Specific growth rate of Chlamydomonas reinhardtii and Chlorella sorokiniana under medium duration light/dark cycles: 13–87 s

The specific growth rate of Chlamydomonas reinhardtii and Chlorella sorokiniana decreased under square-wave light/dark cycles of medium duration, 13–87 s, in comparison to continuous illumination. Three experiments were done in three different turbidostats at saturating and sub-saturating light intensities during the light period, 240–630 μmol m⁻² s⁻¹. Within each experiment the light intensity during the light periods of the intermittent light regimes was equal and this intensity was also applied under continuous illumination. The specific growth rate decreased proportional or more than proportional to the fraction of time the algae were exposed to light; this light fraction ranged from 0.32 to 0.88. We conclude that under these light regimes the chlorophyta C. reinhardtii and C. sorokiniana are not able to store light energy in the light period to sustain growth in the dark period at the same rate as under continuous illumination. C. reinhardtii increased its specific light absorbing surface by increasing its chloropyll-a content under light/dark cycles of 13 s duration and a light fraction of 0.67 at 240 μmol m⁻² s⁻¹; the chloropyll-a content was twice as high under intermittent illumination in comparison to continuous illumination. The combination of a higher specific light absorption together with a lower specific growth rate led to a decrease of the yield of biomass on light energy under intermittent illumination.     

Adjusted light and dark cycles can optimize photosynthetic efficiency in algae growing in photobioreactors

Biofuels from algae are highly interesting as renewable energy sources to replace, at least partially, fossil fuels, but great research efforts are still needed to optimize growth parameters to develop competitive large-scale cultivation systems. One factor with a seminal influence on productivity is light availability. Light energy fully supports algal growth, but it leads to oxidative stress if illumination is in excess. In this work, the influence of light intensity on the growth and lipid productivity of Nannochloropsis salina was investigated in a flat-bed photobioreactor designed to minimize cells self-shading. The influence of various light intensities was studied with both continuous illumination and alternation of light and dark cycles at various frequencies, which mimic illumination variations in a photobioreactor due to mixing. Results show that Nannochloropsis can efficiently exploit even very intense light, provided that dark cycles occur to allow for re-oxidation of the electron transporters of the photosynthetic apparatus. If alternation of light and dark is not optimal, algae undergo radiation damage and photosynthetic productivity is greatly reduced. Our results demonstrate that, in a photobioreactor for the cultivation of algae, optimizing mixing is essential in order to ensure that the algae exploit light energy efficiently.     

Photosynthetic efficiency and oxygen evolution of Chlamydomonas reinhardtii under continuous and flashing light

As a result of mixing and light attenuation in a photobioreactor (PBR), microalgae experience light/dark (L/D) cycles that can enhance PBR efficiency. One parameter which characterizes L/D cycles is the duty cycle; it determines the time fraction algae spend in the light. The objective of this study was to determine the influence of different duty cycles on oxygen yield on absorbed light energy and photosynthetic oxygen evolution. Net oxygen evolution of Chlamydomonas reinhardtii was measured for four duty cycles (0.05, 0.1, 0.2, and 0.5) in a biological oxygen monitor (BOM). Oversaturating light flashes were applied in a square-wave fashion with four flash frequencies (5, 10, 50, and 100 Hz). Algae were precultivated in a turbidostat and acclimated to a low photon flux density (PFD). A photosynthesis–irradiance (PI) curve was measured under continuous illumination and used to calculate the net oxygen yield, which was maximal between a PFD of 100 and 200 μmol m^?2?s^?1. Net oxygen yield under flashing light was duty cycle-dependent: the highest yield was observed at a duty cycle of 0.1 (i.e., time-averaged PFD of 115 μmol m^?2?s^?1). At lower duty cycles, maintenance respiration reduced net oxygen yield. At higher duty cycles, photon absorption rate exceeded the maximal photon utilization rate, and, as a result, surplus light energy was dissipated which led to a reduction in net oxygen yield. This behavior was identical with the observation under continuous light. Based on these data, the optimal balance between oxygen yield and production rate can be determined to maximize PBR productivity.     

钝顶螺旋藻在不同光照条件下的放氧特性

钝顶螺旋藻在持续照光和中等频率 (0.01~20 Hz) 的光/暗交替照光下的放氧特性对光生物反应器的设计和操作具有重要意义。构建了一套可实现光/暗交替的光生物反应器系统对此进行研究,结果显示：根据与放氧速率的关系,可以将光强分为4个区：光限制区 (0~335 μmol/(m2·s)),过渡区 (335~875 μmol/(m2·s)),光饱和区 (875~2 775 μmol/(m2·s)) 以及光抑制区 (2 775 μmol/(m2·s)以上)。提高光/暗频率能否提高微藻光合速率取决于所采用的光强和     

Scale-up aspects of photobioreactors: effects of mixing-induced light/dark cycles

The green micro-algae Chlamydomonas reinhardtiiand Dunaliella tertiolecta were cultivated undermedium-duration square-wave light/dark cycles with acycle time of 15 s. These cycles were used to simulatethe light regime experienced by micro-algae inexternally-illuminated (sunlight) air-lift loopbioreactors with internal draft tube. Biomass yieldin relation to light energy was determined as gprotein per mol of photons (400–700 nm). Between 600and 1200 mol m^-2 s^-1 the yield at a10/5 s light/dark cycle was equal to the yield atcontinuous illumination. Consequently, provided thatthe liquid circulation time is 15 s, a considerabledark zone seems to be allowed in the interior ofair-lift loop photobioreactors (33% v/v) without lossof light utilization efficiency. However, at a 5/10 slight/dark cycle, corresponding to a 67% v/v darkzone, biomass yield decreased. Furthermore, bothalgae, C. reinhardtii and D. tertiolecta,responded similarly to these cycles with respect tobiomass yield. This was interesting because they werereported to exhibit a different photoacclimationstrategy. Finally, it was demonstrated that D.tertiolecta was much more efficient at low (average)photon flux densities (57–370 mol m^-2s^-1) than at high PFDs (> 600 mol m^-2s^-1) and it was shown that D. tertiolectawas cultivated at a sub-optimal temperature (20 ^°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.     

Analysis of light regime in continuous light distributions in photobioreactors

Maximum photobioreactor (PBR) efficiency is a must in applications such as the obtention of microalgae-derived fuels. Improving PBR performance requires a better understanding of the "light regime", the varying irradiance that microalgal cells moving in a dense culture are exposed to. We propose a definition of light regime that can be used consistently to describe the continuously varying light patterns in PBRs as well as in light/dark cycles. Equivalent continuous and light/dark regimes have been experimentally compared and the results show that continuous variations are not well represented by light/dark cycles, as had been widely accepted. It has been shown that a correct light regime allows obtaining photosynthetic rates higher than the corresponding to continuous light, the so-called "flashing light effect" and that this is possible in commercial PBRs. A correct PBR operation could result in photosynthetic efficiency close to the optimum eight quanta per O(2).     

Effect of lectins on auxin-induced elongation and wall loosening in oat coleoptile and azuki bean epicotyl segments

A marine unicellular aerobic nitrogen-fixing cyanobacterium Synechococcus sp. strain Miarni BG 043511 was pretreated with different light and dark regimes in order to induce higher growth synchrony. A pretreatment of two dark and light cycles of 16 h each yielded good synchrony for 3 cell division cycles. Longer dark treatments decreased the degree of synchrony and shorter dark treatments caused irregular cell division. Once synchronous culture was established, distinct phases of cellular carbohydrate accumulation and cellular carbohydrate degradation were observed even under continuous illumination. Changes in carbohydrate content were repeated in a cyclic manner with approximately 20 h intervals, the same as the cell division cycle. This change in carbohydrate metabolism provided a good index of growth synchrony under nitrogen-fixing conditions.
Photosynthetic oxygen evolution and nitrogen fixation capabilities and their activities in near, in situ, culture conditions were measured in well synchronized cultures of this strain under continuous illumination. Distinct oscillations of both photosynthetic oxygen evolution and nitrogen fixation capabilities with ca 20-h intervals, similar to the interval of the cell division cycle, were observed for three cycles. However, the activities of photosynthetic oxygen evolution were inversely correlated with those of nitrogen fixation. During the nitrogen fixation period, net oxygen consumption was observed even in the light under conditions approximating in situ culture conditions. The phase of temporal appearance of nitrogenase activity during the cell division cycle coincided with the phase of carbohydrate net degradation. These data indicate that this unicellular cyanobacterium can grow diazotrophically under conditions of continuous illumination by the segregation of photosynthesis and nitrogen fixation within a cell division cycle.     

Photoperiod and light quality effects on rate of dark respiration and on photosynthetic capacity in developing seedlings of Chenopodium rubrum

Development and acclimation of energy transduction were studied in seedlings of Chenopodium rubrum L. ecotype selection 184 (50° 10' N; 105° 35' W) in response to photomorphogenic and photoperiodic treatments. Dark respiration and photosynthetic capacity [nmol O₂ (pair of cotyledons)⁻¹ h⁻¹] were measured with an oxygen electrode. Changes in chloroplast ultrastructure were analyzed concomitantly. After germination, seedlings were grown at constant temperature either in darkness or in continuous light (white, red, far-red and blue) or were subjected to diurnal cycles of light/dark or changes in light quality. Dark respiration was low in far-red light treated seedlings. In red light treated seedlings dark respiration was high and the mean value did not depend on fluence rate or photoperiod. Blue light stimulated transitorily and modulated dark respiration in photoperiodic cycles. Photosynthetic capacity was reduced by far-red light and increased by red light. In response to blue light photosynthetic capacity increased, with indications of a requirement for continuous energy input. Phytochrome and a separate blue light receptor seemed to be involved. In continuous red light a clear cut circadian rhythm of dark respiration was observed. Blue light had a specific effect on chloroplast structure.     

Photosynthetic efficiency of Chlamydomonas reinhardtii in attenuated, flashing light

As a result of mixing and light attenuation, algae in a photobioreactor (PBR) alternate between light and dark zones and, therefore, experience variations in photon flux density (PFD). These variations in PFD are called light/dark (L/D) cycles. The objective of this study was to determine how these L/D cycles affect biomass yield on light energy in microalgae cultivation. For our work, we used controlled, short light path, laboratory, turbidostat‐operated PBRs equipped with a LED light source for square‐wave L/D cycles with frequencies from 1 to 100 Hz. Biomass density was adjusted that the PFD leaving the PBR was equal to the compensation point of photosynthesis. Algae were acclimated to a sub‐saturating incident PFD of 220 µmol m^?2 s^?1 for continuous light. Using a duty cycle of 0.5, we observed that L/D cycles of 1 and 10 Hz resulted on average in a 10% lower biomass yield, but L/D cycles of 100 Hz resulted on average in a 35% higher biomass yield than the yield obtained in continuous light. Our results show that interaction of L/D cycle frequency, culture density and incident PFD play a role in overall PBR productivity. Hence, appropriate L/D cycle setting by mixing strategy appears as a possible way to reduce the effect that dark zone exposure impinges on biomass yield in microalgae cultivation. The results may find application in optimization of outdoor PBR design to maximize biomass yields. Biotechnol. Bioeng. 2012; 109: 2567–2574. © 2012 Wiley Periodicals, Inc.     

Effect of light/dark cycles on wastewater treatments by microalgae

Chlorella kessleri was cultivated in artificial wastewater using diurnal illumination of 12 h light/12 h dark (L/D) cycles. The inoculum density was 10⁵ cells/mL and the irradiance in light cycle was 45 μmol m² s⁻¹ at the culture surface. As a control culture, another set of flasks was cultivated under continuous illumination. Regardless of the illumination scheme, the total organic carbon (TOC) and chemical oxygen demand (COD) was reduced below 20% of the initial concentration within a day. However, cell concentration under the L/D lighting scheme was lower than that under the continuous illuminating scheme. Thus the specific removal rate of organic carbon under L/D cycles was higher than that under continuous illumination. This result suggested thatC. kessleri grew chemoorganotrophically in the dark periods. After 3 days, nitrate was reduced to 136.5 and 154.1 mg NO₃ ⁻-N/L from 168.1 mg NO₃ ⁻-N/L under continuous illumination and under diurnal cycles, respectively. These results indicate nitrate removal efficiency under continuous light was better than that under diurnal cycles. High-density algal cultures using optimized photobioreactors with diurnal cycles will save energy and improve organic carbon sources removal.     

An energy balance from absorbed photons to new biomass for Chlamydomonas reinhardtii and Chlamydomonas acidophila under neutral and extremely acidic growth conditions

Chlamydomonas is one of the most well-studied photosynthetic organisms that had important biotechnological potential for future bioproductions of biofuels. However, an energy balance from incident photons to the energy stored in the new biomass is still lacking. In this study, we applied a recently developed system to measure the energy balance for steady state growth of Chlamydomonas reinhardtii grown at pH 6.5, and C. acidophila that was grown at pH 6.5 and 2.6. Energy use efficiency was quantified on the basis of light absorption, photosynthetic quantum yield, photosynthetic and respiratory quotient, and electron partitioning into proteins, carbohydrates and lipids. The results showed that lower growth rates of C. acidophila under both pH conditions were not caused by the differences in the photosynthetic quantum yield or in alternative electron cycling, but rather by differences in the efficiency of light absorption and increased dark respiration. Analysis of the macromolecular composition of the cells during the light phase showed that C. acidophila uses biosynthetic electrons preferentially for carbohydrate synthesis but not for synthesis of lipids. This led to a strong diurnal cycle of the C/N ratio and could explain the higher dark respiration of C. acidophila compared with C. reinhardtii .     

Slow fluorescence fluctuations following high light to low light or dark transitions in Chlamydomonas reinhardi.

Slow fluorescence transients in Chlamydomonas reinhardi arise after transitions from high light intensities to low light or dark conditions. Characteristics of the newly described transient phenomena include: (a) A slow biphasic decrease in fluorescence yield occurs in the dark, followed by an even slower, hour long, increase in fluorescence. (b) A similar, but faster, fluorescence yield decrease and subsequent increase also occurs during low intensity illumination periods separating high light intervals, or after transitions from high intensity to low intensity light. (c) Short (several seconds) flashes of light given during a dark period have no effect on the dark fluorescence decay, regardless of the flash frequency. Such flash regimes accurately monitor the dark decline of the M2 level by tracing the parallel decay of flash-generated P2 (Kautsky) peaks. However, flashes during a low light illumination period do influence the decay kinetics. Frequent flashes allow decay similar to that occurring in dark, but less frequent flashes inhibit the decrease in fluorescence yield.     

Influence of high frequency light/dark fluctuations on photosynthetic characteristics of microalgae photoacclimated to different light intensities and implications for mass algal cultivation

Oxygen evolution from aScenedesmus obliquus dominated outdoor culture was followed in a small volume chamber, irradiated either by continuous white light or under light/dark frequencies between 0.05 to 5000 Hz, using arrays of high intensity red light emitting diodes (LED's). By placing neutral density filters in the path of the white light, light saturation curves of the oxygen evolution (P/I curves) were measured using diluted aliquots of algal cultures. The results clearly showed that photosynthetic rates increased exponentially with increasing light/dark frequencies, that a longer dark period in relation to the light period does not necessarily lead to higher photosynthetic rates (efficiencies), and that algae do not acclimate to a specific light/dark frequency. One of the most important factors that influenced photosynthetic rates, either under continuous illumination or intermittent, was whether the algae were dark or light acclimated. Low light/dark frequencies were perceived by the algae as low light conditions, whilst the opposite was true for high frequencies. The light utilisation efficiency in a fluctuating light/dark environment depended on the acclimated state of the algae, the specific frequency of the fluctuations and the duration of the exposure. Since the frequencies determined the perceived quantities of light, dark reactions played an important role in determining the average photosynthetic efficiencies. These results have important implications for algal biotechnology.     

间作对不同类型饲料作物光能利用特征及生产能力的影响

为探究豆-禾间作模式中的光能利用特征及产量效应,本研究以苜蓿、小黑麦、燕麦、玉米、甜高粱5种单作模式为参照,对苜蓿-小黑麦、苜蓿-燕麦、苜蓿-玉米和苜蓿-甜高粱4种间作模式开展田间试验,研究其光能利用特征和生产能力,并采用通径分析法探讨光能利用各指标对产量形成的贡献程度.结果表明: 光能利用各指标对产量的贡献由大到小依次是叶面积指数(LAI,1.236)、净光合速率(P_n,0.519)、冠层开度(DIFN,0.302)、胞间CO₂浓度(C_i,-0.026)、气孔导度(g_s,-0.116)、蒸腾速率(T_r,-0.188)、光合有效辐射(PAR)截获率(FIPAR,-1.708);光能利用率(LUE)作为光能利用各指标的综合体现,其数值最大(1.367).与单作相比,间作提高了4种禾本科牧草的LAI、P_n、g_s、T_r和FIPAR,降低了DIFN和C_i,而苜蓿则呈相反的变化趋势.4种禾本科牧草间作下LUE与单作相比均显著提高,其中,小黑麦和燕麦的平均增幅(35.2%和30.4%)高于玉米和甜高粱(28.7%和26.3%),且与小黑麦、燕麦间作的苜蓿其LUE的平均降幅(6.1%和8.3%)明显低于与玉米、甜高粱间作(21.8%和24.5%).4种间作模式的土地当量比(LER)均大于1,其中,苜蓿-小黑麦和苜蓿-燕麦的LER值显著大于苜蓿-玉米和苜蓿-甜高粱.LAI对产量的直接贡献最大,其次是P_n.4种间作模式中,苜蓿-小黑麦和苜蓿-燕麦的光能利用提升潜力和增产潜力更大.     

Effect of light nitrogenase function and synthesis in Rhodopseudomonas capsulata.

The metabolic versatility of the purple nonsulfur photosynethetic bacterial permits the expression of either a phototrophic or a dark aerobic mode of growth. These organism also possess nitrogenase activity which may function under semiaerboic conditions. On the basis of these important properties, the light dependence of nitrogenase function and synthesis in Rhodopseudomonas capsulata was investigated. Nitrogenase activity was strictly dependent on light; no activity was observed in the dark, even when energy (ATP) was supplied by oxidative phosphorylation. It was concluded that the low-potential reducing agent required by the nitrogenase-catalyzed reaction could only be generated by a photochemical reaction. Nitrogenase biosynthesis was also largely dependent on light; however, a small amount of synthesis was observed in resting cells incubated in the dark. Resting cells prepared from dark-grown cultures synthesized nitrogenase at high rates upon illumination. The highest stability of nitrogenase in these resting cells was observed when suspensions were exposed to a diurnal pattern of illumination rather than continuous light. Although nitrogenase function and synthesis are closely coupled to photosynthetic activity, the biosyntheses of bacteriochorophyll and nitrogenase are independent of each other and are most probably subject to different regulatory mechanisms by light.     

Light-induced changes in GTP and ATP in frog rod photoreceptors. Comparison with recovery of dark current and light sensitivity during dark adaptation

Light decreases GTP and ATP levels in purified suspensions of physiologically active frog rod outer segments still attached to their inner segment ellipsoids (OS-IS). (a) The GTP decrease is slower in OS-IS (t1/2 = 40 s) than in isolated outer segments (t1/2 = 7 s), which suggests there is more effective buffering in OS-IS. (b) The GTP decrease becomes detectable only at intensities greater than those required to saturate the photoresponse. As the intensity of a continuous light is increased over 4 log units, GTP levels decrease linearly with log intensity by as much as 60%. GTP is reduced to steady intermediate levels during extended illumination of intermediate intensity. (c) At levels of illumination bleaching greater than 0.003% of the rhodopsin, a decrease in ATP levels becomes detectable. (d) Following a flash, GTP levels fall and then rise with a recovery time dependent on the intensity of the flash. (e) After both 0.2 and 2% flash bleaches, the recovery of GTP levels parallels the recovery of light sensitivity, which is slower than the recovery of the dark current. This raises the possibility of a link between GTP levels and light sensitivity.     

The flexible interrelation between AOX respiratory pathway and photosynthesis in rice leaves.

Alternative respiratory pathway was investigated in rice seedlings grown under total darkness, light/dark cycle, or continuous light. The capacity of the alternative pathway was relatively higher in leaves that had longer light exposure. An analysis of rice AOX1 multigene family revealed that AOX1c, but not AOX1a and AOX1b, had a light-independent expression. The alternative oxidase (AOX) inhibitor, salicylhydroxamic acid (SHAM, 1mM), inhibited nearly 68% of the capacity of the alternative pathway in leaves grown under different light conditions. The plants grown under different light periods were treated with SHAM and then were exposed to illumination for 4h. The transition from dark to 4h of light stimulated the capacity of alternative pathway in etiolated rice seedlings and in those grown under light/dark cycle, whereas the capacity of the alternative pathway was constant in seedlings grown under continuous light with additional 4h of illumination. Etiolated leaves did not show any CO(2) fixation after 4h of illumination, and the increase in chlorophyll content was delayed by the SHAM pretreatment. When seedlings grown under light/dark cycle were moved from dark and exposed to 4h of light, increases in chlorophyll content and CO(2) fixation rate were reduced by SHAM. Although these parameters were stable in plants grown under continuous light, SHAM decreased CO(2) fixation rate but not the chlorophyll content. These results indicate that the role and regulation of AOX in light are determined by the developmental stage of plant photosynthetic apparatus.     

Differential photoinhibition of bacterial and archaeal ammonia oxidation

Inhibition by light potentially influences the distribution of ammonia oxidizers in aquatic environments and is one explanation for nitrite maxima near the base of the euphotic zone of oceanic waters. Previous studies of photoinhibition have been restricted to bacterial ammonia oxidizers, rather than archaeal ammonia oxidizers, which dominate in marine environments. To compare the photoinhibition of bacterial and archaeal ammonia oxidizers, specific growth rates of two ammonia-oxidizing archaea (Nitrosopumilus maritimus and Nitrosotalea devanaterra) and bacteria (Nitrosomonas europaea and Nitrosospira multiformis) were determined at different light intensities under continuous illumination and light/dark cycles. All strains were inhibited by continuous illumination at the highest intensity (500 μE m(-2) s(-1)). At lower light intensities, archaeal growth was much more photosensitive than bacterial growth, with greater inhibition at 60 μE m(-2) s(-1) than at 15 μE m(-2) s(-1), where bacteria were unaffected. Archaeal ammonia oxidizers were also more sensitive to cycles of 8-h light/16-h darkness at two light intensities (60 and 15 μE m(-2) s(-1)) and, unlike bacterial strains, showed no evidence of recovery during dark phases. The findings provide evidence for niche differentiation in aquatic environments and reduce support for photoinhibition as an explanation of nitrite maxima in the ocean.