Editor Dr Michael Lesser     

Isolation,identification and growth conditions of photosynthetic bacteria found in seafood processing wastewater

Four photosynthetic bacteria, isolated from 14 samples taken from seafood processing plants, were identified as species of Rhodocyclus gelatinosus, belonging to the purple, non-sulphur bacteria of the family Rhodospirillaceae. Cultivation in synthetic medium under four different conditions indicated that all four strains gave maximum carotenoid and bacteriochlorophyll synthesis under anaerobic conditions in the light, with values of 11 to 12.6 and 102 to 108 mg/g dry cell wt, respectively. These values are 87% higher than the pigment content obtained from aerobic cultivation, although the cell biomass of all strains (1.7 to 2.3 g/l) was 22 to 38% higher under aerobic conditions. Protein content was always between 32 and 43%. The specific growth rates of all isolates in aerobic cultivation (0.04 to 0.06 h^-1) were twice those in anaerobic conditions in the light. No growth occurred in anaerobic conditions in the dark.     

EFFECTS OF PHOTON FLUENCE RATE AND LIGHT SPECTRUM COMPOSITION ON GROWTH,PHOTOSYNTHESIS AND PIGMENT RELATIONS IN GRACILARIA SP.1

The optimal photon fluence rate for growth of tha llus tips of Gracilaria sp. was low (about 100 μE·^–2·¹); higher photon fluence rates inhibited growth. Both phycoerythrin (PE) and chlorophyll (chl) contents decreased with increasing photon fluence rates (up to 100 μE·–m^–2s^–1) in a fashion inverse to the growth response. Chl/PE ratios varied directly as the growth response over a larger photon fluence rate range. The peak chl/PE ratios were obtained at a photon fluence rate optimal for growth, suggesting that this parameter may be used to estimate in situ growth rates. A low compensation point (about 7 μE·^–2s^–1) was observed for low light (15 μE·^–2s^–1) grown plants. This compensation point was also obtained for growth in the long–term (5–6 weeks) experiments. Plants grown at 60 and 140 μE·^–2s^–1 showed higher light compensation and saturation points, suggesting that the variations in pigment composition found between the different treatments determine the photosynthetic responses at sub–optimal photon fluence rates. Photosynthetic rates at light saturation were the same, on a biomass basis, for plants grown at the various photon fluence rates. Thus, the photosynthetic dark reactions were not influenced by previous light regimes. It is suggested that maximal photosynthetic rates expressed on a biomass basis better reflect the potential productivity at tight saturation than if expressed on a pigment basis. Gracilaria sp. grew better under non–filtered fluorescent and greenish than under reddish and blue–enriched light of equal and sub–optimal photon, fluence rate. However, the pigment relations of the algae did not change in a direction complementary to the light composition at which they grew. This, together with the relatively higher photosynthetic rates under reddish and blueish light for plants previously grown under reddish and blueish light, suggests that adaptations to variouslight spectra are based on mechanisms different from complementary chromatic adaptation of the pigments.     

The differing responses of two <Emphasis Type="Italic">Arabidopsis</Emphasis> ecotypes to ammonium are modulated by the photoperiod regime

Responses to excessive ammonium (NH₄ ⁺) were compared between two Arabidopsis ecotypes (Col-0, JA22) with respect to different photoperiods in hydroponics. In this study, we showed that external extra NH₄ ⁺ led to severe growth suppression, accumulations of free NH₄ ⁺ and amino acids and increased the activities of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in shoots of the two Arabidopsis ecotypes. However, the levels of free NH₄ ⁺ and total amino acids increased, whereas the activities of GS, NADH-dependent glutamate synthase and GDH decreased under the continuous light when compared with the light (16 h)–dark (8 h) cycle photoperiod. Statistical analyses suggested that strong correlations exist among the growth reduction, accumulations of free NH₄ ⁺, total amino acids and levels of GS activity in shoots under the high NH₄ ⁺ stress regardless of the photoperiod regimes. Interestingly, under the continuous light, Col-0 showed more resistant to such growth reduction and maintained about onefold higher capability of converting excess free NH₄ ⁺ into amino acids, with onefold higher GS activity induced by the external NH₄ ⁺ when compared with JA22. In contrast, these differences were abolished between Col-0 and JA22 under the light–dark cycle condition. Taken together, our results conclude that the sensitivity to NH₄ ⁺ of Col-0 and JA22 is changed between the continuous light and the light–dark cycle photoperiod, which is correlative to the alteration of the GS activity in shoots.     

Dynamics of Redox Changes of Iron Caused by Light–dark Variations in Littoral Sediment of a Freshwater Lake

Depth profiles of oxygen concentration and the redox status of acid-extractable iron were measured in littoral sediment cores of Lake Constance incubated under a light–dark regimen of 12 h. While oxygen penetrated to 3.4±0.2 mm depth in the dark, photosynthetic oxygen production shifted the oxic–anoxic interface down to 4.0±0.2 mm or 5.9±1.6 mm depth, at low or high light intensity, respectively, and caused a net oxygen efflux into the water column. After a light–dark or dark–light transition, the oxygen concentration at the sediment surface reached a new steady state within about 20 min. The redox state of the bioavailable iron was determined in 1-mm slices of sediment subcores. After a dark period of 12 h, 85% of the acid-extractable iron (10.5 μmol cm⁻³ total) in the uppermost 8 mm was in the reduced state. Within 12 h at low or high light intensity, the proportion of ferrous iron decreased to 82 or 75%, respectively, corresponding to net rates of iron oxidation in the range of 244 and 732 nmol cm⁻³ h⁻¹, respectively. About 55 or 82% of the iron oxidation at low or high light intensity occurred in the respective oxic zone of the sediment; the remaining part was oxidized in the anoxic zone, probably coupled to nitrate reduction. The areal rates of iron oxidation in the respective oxic layer (21 or 123 nmol cm⁻² h⁻¹ at low or high light intensity, respectively) would account for 4 and 23% of the total electron flow to oxygen, respectively. Light changes caused a rapid migration of the oxic–anoxic interface in the sediment, followed by a slow redox reaction of biologically available iron, thus providing temporal niches for aerobic iron oxidizers and anaerobic iron reducers.     

Photosynthetic inorganic carbon utilization and growth of Porphyra linearis (Rhodophyta)

Photosynthetic (oxygen evolution) and growth (biomass increase) responses to ambient pH and inorganic carbon (Ci) supply were determined for Porphyralinearis grown in 0.5 L glass cylinders in the laboratory, or in 40 L fibreglass outdoor tanks with running seawater. While net photosynthetic rates were uniform at pH 6.0–8.0, dropping only at pH 8.7, growth rates were significantly affected by pH levels other than that of seawater (c. pH 8.3). In glass cylinders, weekly growth rates averaged 76% at external pH 8.0, 13% at pH 8.7 and 26% at pH 7.0. Photosynthetic O₂ evolution on a daily basis(i.e. total O₂ evolved during day time less total O₂ consumed during night time) was similar to the growth responses at all experimental pH levels, apparently due to high dark respiration rates measured at acidic pH. Weekly growth rates averaged 53% in algae grown in fibreglass tanks aerated with regular air (360 mg L_-1 CO₂) and 28% in algae grown in tanks aerated with CO₂-enriched air (750 mg L^-1 CO₂). The pH of the seawater medium in which P. linear is was grown increased slightly during the day and only rarely reached 9.0. The pH at the boundary layer of algae submerged in seawater increased in response to light reaching, about pH 8.9 within minutes, or remained unchanged for algae submerged in a CO₂-free artificial sea water medium. Photosynthesis of P. linearissaturated at Ci concentrations of seawater (K_0.5560 μM at pH 8.2) and showed low photosynthetic affinity for CO₂(K_0.5 61 μM) at pH 6.0. It is therefore concluded that P. linearisuses primarily CO₂ with HCO₃ ^- being an alternative source of Ci for photosynthesis. Its fast growth could be related to the enzyme carbonic anhydrase whose activity was detected intra- and extracellularly. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photosynthetic acclimation to different light levels in the brown marine macroalga, <Emphasis Type="Italic">Hizikia fusiformis</Emphasis> (Sargassaceae,Phaeophyta)

Hizikia fusiformis thalli experience dynamic incident light conditions during the period of growth. The present study was designed to examine how changing photon irradiance affects the photosynthesis both in the short and long terms by culturing H. fusiformis under three different light levels: 35 μmol photons m^-2 s^-1 (low light, LL), 85 μmol photons m^-2 s^-1 (intermediate light, IL), and 165 μmol photons m^-2 s^-1 (high light, HL). A similar relative growth rate was observed between IL- and HL-grown algae, but the growth rate was significantly reduced in LL-grown algae. The photosynthetic rates (P _n) measured at their respective growth light levels were found to be lowest in the thalli grown at LL and highest at HL. However, LL-grown algae exhibited much higher P _n in comparison with IL- and the HL-grown thalli at the same measuring photosynthetic photon flux density, indicating the photosynthetic acclimation to low growth light in H. fusiformis. The photosynthesis–light curves showed that LL-grown algae had a highest light-saturating maximum P _n (P _max) in comparison with IL- or HL-grown algae when the photosynthetic rates were expressed on the biomass basis. However, P _max was highest in HL-grown algae compared to IL- or LL-grown algae when the rates were normalized to chlorophyll a. The photosynthesis–inorganic carbon (Ci) response curves were also significantly affected by the growth light conditions. The highest value of apparent photosynthetic conductance occurred in LL-grown algae while the lowest value in HL-grown algae. Additionally, the activity of external carbonic anhydrase (CA) tended to increase while the total CA activity inclined to decrease in H. fusiformis thalli when the growth light level altered from 35 to 165 μmol photons per square meter per second. The external CA inhibitors showed a higher inhibition in HL-grown algae compared with LL-grown algae. It was proposed that photosynthetic acclimation to low light condition in H. fusiformis was achieved through an increase in the number of reaction centers and increased capacities of electron transport and of Ci transport within cells. The ability of photosynthetic acclimation to low light confers H. fusiformis thalli to overcome the environmental low light condition as a result of the attenuation of seawater or self-shading through enhancing its photosynthetic performance and carbon assimilation necessary for growth.     

Growth characteristics of the cyanobacterium Nostoc flagelliforme in photoautotrophic, mixotrophic and heterotrophic cultivation

Nostoc flagelliforme is a terrestrial cyanobacterium with high economic value. Dissociated cells separated from a natural colony of N. flagelliforme were cultivated for 7 days under either phototrophic, mixotrophic or heterotrophic culture conditions. The highest biomass, 1.67 g L⁻¹ cell concentration, was obtained under mixotrophic culture, representing 4.98 and 2.28 times the biomass obtained in phototrophic and heterotrophic cultures, respectively. The biomass in mixotrophic culture was not the sum as that in photoautotrophic and heterotrophic cultures. During the first 4 days of culture, the cell concentration in mixotrophic culture was lower than the sum of those in photoautotrophic and heterotrophic cultures. However, from the 5th day, the cell concentration in mixotrophic culture surpassed the sum of those obtained from the other two trophic modes. Although the inhibitor of photosynthetic electron transport DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea] efficiently inhibited autotrophic growth of N. flagelliforme cells, under mixotrophic culture they could grow by using glucose. The addition of glucose changed the response of N.flagelliforme cells to light. The maximal photosynthetic rate, dark respiration rate and light compensation point in mixotrophic culture were higher than those in photoautotrophic cultures. These results suggest that photoautotrophic (photosynthesis) and heterotrophic (oxidative metabolism of glucose) growth interact in mixotrophic growth of N. flagelliforme cells.     

Growth, biomass allocation and photosynthesis of invasive and native Hawaiian rainforest species

Growth, biomass allocation, and photosynthetic characteristics of seedlings of five invasive non-indigenous and four native species grown under different light regimes were studied to help explain the success of invasive species in Hawaiian rainforests. Plants were grown under three greenhouse light levels representative of those found in the center and edge of gaps and in the understory of Hawaiian rainforests, and under an additional treatment with unaltered shade. Relative growth rates (RGRs) of invasive species grown in sun and partial shade were significantly higher than those for native species, averaging 0.25 and 0.17 g g⁻¹ week⁻¹, respectively, while native species averaged only 0.09 and 0.06 g g⁻¹ week⁻¹, respectively. The RGR of invasive species under the shade treatment was 40% higher than that of native species. Leaf area ratios (LARs) of sun and partial-shade-grown invasive and native species were similar but the LAR of invasive species in the shade was, on average, 20% higher than that of native species. There were no differences between invasive and native species in biomass allocation to shoots and roots, or in leaf mass per area across light environments. Light-saturated photosynthetic rates (Pmax) were higher for invasive species than for native species in all light treatments. Pmax of invasive species grown in the sun treatment, for example, ranged from 5.5 to 11.9 μmol m⁻² s⁻¹ as compared with 3.0−4.5 μmol m⁻² s⁻¹ for native species grown under similar light conditions. The slope of the linear relationship between Pmax and dark respiration was steeper for invasive than for native species, indicating that invasive species assimilate more CO₂ at a lower respiratory cost than native species. These results suggest that the invasive species may have higher growth rates than the native species as a consequence of higher photosynthetic capacities under sun and partial shade, lower dark respiration under all light treatments, and higher LARs when growing under shade conditions. Overall, invasive species appear to be better suited than native species to capturing and utilizing light resources, particularly in high-light environments such as those characterized by relatively high levels of disturbance. Received: 30 December 1997 / Accepted: 1 September 1998     

Seasonality and controls of phytoplankton productivity in the middle Cape Fear River, USA

Our 1 year study was aimed at assessing seasonal patterns and controls on phytoplankton primary production (PPR) and biomass (chlorophyll a) in a fourth order section of the middle Cape Fear River in North Carolina, USA, and to determine the impact of three low-head lock and dam (LD) structures on these variables within the 70 km study reach of this coastal river. Mean concentrations of NO₃ ⁻–N, NH₄ ⁺–N and soluble reactive phosphorus (SRP) averaged 52.9, 6.0, and 3.6 μmol l⁻¹ in monthly sampling, while the average light attenuation coefficient was 2.4 m⁻¹. The average euphotic depth was 2.1 m. Nutrient concentrations and attenuation coefficients were not significantly different above versus below each LD, or along the entire study reach. Significantly higher concentrations of dissolved O₂ below versus above each LD were attributed to re-aeration during spillway transit. No seasonal pattern in physicochemical properties was apparent. Phytoplankton chlorophyll a concentrations ranged from <1 to 36 μg l⁻¹, while rates of primary production ranged from 18 to 2,580 mg C m⁻² day⁻¹, with values for both variables peaking in the spring and early summer. Chlorophyll a and primary productivity values were consistently higher above versus below each LD in May and June suggesting a seasonal effect, but values were otherwise similar such that overall means were not significantly different. Several factors point to light as the primary control on phytoplankton in the middle Cape Fear River: high nutrient concentrations; a low ratio of euphotic : mixing depth (0.46); progressive increases in chlorophyll a and radiocarbon uptake in all treatments in quarterly nutrient enrichment bioassays conducted at levels of irradiance elevated relative to in situ river values; and consistently low quarterly values of (maximum rate of chlorophyll-normalized C uptake; ≤3.7 mg C mg chl a⁻¹ h⁻¹) and I _k (light saturation parameter; ≤104 μmol photons m⁻² s⁻¹) for photosynthetic light–response (P–I) curves. Handling editor: L. Naselli-Flores     

Modeling growth and photosynthetic response in <Emphasis Type="Italic">Arthrospira platensis</Emphasis> as function of light intensity and glucose concentration using factorial design

Combined effect of light intensity and glucose concentration on Arthrospira platensis growth and photosynthetic response was evaluated using a 3² factorial design. This design was carried out with light levels of 50, 100, and 150 μmol photons m⁻² s⁻¹ and glucose concentrations of 0.5, 1.5, and 2.5 g L⁻¹. Results from the response surface methodology were that the highest level of light intensity and glucose concentration improved biomass (1.33 g L⁻¹), maximum specific growth rate (0.49 day⁻¹), and net photosynthetic rate (139.89 μmol O₂ mg Chl⁻¹ h⁻¹). Furthermore, the interaction of both factors showed that at low light, glucose had a low effect on maximum biomass and maximal net photosynthetic rate. However, at the highest light levels, the effect of glucose was more sensitive and the increase of glucose concentration increased the levels of all responses. The rates of the instantaneous relative growth, net photosynthesis, and dark respiration of growth cultures showed two different phases in mixotrophic condition. The first was distinguished by the preponderance of the photoautotrophic mode; the second was based mainly on photoheterotrophy.     

Influence of Spectral Range and Carbon and Nitrogen Sources on Oxygen Evolution and Emerson Enhancement in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii was grown in medium with different carbon (acetate, CO₂, or both), and nitrogen (ammonium chloride, peptone, urea) sources and under light of different spectral composition. The light-dark cycles were found more suitable for mixotrophic growth than continuous irradiation. Both blue (BR) and red (RR) radiations decreased photosynthetic capacity of mixotrophic cells compared to “white light” (WL). Effect of RR was associated with photon distribution favouring photosystem 1 (PS1) suggesting increased cyclic phosphorylation. Mixotrophic growth in 10 mM NH₄Cl increased photosynthetic oxygen evolution compared to standard concentration of 5 mM NH₄Cl used for growing C. reinhardtii. Autotrophic growth stimulated the photosynthetic capacity compared to mixotrophic one. However, higher photosynthetic capacity was achieved for mixotrophic cells by growing them at high NH₄ ⁺/K⁺ ratio and high phosphate concentration. This revised version was published online in July 2006 with corrections to the Cover Date.     

Productivity correlated to photobiochemical performance of <Emphasis Type="Italic">Chlorella</Emphasis> mass cultures grown outdoors in thin-layer cascades

This work aims to: (1) correlate photochemical activity and productivity, (2) characterize the flow pattern of culture layers and (3) determine a range of biomass densities for high productivity of the freshwater microalga Chlorella spp., grown outdoors in thin-layer cascade units. Biomass density, irradiance inside culture, pigment content and productivity were measured in the microalgae cultures. Chlorophyll-fluorescence quenching was monitored in situ (using saturation-pulse method) to estimate photochemical activities. Photobiochemical activities and growth parameters were studied in cultures of biomass density between 1 and 47 g L⁻¹. Fluorescence measurements showed that diluted cultures (1–2 g DW L⁻¹) experienced significant photostress due to inhibition of electron transport in the PSII complex. The highest photochemical activities were achieved in cultures of 6.5–12.5 g DW L⁻¹, which gave a maximum daylight productivity of up to 55 g dry biomass m⁻² day⁻¹. A midday depression of maximum PSII photochemical yield (F _v/F _m) of 20–30% compared with morning values in these cultures proved to be compatible with well-performing cultures. Lower or higher depression of F _v/F _m indicated low-light acclimated or photoinhibited cultures, respectively. A hydrodynamic model of the culture demonstrated highly turbulent flow allowing rapid light/dark cycles (with frequency of 0.5 s⁻¹) which possibly match the turnover of the photosynthetic apparatus. These results are important from a biotechnological point of view for optimisation of growth of outdoor microalgae mass cultures under various climatic conditions.     

Photosynthesis and respiration ofPinus pumila needles in relation to needle age and season

Rates of net photosynthesis and dark respiration were measured for detached needles ofPinus pumila trees growing on the Kiso mountain range in central Japan in 1987. Dependency of photosynthesis on light and temperature was examined in relation to needle age and season. The light saturation point of net photosynthesis was lower in 3- and 4-yr-old needles than that in current (flushed in 1987), 1- and 2-yr-old needles.P _nmax, net photosynthetic rates at 1000 μmol m⁻² s⁻¹ and 15°C, of needles from 1- to 4-yr-old generally decreased with needle age.P _nmax of 1- to 4-yr-old needles became higher in August than in other months, andP _nmax of current needles did so in September. Current needles showed high respiration rates (at 15°C) only in August. Optimum air temperatures for net photosynthesis at 1000 μmol m⁻² s⁻¹ were between 10 and 15°C for current and 1-yr-old needles. The temperature coefficient of dark respiration rates was 2.3–3.3 for current needles from August to October, and 2.2 for 1-yr-old needles in mid-July.     

In vitro production of microtubers for conservation of potato germplasm: Effect of genotype,abscisic acid,and sucrose

Summary With the objective of using microtubers for conservation of potato germplasm, the main effects of genotype, abscisic acid (ABA), and sucrose level, and of their interactions on biomass production, microtuberization, microtuber dormancy, and dry matter content, were studied. ABA decreased both microtuber production and microtuber dormancy, whereas higher concentrations (60–80 gl⁻¹) of sucrose promoted biomass production, microtuber production as well as microtuber dry matter content. Microtubers stored under diffused light had longer dormancy than those kept continuously in the dark. Interactions among various factors conditioned the main effects for some characters. In vitro performance of the genotypes studied was related to their known performance under in vivo conditions for most of the characters. Microtubers produced on media devoid of ABA and containing high sucrose concentrations and N⁶-benzyladenine (44.38 μM) could be stored for 12 mo. under diffused light at 6±1°C.     

Photosynthetic efficiency of Chlamydomonas reinhardtii in flashing light

Efficient light to biomass conversion in photobioreactors is crucial for economically feasible microalgae production processes. It has been suggested that photosynthesis is enhanced in short light path photobioreactors by mixing‐induced flashing light regimes. In this study, photosynthetic efficiency and growth of the green microalga Chlamydomonas reinhardtii were measured using LED light to simulate light/dark cycles ranging from 5 to 100 Hz at a light‐dark ratio of 0.1 and a flash intensity of 1000 µmol m⁻² s⁻¹. Light flashing at 100 Hz yielded the same photosynthetic efficiency and specific growth rate as cultivation under continuous illumination with the same time‐averaged light intensity (i.e., 100 µmol m⁻² s⁻¹). The efficiency and growth rate decreased with decreasing flash frequency. Even at 5 Hz flashing, the rate of linear electron transport during the flash was still 2.5 times higher than during maximal growth under continuous light, suggesting storage of reducing equivalents during the flash which are available during the dark period. In this way the dark reaction of photosynthesis can continue during the dark time of a light/dark cycle. Understanding photosynthetic growth in dynamic light regimes is crucial for model development to predict microalgal photobioreactor productivities. Biotechnol. Bioeng. 2011;108: 2905–2913. © 2011 Wiley Periodicals, Inc.     

Equilibration kinetics in isolated and membrane-bound photosynthetic reaction centers upon illumination: a method to determine the photoexcitation rate

Kinetics of electron transfer, following variation of actinic light intensity, for photosynthetic reaction centers (RCs) of purple bacteria (isolated and membrane-bound) were analyzed by measuring absorbance changes in the primary photoelectron donor absorption band at 865 nm. The bleaching of the primary photoelectron donor absorption band in RCs, following a sudden increase of illumination from the dark to an actinic light intensity of I _exp, obeys a simple exponential law with the rate constant , in which α is a parameter relating the light intensity, measured in mW/cm², to a corresponding theoretical rate in units of reciprocal seconds, and k _rec is the effective rate constant of the charge recombination in the photosynthetic RCs. In this work, a method for determining the α parameter value is developed and experimentally verified for isolated and membrane-bound RCs, allowing for rigorous modeling of RC macromolecule dynamics under varied photoexcitation conditions. Such modeling is necessary for RCs due to alterations of the forward photoexcitation rates and relaxation rates caused by illumination history and intramolecular structural dynamics effects. It is demonstrated that the classical Bouguer–Lambert–Beer formalism can be applied for the samples with relatively low scattering, which is not necessarily the case with strongly scattering media or high light intensity excitation. An erratum to this article can be found at     

Photic induction of locomotor activity is correlated with photic habitat in <Emphasis Type="Italic">Anolis</Emphasis> lizards

A variety of ecologically important behaviors, including circadian rhythms and seasonal reproduction, are influenced by non-visual responses to light, yet very little is known about the relationship between photic habitat and non-visual photoreception. Puerto Rican Anolis lizards have diverged into multiple photic niches, making them a good model for non-visual photosensory ecology. We investigated the photic induction of locomotor activity, a non-visual response to light, in four species of Anolis comprising two pairs of closely related, ecomorphologically similar species whose microhabitats differ in solar irradiance. We developed a device for continuous, automated detection and recording of anole locomotor activity, and used it to characterize activity under 12:12 h light–dark cycles. Next, we administered a series of 2-h light pulses during the dark period of the light–dark cycle and measured the increase in locomotor activity relative to baseline dark activity. Five different irradiances (ranging from very dim to daytime levels) were given to each individual lizard on separate nights. As expected, light caused an irradiance-dependent increase in locomotor activity in all four species. The responses at the highest irradiances were significantly greater in species occupying relatively more shaded habitats, suggesting that non-visual photoreception may be adapted to habitat light in Anolis lizards.     

Carbon balance in a monospecific stand of an annual herb <Emphasis Type="Italic">Chenopodium album</Emphasis> at an elevated CO<Subscript>2</Subscript> concentration

Elevated CO₂ enhances carbon uptake of a plant stand, but the magnitude of the increase varies among growth stages. We studied the relative contribution of structural and physiological factors to the CO₂ effect on the carbon balance during stand development. Stands of an annual herb Chenopodium album were established in open-top chambers at ambient and elevated CO₂ concentrations (370 and 700 μmol mol⁻¹). Plant biomass growth, canopy structural traits (leaf area, leaf nitrogen distribution, and light gradient in the canopy), and physiological characteristics (leaf photosynthesis and respiration of organs) were studied through the growing season. CO₂ exchange of the stand was estimated with a canopy photosynthesis model. Rates of light-saturated photosynthesis and dark respiration of leaves as related with nitrogen content per unit leaf area and time-dependent reduction in specific respiration rates of stems and roots were incorporated into the model. Daily canopy carbon balance, calculated as an integration of leaf photosynthesis minus stem and root respiration, well explained biomass growth determined by harvests (r ² = 0.98). The increase of canopy photosynthesis with elevated CO₂ was 80% at an early stage and decreased to 55% at flowering. Sensitivity analyses suggested that an alteration in leaf photosynthetic traits enhanced canopy photosynthesis by 40–60% throughout the experiment period, whereas altered canopy structure contributed to the increase at the early stage only. Thus, both physiological and structural factors are involved in the increase of carbon balance and growth rate of C. album stands at elevated CO₂. However, their contributions were not constant, but changed with stand development.     

Isolate-specific effects of ultraviolet radiation on photosynthesis,growth and mycosporine-like amino acids in the microbial mat-forming cyanobacterium <Emphasis Type="Italic">Microcoleus </Emphasis><Emphasis Type="Italic">chthonoplastes</Emphasis>

Microcoleus chthonoplastes constitutes one of the dominant microorganisms in intertidal microbial mat communities. In the laboratory, the effects of repeated daily exposure to ultraviolet radiation (16:8 light:dark cycle) was investigated in unicyanobacterial cultures isolated from three different localities (Baltic Sea = WW6; North Sea = STO and Brittany = BRE). Photosynthesis and growth were measured in time series (12–15 days) while UV-absorbing mycosporine-like amino acids (MAAs) and cellular integrity were determined after 12 and 3 days exposure to three radiation treatments [PAR (22 μmol photon m⁻² s⁻¹) = P; PAR + UV-A (8 W m⁻²) = PA; PAR + UV-A + UV-B (0.4 W m⁻²) = PAB]. Isolate-specific responses to UVR were observed. The proximate response to radiation stress after 1-day treatment showed that isolate WW6 was the most sensitive to UVR. However, repeated exposure to radiation stress indicated that photosynthetic efficiency (F _v/F _m) of WW6 acclimated to UVR. Conversely, although photosynthesis in STO exhibited lower reduction in F _v/F _m during the first day, the values declined over time. The BRE isolate was the most tolerant to radiation stress with the lowest reduction in F _v/F _m sustained over time. While photosynthetic efficiencies of different isolates were able to acclimate to UVR, growth did not. The discrepancy seems to be due to the higher cell density used for photosynthesis compared to the growth measurement. Apparently, the cell density used for photosynthesis was not high enough to offer self-shading protection because cellular damage was also observed in those filaments under UVR. Most likely, the UVR acclimation of photosynthesis reflects predominantly the performance of the surviving cells within the filaments. Different strategies were observed in MAAs synthesis. Total MAAs content in WW6 was not significantly different between all the radiation treatments. In contrast, the additional fluence of UV-A and UV-B significantly increased MAAs synthesis and accumulation in STO while only UV-B fluence significantly increased MAAs content in BRE. Regardless of the dynamic photosynthetic recovery process and potential UV-protective functions of MAAs, cellular investigation showed that UV-B significantly contributed to an increased cell mortality in single filaments. In their natural mat habitat, M. chthonoplastes benefits from closely associated cyanobacteria which are highly UVR-tolerant due to the production of the extracellular UV-sunscreen scytonemin.