Photosynthetic O2 Evolution Rate Research Articles

Taxonomic and Morpho-Functional Photosynthetic Patterns of 18 Intertidal Macroalgal Species in the Guangdong–Hong Kong–Macao Greater Bay Area, China

Macroalgae provide food for microbial, meio- and macro-faunal communities in coastal ecosystems, thus mediating nutrient dynamics and functions in these ecosystems. Because of this vital role, it is important to clarify physiological information about macroalgae as it reflects their growth potential in the field. In this study, we examined the biomass, pigment content, and photosynthetic O2 evolution rate versus irradiance curves of 18 macroalgal species from the intertidal zone of the Guangdong–Hong Kong–Macao Greater Bay Area, China, and investigated their photosynthetic patterns in relation to phyla characteristics, morphology, and growth locations. The results showed that green algae had the highest maximum photosynthetic O2 evolution rate (Pmax), light utilization efficiency (α), and dark respiration (Rd) among the three macroalgal phyla; the sheet-like macroalgal species had the highest Pmax, α, and Rd among the four morphological categories. The macroalgal species in the upper intertidal zone showed higher Pmax and α and lower saturation irradiance (EK) and compensation irradiance (EC) than those species in the lower intertidal location. The PCA results showed that the biomass of sheet-like macroalgal species was positively correlated with factor PC1 (50.34%), and that of finely branched species was negatively correlated with factor PC2 (25.17%). In addition, our results indicate that the light absorption and utilization capabilities of macroalgae could determine whether they could dominate the intertidal zone and that their photosynthetic characteristics could be used as a potential indicator of their biomass distribution in the Greater Bay Area.

Different Responses of Photosynthesis to Nitrogen Starvation Between Highly Oil-Accumulative Diatoms, Fistulifera solaris and Mayamaea sp. JPCC CTDA0820.

Highly oil-accumulative diatoms are expected to be a promising biomass for the production of biofuel. To harvest the diatom oils at high yields, it is critical to elucidate the relationship of oil accumulation with photosynthesis under fluctuating environmental conditions. Here, we characterized the physiological responses of the growth and photosynthesis in the mesophilic diatom Fistulifera solaris and the cold-tolerant one Mayamaea sp. JPCC CTDA0820 to nitrogen starvation, one of the most notable abiotic stresses, where most eukaryotic algae decrease their photosynthetic activity and accumulate oil in the cells. While F. solaris started showing growth retardation at NaNO3 levels less than 50% of a normal F/2 artificial seawater (ASW) medium, Mayamaea sp. sustained normal growth even at a NaNO3 level 10% of normal F/2ASW, indicating the sharp contrast of nitrogen requirement between these two diatom species. In the transition from 100 to 0% nitrogen conditions in the modified F/2ASW, F. solaris showed a clear suppression of chlorophyll (Chl)-based photosynthetic O2 evolution rate and relative electron transport rate at photosystem II, which were negatively correlated to the capacity of non-photochemical quenching. Meanwhile, there was no change in these Chl-based parameters observed in nitrogen-starved Mayamaea sp. Instead, Mayamaea sp. showed a significant decrease in the Chl a amount per cells. These data suggested the occurrence of two types of photosynthetic responses to nitrogen starvation in oleaginous diatoms; that is, (1) suppression of photosynthetic activity per Chl with enhancing heat dissipation of excess light energy and (2) synchronous suppression of cellular photosynthetic activity with Chl amounts.

Dark fermentative hydrogen production and transcriptional analysis of genes involved in the unicellular halotolerant cyanobacterium Aphanothece halophytica under nitrogen and potassium deprivation.

The unicellular halotolerant cyanobacterium Aphanothece halophytica is known as a potential hydrogen (H2) producer. This study aimed to investigate the enhancement of H2 production under nutrient deprivation. The results showed that nitrogen and potassium deprivation induced dark fermentative H2 production by A. halophytica, while no differences in H2 production were found under sulfur and phosphorus deprivation. In addition, deprivation of nitrogen and potassium resulted in the highest H2 production in A. halophytica due to the stimulation of hydrogenase activity. The effect of adaptation time under nitrogen and potassium deprivation on H2 production was investigated. The results showed that the highest H2 accumulation of 1,261.96 ± 96.99µmol H2 g dry wt-1 and maximum hydrogenase activity of 179.39 ± 8.18µmol H2 g dry wt-1 min-1 were obtained from A. halophytica cells adapted in the nitrogen- and potassium-deprived BG11 medium supplemented with Turk Island salt solution (BG110-K) for 48h. An increase in hydrogenase activity was attributed to the decreased O2 concentration in the system, due to a reduction of photosynthetic O2 evolution rate and a promotion of dark respiration rate. Moreover, nitrogen and potassium deprivation stimulated glycogen accumulation and decreased specific activity of pyruvate kinase. Transcriptional analysis of genes involved in H2 metabolism using RNA-seq confirmed the above results. Several genes involved in glycogen biosynthesis (glgA, glgB, and glgP) were upregulated under both nitrogen and potassium deprivation, but genes regulating enzymes in the glycolytic pathway were downregulated, especially pyk encoding pyruvate kinase. Interestingly, genes involved in the oxidative pentose phosphate pathway (OPP) were upregulated. Thus, OPP became the favored pathway for glycogen catabolism and the generation of reduced nicotinamide adenine dinucleotide phosphate (NADPH), which resulted in an increase in H2 production under dark anaerobic condition in both nitrogen- and potassium-deprived cells.

Tight relationship between two photosystems is robust in rice leaves under various nitrogen conditions.

Leaf nitrogen (N) level affects not only photosynthetic CO2 assimilation, but also two photosystems of the photosynthetic electron transport. The quantum yield of photosystem II [Y(II)] and the non-photochemical yield due to the donor side limitation of photosystem I [Y(ND)], which denotes the fraction of oxidized P700 (P700+) to total P700, oppositely change depending on leaf N level, and the negative correlation between these two parameters has been reported in leaves of plants cultivated at various N levels in growth chambers. Here, we aimed to clarify whether this correlation is maintained after short-term changes in leaf N level, and what parameters are the most responsive to the changes in leaf N level under field conditions. We cultivated rice varieties at two N fertilization levels in paddy fields, treated additional N fertilization to plants grown at low N, and measured parameters of two photosystems of mature leaves. In rice leaves under low N condition, the Y(ND) increased and the photosynthetic linear electron flow was suppressed. In this situation, the accumulation of P700+ can function as excess energy dissipation. After the N addition, both Y(ND) and Y(II) changed, and the negative correlation between them was maintained. We used a newly-developed device to assess the photosystems. This device detected the similar changes in Y(ND) after the N addition, and the negative correlation between Y(ND) and photosynthetic O2 evolution rates was observed in plants under various N conditions. This study has provided strong field evidence that the Y(ND) largely changes depending on leaf N level, and that the Y(II) and Y(ND) are negatively correlated with each other irrespective of leaf N level, varieties and annual variation. The Y(ND) can stably monitor the leaf N status and the linear electron flow under field conditions.

Photosynthetic Characteristics of Macroalgae Ulva fasciata and Sargassum thunbergii in the Daya Bay of the South China Sea, with Special Reference to the Effects of Light Quality

The changes in underwater light in field usually occur not only in intensity but in spectrum, affecting the photophysiology of marine photoautotrophs. In this study, we comparably examined the photosynthesis of two dominating macroalgae in the Daya Bay, Chlorophyta Ulva fasciata and Phaeophyta Sargassum thunbergii, under white light, as well as under red, green and blue light. The results showed that the net photosynthetic O2 evolution rate (Pn) of U. fasciata under field light increased from 25.2 ± 3.06 to 168 ± 1.2 µmol O2 g FW−1 h−1 from dawn to noon, then decreased to 42.4 ± 0.20 µmol O2 g FW−1 h−1 at dusk. The Pn of S. thunbergii exhibited a similar diel change pattern, but was over 50% lower than that of U. fasciata. The maximal photosynthetic rate (Pmax) of U. fasciata derived from the photosynthesis vs. irradiance curve under white light (i.e., 148 ± 15.8 µmol O2 g FW−1 h−1) was ~30% higher than that under blue light, while the Pmax of S. thunbergii under white light (i.e., 39.2 ± 3.44 µmol O2 g FW−1 h−1) was over 50% lower than that under red, green and blue light. Furthermore, the daily primary production (PP) of U. fasciata was ~20% higher under white than blue light, while that of S. thunbergii was 34% lower, indicating the varied light spectral compositions influence algal photosynthetic ability and thus their primary production in field, and such an influence is species-specific.

Physicochemical behavioral changes in consort with nitrogen metabolism of cyanobacterium Anabaena PCC 7120 under arsenite regimes.

The present study was undertaken to investigate the arsenite (As III)-induced changes in the diazotrophic cyanobacterium Anabaena PCC 7120. It was observed that the growth of cyanobacterial decreased with increase in As (III) concentration. The cells exposed to As (III) showed morphological variation (deformity) due to the formation of deeper constrictions in vegetative cells. Strain showed increased heterocyst differentiation (1.6-fold higher) whereas decreased nitrogenase activity at the concentration of 40ppm As (III). The activities of NR, NiR, urease and GS decreased with increase in As (III) concentrations and attained their minimum levels at 40ppm of As (III). The Ca2+-dependent ATPase activity increased with increase in As (III) concentration and attained its about 2.72-fold higher level at 40ppm of As (III). In contrast, sharp decline in Mg2+-dependent ATPase activity (28%) was recorded at 1ppm of As (III) over untreated control. The rates of photosynthetic O2 evolution and respiration decreased with increase in As (III) concentration and attained its minimal level at 40ppm of As (III). Therefore, this study highlighted arsenite regimes efficiently correlated with behavioral changes in consort with strain.

High Salinity Reduces Plant Growth and Photosynthetic Performance but Enhances Certain Nutritional Quality of C4 Halophyte Portulaca oleracea L. Grown Hydroponically Under LED Lighting.

Portulaca oleracea L. (known as purslane) is one of the most nutritious leafy vegetables owing to its high content of antioxidants. In this study, all plants were grown indoors hydroponically with different NaCl salinities. Photosynthetic photo flux density (PPFD) at 200 μmol m−2 s−1 (12 h) was provided to all plants by LED with red:blue ratio of 2.2. Thirty days after transplanting, plants grown with100 mM NaCl had the highest productivity and the fastest leaf growth followed by those with 0, 200 and 300 mM NaCl. Grown with 300 mM NaCl, purslane had the lowest specific leaf area due to its highest leaf dry matter content and its lowest water content. All plants had similar values of leaf succulence except for those with 300 mM NaCl. Total chlorophyll and carotenoids contents were significantly higher in plants grown with 0 and 100 mM NaCl than with 200, and 300 mM NaCl. All plants had Fv/Fm ratios close to 0.8. However, electron transport rate and ΔF/Fm′ were significantly higher in plants grown with 0 and 100 mM NaCl than with 200 and 300 mM NaCl. CAM-induced purslane with 300 mM NaCl had higher non-photochemical quenching. Maximum net photosynthetic O2 evolution rate and Cyt b6f concentration were significantly lower with 300 mM NaCl compared to all other plants while all plants had similar PS II concentration. Proline concentration increased with increasing salinities. All plants had similar levels of total soluble sugars. Plants grown with 0 and 100 mM NaCl had significantly higher concentrations of NO3−, total reduced nitrogen, total leaf soluble protein, Rubisco protein, total ascorbic acid, and total phenolic compounds than with 200 and 300 mM NaCl. The highest concentrations of K, Ca, and Mg were found in purslane grown under 0 mM NaCl. Statistically, no significant differences in Fe concentrations were observed among all plants. However, salinity seems to increase Fe concentration. In conclusion, it is feasible to grow purslane under 100 mM NaCl as it is the most optimal condition to achieve higher productivity and better quality. However, the production of antioxidants may depend on not only salinity but also other growth conditions.

Quantity of supplementary LED lightings regulates photosynthetic apparatus, improves photosynthetic capacity and enhances productivity of Cos lettuce grown in a tropical greenhouse.

Although cooling their rootzone allows year-round (temperate) vegetable production in Singapore's warm climate, these crops have frequently experienced increasingly unpredictable cloudy and hazy weather. Supplementary lighting with light-emitting diodes (LEDs) could be used to reduce the impacts of low light intensity. This study investigated the responses of temperate Cos lettuce (Lactuca sativa L.) to different quantities (photosynthetic photon flux density, PPFD of 0, 150, 300µmolm-2s-1) of supplementary LED lightings in the tropical greenhouse. Increasing light intensity significantly increased total leaf area, shoot and root fresh weight (FW) and dry weight (DW), total chlorophyll (Chl) and carotenoids (Car) contents, light-saturated photosynthetic CO2 assimilation rate (Asat) and transpiration rate (Tr). There were no significant differences in Fv/Fm ratio, total reduced nitrogen, specific leaf area (SLA) and PSII concentration among the three light treatments. However, there was an increasing trend with increasing light intensity for Chl a/b ratio, net photosynthetic O2 evolution rate (PN), cytochrome b6f (Cyt b6f), leaf total soluble protein and Rubisco concentrations. This study provides the basic understanding of photosynthetic apparatus and capacity of temperate crops grown under different supplementary LED lightings in the tropical greenhouse.

Characterization of Light-Enhanced Respiration in Cyanobacteria

In eukaryotic algae, respiratory O2 uptake is enhanced after illumination, which is called light-enhanced respiration (LER). It is likely stimulated by an increase in respiratory substrates produced during photosynthetic CO2 assimilation and function in keeping the metabolic and redox homeostasis in the light in eukaryotic cells, based on the interactions among the cytosol, chloroplasts, and mitochondria. Here, we first characterize LER in photosynthetic prokaryote cyanobacteria, in which respiration and photosynthesis share their metabolisms and electron transport chains in one cell. From the physiological analysis, the cyanobacterium Synechocystis sp. PCC 6803 performs LER, similar to eukaryotic algae, which shows a capacity comparable to the net photosynthetic O2 evolution rate. Although the respiratory and photosynthetic electron transports share the interchain, LER was uncoupled from photosynthetic electron transport. Mutant analyses demonstrated that LER is motivated by the substrates directly provided by photosynthetic CO2 assimilation, but not by glycogen. Further, the light-dependent activation of LER was observed even with exogenously added glucose, implying a regulatory mechanism for LER in addition to the substrate amounts. Finally, we discuss the physiological significance of the large capacity of LER in cyanobacteria and eukaryotic algae compared to those in plants that normally show less LER.

In situ Photosynthetic Activities and Associated Biogeochemical Changes in Three Tropical Seagrass Species

Tropical seagrasses, experience considerable spatio-temporal changes in light and other biogeochemical conditions and adopt species-specific acclimatization strategies. Variation in photo-acclimatory responses of three tropical seagrass species (Cymodocea serrulata, Thalassia hemprichii and Enhalus acoroides) were studied by measuring photosynthetic electron transport rates (ETR) using Pulse Amplitude Modulated (PAM) fluorometry technique. Quantitative values of ETR and rates of photosynthetic O2 evolution (net O2 exchange corrected for dark respiration) were compared to establish species-specific relationships in these shallow water conditions. The apparent average molar ratio of O2 evolution to ETR for Cymodocea serrulata, Thalassia hemprichii and Enhalus acoroides were estimated as 0.23, 0.19 and 0.20, respectively. The highest photosynthetic activity (ETRmax) varied significantly among the three studied species in the decreasing order as: E. acoroides (59.27); T. hemprichii (54.06) and C. serrulata (46.72). The effective quantum yield (Y) of PS II (Photosystem II), one of the most useful indicators of stress conditions for seagrass, was observed to be significantly higher for E. acoroides compared to the other two species. Variability of Y in the ambient conditions are explained by the difference in water temperature and pCO2 for the three seagrass species. This study is useful in predicting the photo-acclimation strategies to any change in light availability and subsequent biogeochemical changes at the bottom surface, by these shallow water tropical seagrass species.

Growth and photosynthetic characteristics of sweet potato (Ipomoea batatas) leaves grown under natural sunlight with supplemental LED lighting in a tropical greenhouse

Leaf growth and photosynthetic characteristics of sweet potato (Ipomoea batatas var. Biru Putih) grown under different light quantities were studied in a tropical greenhouse. The stem cuttings of I. batataswith adventitious roots were grown hydroponically under (1) only natural sunlight (SL); (2) SL with supplemental LED at a PPFD of 150 μmol m−2 s–1 (SL + L-LED); and (3) SL with supplemental LED at a PPFD of 300 μmol m−2 s–1 (SL + H-LED). One week after emergence, all leaves had similar area and water content. However, leaf fresh weight and dry weight were significantly higher in plants grown under SL+L-LED and SL + H-LED than under SL due to their thicker leaves reflected by the lower specific leaf area. Plants grown under SL had significantly lower concentrations of total chlorophyll (Chl) and total carotenoids (Car) but higher Chl a/b ratio than under SL + L-LED and SL + H-LED. However, all plants had similar Chl/Car ratios. Although midday Fv/Fm ratio was the lowest in leaves grown under SL+ H-LED followed by SL + L-LED and SL, predawn Fv/Fm ratios of all leaves were higher than 0.8. Increasing growth irradiance with supplemental LED resulted in higher electron transport rate and photochemical quenching but lower non-photochemical quenching compared to those of plants grown under SL. Measured under their respective growth irradiance in the greenhouse, attached leaves grown under SL + L-LED and SL+H-LED had significantly higher photosynthetic CO2 assimilation rate and stomatal conductance than under SL. However, measuring the detached leaves at 25 °C in the laboratory, there were no significant differences in PS II and Cyt b6f concentrations although light- and CO2-statured photosynthetic O2 evolution rates were slightly higher in leaves grown under SL+ H-LED than under SL. Impacts of supplemental LED on leaf growth and photosynthetic characteristics were discussed.

Rotifers release a lipid-soluble agent that inhibits photosynthetic electron transport in Chlorella sp

Rotifer contamination, as a severe constraint in mass cultures, impedes industrial-scale cultivation of microalgae. Once contamination occurred, the rotifers released something that inhibited microalgal cell growth in addition to algal reduction due to predation. With the aim of further identifying the type and effects of the inhibitor released from rotifers, Chlorella sp. was cultured in fresh modified F/2 medium and in media containing various proportions of boiled rotifer culture-medium filtrate (RCF), as well as in media containing water-soluble or lipid-soluble fractions isolated from RCF. Then, the growth and photosynthesis of Chlorella sp. were measured. The rotifer inhibition of Chlorella growth could not be eliminated by boiling the RCF, indicating that the inhibitor was not likely a protein. The growth and photosynthesis of Chlorella sp. was not inhibited by the water-soluble RCF fraction, but they were significantly inhibited by the lipid-soluble RCF fraction, in a dose-dependent manner. Further, the lipid-soluble fraction decreased energy conservation and photosynthetic electron transport, which induced a severe decrease in PSII activity and a decrease in the net photosynthetic O2 evolution rate. Based on these physiological responses of Chlorella cells, the lipid-soluble fraction rather than the protein or water-soluble fractions was determined to contain the responsible inhibitor, suggesting the direction of further studies.

Transcriptomic analysis of hydrogen photoproduction in Chlorella pyrenoidosa under nitrogen deprivation

H2 photoproduction by green algae is a very attractive way to produce clean and renewable energy. In our previous study, Chlorella pyrenoidosa strain IOAC707S was found to produce hydrogen under nitrogen deprivation in seawater medium; however the mechanism is not entirely clear. In this work, it was found that under nitrogen deprivation, the efficiency of photosystem II photochemical activity and the photosynthetic O2 evolution rate of C. pyrenoidosa strain IOAC707S decreased, while the respiratory rate increased, which were favorable for the establishment of anoxia and H2 photoproduction. Transcriptomic analysis indicated that photosynthetic activity did not simply decline but underwent a complex adjustment to prepare for H2 photoproduction. The activities of glycolysis and the pentose phosphate pathway were up-regulated, suggesting more electron transport from endogenous substrate to H2 photoproduction. The decrease of the tricarboxylic acid cycle activity resulted in decreased levels of CO2, which would limit Calvin cycle activity, and subsequently lead to the accumulation of NADPH and a reduction of the photosynthetic electron transport chain. Under these conditions, the induced hydrogenase served as an electron valve, and H2 was produced.

Algal density mediates the photosynthetic responses of a marine macroalga Ulva conglobata (Chlorophyta) to temperature and pH changes

Growing of macroalgae increases their biomass densities in natural habitats. To explore how the altered algal density impacts their photosynthetic responses to changes of environmental factors, we compared the photosynthesis versus irradiance characteristics of a marine green macroalga Ulva conglobata under low [2.0 g fresh weight (FW) L−1], medium (6.0 g FW L−1) and high biomass densities (12.0 g FW L−1), and under a matrix of temperatures (20, 25, 30 and 35 °C) and pH levels (7.8, 8.2 and 8.6). Increased algal densities decreased the photosynthetic O2 evolution rate among all combined temperature and pH treatments, in parallel with the decrease of light-utilizing efficiency (α, the initial slope) and maximum photosynthetic rate (Pmax) and the increase of light saturation point (EK). Rising temperature interacted with lowered pH to increase the α under low but not under high algal densities. Rising temperature increased the Pmax and decreased the EK under low algal density, but not under high density. Lowered pH promoted the Pmax and EK under all three algal densities. The increased temperature enhanced the dark respiration (Rd) and light compensation point (EC), while the altered pH showed a limited effect. Moreover, the increased algal density reduced the Rd, and had a limited effect on the EC. In addition, our results indicate that changing algal densities caused the complex photophysiological changes in responses to the temperature and pH changes, and these complex responses resolved into a close relation between Rd and Pmax across the matrix of temperatures and pH levels.

Role of nano-powder of Azadirachta indica leaves to regulate the physiological responses and metal uptake in Triticum aestivum seedlings

ABSTRACTIn the present study different doses (0.05, 2.0 and 5.0 mg per 30 ml nutrient medium) of nano-powder (Azadirachta indica leaves) were applied in Cd contaminated (6 ppm) hydroponic system to regulate the metal uptake in Triticum aestivum (wheat) seedlings. Other physiological attributes including oxidative biomarkers, antioxidants and photosynthetic responses were also assessed. The level of Cd was maximally reduced at the dose of 2.0 mg nano powder per 30 ml nutrient medium by 45 and 49% in the shoot and root, respectively. With the maximum reduction in the Cd uptake at this dose, the generation of oxidative stress markers such as H2O2 (12%), MDA (26%) and SOR (20%) content showed maximum reduction in treated seedlings. At different doses of nano-powder, the activities of antioxidative enzymes were also showed significant variation. Further, the photosynthetic O2 evolution rate was improved with the treatment of nano-powder and the best response was noted at 2.0 mg per 30 ml nutrient medium with the maximum value of fresh shoot biomass (38%). The overall results suggest that, this technique could be easily applied for reducing the metal content and increasing the quality of agricultural crops.

A thylakoid-located carbonic anhydrase regulates CO2 uptake in the cyanobacterium Synechocystis sp. PCC 6803.

Carbonic anhydrases (CAs) are involved in CO2 uptake and conversion, a fundamental process in photosynthetic organisms. Nevertheless, the mechanism underlying the regulation of CO2 uptake and intracellular conversion in cyanobacteria is largely unknown. We report the characterization of a previously unrecognized thylakoid-located CA Slr0051 (EcaB) from the cyanobacterium Synechocystis sp. PCC 6803, which possesses CA activity to regulate CO2 uptake. Inactivation of ecaB stimulated CO2 hydration in the thylakoids, suppressed by the classical CA inhibitor acetazolamide. Absence of ecaB increased the reduced state of the photosynthetic electron transport system, lowered the rate of photosynthetic O2 evolution at high light (HL) and pH, and decreased the cellular affinity for extracellular inorganic carbon. Furthermore, EcaB was upregulated in cells grown at limiting CO2 concentration or HL in tandem with CupA. EcaB is mainly located in the thylakoid membranes where it interacts with CupA and CupB involved in CO2 uptake by converting it to bicarbonate. We propose that modulation of the EcaB level and activity in response to CO2 changes, illumination or pH reversibly regulates its conversion to HCO3 by the two CO2 -uptake systems (CupA, CupB), dissipating the excess HCO3- and alleviating photoinhibition, and thereby optimizes photosynthesis, especially under HL and alkaline conditions.

Correlation of carbonic anhydrase and Rubisco in the growth and photosynthesis in the diatom Phaeodactylum tricornutum

Carbonic anhydrase (CA) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) are critical enzymes involved in the CO2-concentrating mechanism (CCM) and carbon fixation in phytoplankton. These enzymes play key roles in photosynthetic carbon acquisition and fixation in diatoms. However, the potential synergistic interactions of the said enzymes remain little understood. In the present study, effects of CA inhibitors on the growth, photosynthetic rate, and Rubisco activities in the diatom Phaeodactylum tricornutum were investigated. Our results showed that the growth and photosynthetic O2 evolution rate of P. tricornutum were significantly inhibited by the intracellular CA (iCA) inhibitor ethoxzolamide (EZ), but not by extracellular CA (eCA) inhibitor acetazolamide (AZ) in the long term. This suggests that the eCA was not necessarily essential for growth and photosynthesis, but rather it was the iCA activity that was more crucial in P. tricornutum. We also found that there is a significant positive correlation between Rubisco and CA activities, suggesting that the two carbon acquisition and fixation components of photosynthesis may have undergone co-evolution to optimize fundamental trade-offs. These results further indicate that the roles of eCA and iCA may also differ among diatom species or strains, and that their functions are tightly correlated with other carbon fixation components.

A Carbon Dioxide Limitation-Inducible Protein, ColA, Supports the Growth of Synechococcus sp. PCC 7002.

A limitation in carbon dioxide (CO2), which occurs as a result of natural environmental variation, suppresses photosynthesis and has the potential to cause photo-oxidative damage to photosynthetic cells. Oxygenic phototrophs have strategies to alleviate photo-oxidative damage to allow life in present atmospheric CO2 conditions. However, the mechanisms for CO2 limitation acclimation are diverse among the various oxygenic phototrophs, and many mechanisms remain to be discovered. In this study, we found that the gene encoding a CO2 limitation-inducible protein, ColA, is required for the cyanobacterium Synechococcus sp. PCC 7002 (S. 7002) to acclimate to limited CO2 conditions. An S. 7002 mutant deficient in ColA (ΔcolA) showed lower chlorophyll content, based on the amount of nitrogen, than that in S. 7002 wild-type (WT) under ambient air but not high CO2 conditions. Both thermoluminescence and protein carbonylation detected in the ambient air grown cells indicated that the lack of ColA promotes oxidative stress in S. 7002. Alterations in the photosynthetic O2 evolution rate and relative electron transport rate in the short-term response, within an hour, to CO2 limitation were the same between the WT and ΔcolA. Conversely, these photosynthetic parameters were mostly lower in the long-term response of a few days in ΔcolA than in the WT. These data suggest that ColA is required to sustain photosynthetic activity for living under ambient air in S. 7002. The unique phylogeny of ColA revealed diverse strategies to acclimate to CO2 limitation among cyanobacteria.

Characterization of a heat-tolerant Chlorella sp. GD mutant with enhanced photosynthetic CO2 fixation efficiency and its implication as lactic acid fermentation feedstock

BackgroundFermentative production of lactic acid from algae-based carbohydrates devoid of lignin has attracted great attention for its potential as a suitable alternative substrate compared to lignocellulosic biomass.ResultsA Chlorella sp. GD mutant with enhanced thermo-tolerance was obtained by mutagenesis using N-methyl-N′-nitro-N-nitrosoguanidine to overcome outdoor high-temperature inhibition and it was used as a feedstock for fermentative lactic acid production. The indoor experiments showed that biomass, reducing sugar content, photosynthetic O2 evolution rate, photosystem II activity (Fv/Fm and Fv′/Fm′), and chlorophyll content increased as temperature, light intensity, and CO2 concentration increased. The mutant showed similar DIC affinity and initial slope of photosynthetic light response curve (α) as that of the wild type but had higher dissolved inorganic carbon (DIC) utilization capacity and maximum photosynthesis rate (Pmax). Moreover, the PSII activity (Fv′/Fm′) in the mutant remained normal without acclimation process after being transferred to photobioreactor. This suggests that efficient utilization of incident high light and enhanced carbon fixation with its subsequent flux to carbohydrates accumulation in the mutant contributes to higher sugar and biomass productivity under enriched CO2 condition. The mutant was cultured outdoors in a photobioreactor with 6% CO2 aeration in hot summer season in southern Taiwan. The harvested biomass was subjected to separate hydrolysis and fermentation (SHF) for lactic acid production with carbohydrate concentration equivalent to 20 g/L glucose using the lactic acid-producing bacterium Lactobacillus plantarum 23. The conversion rate and yield of lactic acid were 80% and 0.43 g/g Chlorella biomass, respectively.ConclusionsThese results demonstrated that the thermo-tolerant Chlorella mutant with high photosynthetic efficiency and biomass productivity under hot outdoor condition is an efficient fermentative feedstock for large-scale lactic acid production.

Plant Growth and Photosynthetic Characteristics of Mesembryanthemum crystallinum Grown Aeroponically under Different Blue- and Red-LEDs.

Mesembryanthemum crystallinum is a succulent, facultative crassulacean acid metabolism (CAM) plant. Plant growth and photosynthetic characteristics were studied when M. crystallinum plants were grown indoor under light emitting diodes (LED)-lighting with adequate water supply. Plants were cultured aeroponically for a 16-h photoperiod at an equal photosynthetic photon flux density of 350 μmol m-2 s-1 under different red:blue LED ratios: (1) 100:0 (0B); (2) 90:10 (10B); (3) 80:20 (20B); (4) 70:30 (30B); (5) 50:50 (50B); and (6)100:0 (100B). M. crystallinum grown under 10B condition had the highest shoot and root biomass and shoot/root ratio while those grown under 0B condition exhibited the lowest values. Compared to plants grown under 0B condition, all other plants had similar but higher total chlorophyll (Chl) and carotenoids (Car) contents and higher Chl a/b ratios. However, there were no significant differences in Chl/Car ratio among all plants grown under different red- and blue-LEDs. Photosynthetic light use efficiency measured by photochemical quenching, non-photochemical quenching, and electron transport rate, demonstrated that plants grown under high blue-LED utilized more light energy and had more effective heat dissipation mechanism compared to plants grown under 0B or lower blue-LED. Statistically, there were no differences in photosynthetic O2 evolution rate, light-saturated CO2 assimilation rate (Asat), and light-saturated stomatal conductance (gssat) among plants grown under different combined red- and blue-LEDs but they were significantly higher than those of 0B plants. No statistically differences in total reduced nitrogen content were found among all plants. For the total soluble protein, all plants grown under different combined red- and blue-LEDs had similar values but they were significantly higher than that of plants grown under 0B condition. However, plants grown under higher blue-LEDs had significant higher ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) protein than those plants grown under lower blue-LED. High Asat and gssat but very low CAM acidity of all M. crystallinum plants during light period, imply that this facultative CAM plant performed C3 photosynthesis when supplied with adequate water. Results of this study suggest that compared to red- or blue-LED alone, appropriate combination of red- and blue-LED lighting enhanced plant growth and photosynthetic capacities of M. crystallinum.