Photosynthetic Oxygen Release Research Articles

Realizing UV driven microalgae photosynthesis to bio-fix CO2 by spectrum conversion

Microalgae, as efficient photosynthetic autotrophs, hold significant potential for carbon sequestration. However, their photosynthesis is constrained by the selective use of the solar spectrum. To enhance the utilization of natural light energy, a dual functional substrate with ultraviolet (UV) light conversion and microalgae biofilm support was proposed by coating a rare earth complexes phosphor agent onto transparent acrylic in this study. By doing this, UV light could directly drive microalgae biofilm photosynthesis by converting it to red light, resulting in a maximum 2.81 times increase in the carbon fixation rate. The UV conversion substrate increased the electron transport rate of microalgal photosystem II by 58.08%, effectively enhancing the photosynthetic rate and oxygen release, promoting the formation of a porous biofilm structure, and strengthening the material transfer within the biofilm. Additionally, reactive oxygen species (ROS) can cause oxidative stress, affecting cell growth and metabolic activity. With the use of the UV conversion substrate, the levels of ROS and malondialdehyde (MDA) in microalgae decreased by 84.56% and 42.35%, respectively. Consequently, UV-induced damage to microalgae cells was reduced, leading to a 52.52% increase in the photosynthesis rate and achieving UV driven photosynthetic carbon sequestration. This study demonstrated the UV conversion substrate’s effectiveness in reducing UV-induced damage in microalgae and enhancing photosystem activity, showing significant potential for carbon fixation within microalgal biofilms.

Abrupt and acclimation responses to changing temperature elicit divergent physiological effects in the diatom Phaeodactylum tricornutum.

Growth rates and other biomass traits of phytoplankton are strongly affected by temperature. We hypothesized that resulting phenotypes originate from deviating temperature sensitivities of underlying physiological processes. We used membrane-inlet mass spectrometry to assess photosynthetic and respiratory O2 and CO2 fluxes in response to abrupt temperature changes as well as after acclimation periods in the diatom Phaeodactylum tricornutum. Abrupt temperature changes caused immediate over- or undershoots in most physiological processes, that is, photosynthetic oxygen release ( ), photosynthetic carbon uptake ( ), and respiratory oxygen release ( ). Over acclimation timescales, cells were, however, able to re-adjust their physiology and revert to phenotypic 'sweet spots'. Respiratory CO2 release ( ) was generally inhibited under high temperature and stimulated under low-temperature settings, on abrupt as well as acclimation timescales. Such behavior may help mitochondria to stabilize plastidial ATP : NADPH ratios and thus maximize photosynthetic carbon assimilation.

Optimal root oxygen release from two macrophytes Saururus cernuus L. and Pistia stratiotes L. varies with light and temperature in simulated constructed wetlands microcosms

Different environmental factors influence photosynthesis, transpiration, stomatal conductance, and oxygen released from macrophytes' roots. We measured several physiological parameters of Saururus cernuus L. (Lizard's Tail) and Pistia stratiotes L. (Water Cabbage) by exposing the plants to various controlled environmental conditions. The oxygen released from the plant roots was also used to treat the chemical and biological oxygen demand in wastewater in microcosms. This work determined that photosynthetic rate and oxygen release were dependent not only on the plant species but also on the light intensity exposed. The highest rate of photosynthesis differed considerably between the two plants, as S. cernuus required more light intensity than P. stratiotes. Maximum root oxygen release was at peak photosynthesis, thus correlated with light intensity. The temperature had an effect on stomatal conductance, photosynthesis rate, and transpiration rate was affected by the wind speed. For the highest stomatal conductance and photosynthesis, P. stratiotes required a higher temperature than S. cernuus. Stomatal conductance also influenced the oxygen released from plant roots. Furthermore, a relationship was determined between physiological processes and their potential to release oxygen from the roots where the reduction in COD and BOD measured at the end of the experiment was largely attributed to the oxygen that the macrophytes added to the substrate through their roots. The results of this study provide a good benchmark for plant scientists to predict the physiological processes of plants in response to different environmental conditions to control the negative impacts of harsh environmental conditions on plants' ability to release oxygen in their rhizosphere and to improve the efficiency of CW.

Transcriptomic Analysis of Ulva prolifera in Response to Salt Stress

Since 2007, Ulva prolifera, a green macroalgal species that blooms periodically in the Yellow Sea of China, has caused the world’s biggest green tide, which induced harmful ecological impacts and economic losses. When the alga is subject to prolonged exposure to air, it suffers abiotic stresses. To explore the physiological and molecular mechanisms of salt stress, the transcriptome data of U. prolifera at different salinities (30, 50, and 90 psu) were obtained by high-throughput sequencing using the Illumina HiSeq platform, and photosynthetic physiological parameters were also measured. The results showed that a total of 89,626 unigenes were obtained after de novo assembly, of which 60,441 unigenes were annotated in the databases (NR, NT, KO, SWISS-PROT, PFAM, GO, and KOG). GO functional enrichment analysis revealed that the enrichment of differentially expressed genes (DEGs) was mainly in cellular, cell, and binding processes. KEGG metabolic pathway enrichment analysis showed that the most frequently enriched pathways of DEGs included glycolysis, pyruvate metabolism, peroxisome, and fatty acid biosynthesis. In addition, resistance-associated proteins, such as heat-shock proteins, microtubule-associated proteins, ubiquitin-associated proteins, abscisic-acid-signaling-pathway-associated proteins, and antioxidant-related proteins are upregulated under salt stress. Genes associated with photosynthesis and carbon fixation pathways are also upregulated, accompanied by an increase in photosynthetic oxygen release rates. These findings provide a basis for understanding the molecular mechanisms of the response of U. prolifera to salinity change, thus providing a theoretical basis for the analysis of the green tide outbreak mechanism.

Effects of nutrients on the utilization pattern of photosynthetic inorganic carbon of the tropical <italic>Thalassia hemprichii</italic>

<p indent="0mm">Studies on the utilization strategy of photosynthetic inorganic carbon of seagrass can reveal its ecological adaptation mechanism. <italic>Thalassia hemprichii</italic>, a widely distributed seagrass species in the Indo-Pacific and Atlantic coastal seas, needs to evolve several utilization patterns of inorganic carbon to adapt different environments and meet its growth, development and reproduction. However, the effect of nutrient on the photosynthetic inorganic carbon utilization strategy in seagrass is still unknown and further research is urgently needed. Therefore, using oxygen electrode technology, chlorophyll fluorescence technology, and non-invasive micro-test technology, this study examined the similarity and difference in the utilization patterns of photosynthetic inorganic carbon of <italic>T</italic>.<italic> hemprichii </italic>from two typical tropical seagrass beds (Tanmen, Hainan Island, and Yongxing Island, Xisha), that were affected by varying degrees of human activity. Experiments showed that the photosynthetic oxygen release rate, maximum relative electron transfer rate, and half-saturated light intensity of <italic>T</italic>.<italic> hemprichii</italic> in Yongxing Island waters were 24.95%, 24.54%, and 28.73% higher than the corresponding in Tanmen, Hainan Island. This indicated that the growth status of <italic>T</italic>.<italic> hemprichii</italic> in the waters of Yongxing Island was better with higher photosynthetic efficiency. The addition of acetazolamide (AZ) and ethoxyzolamide (EZ) significantly reduced the photosynthetic oxygen release rate of <italic>T</italic>.<italic> hemprichii</italic>, with an insignificant difference in the inhibition between AZ and EZ. This indicated that it mainly absorbed HCO₃⁻ via extracellular carbonic anhydrase catalysis. Meanwhile, hydroxymethyl (Tris) inhibited the photosynthetic oxygen release of <italic>T</italic>.<italic> hemprichii</italic> in Tanmen by up to 100% and decreased the maximum relative electron transfer rate and the H⁺ inward flow rate, implying a synergistic H⁺/HCO₃⁻ transport mode. On the other hand, the rate of photosynthetic oxygen release and the maximum electron transfer rate of <italic>T</italic>.<italic> hemprichii</italic> in Xisha did not change significantly with the addition of AZ and EZ, while Tris buffer inhibited photosynthetic oxygen release by up to 100% and significantly reduced the maximum electron transfer rate. This suggested <italic>T</italic>.<italic> hemprichii </italic>in Xisha mainly relied on the H⁺/HCO₃⁻ synergistic transport. Therefore, <italic>T</italic>.<italic> hemprichii </italic>could uptake inorganic carbon in an environment with greater anthropogenic activity, larger nutrient load and lower light availability on Hainan Island by catalyzing HCO₃⁻ into CO₂ with extracellular carbonic anhydrase and the H⁺/HCO₃⁻ synergistic transport. In contrast, <italic>T</italic>.<italic> hemprichii</italic> in an environment with less anthropogenic activity, lower nutrient loading, and greater light availability, relied chiefly on H⁺/HCO₃⁻ synergistic transport. Facing a relatively unfavorable high-nutrient and low-light growth environment, <italic>T</italic>.<italic> hemprichii</italic> could increase the photosynthetic inorganic carbon utilization strategy (such as the activity of extracellular carbonic anhydrase) to improve its utilization ability of inorganic carbon to overcome stress and growth limitation. This suggested that <italic>T</italic>.<italic> hemprichii</italic> could adjust its inorganic carbon utilization patterns according to its geographical environment. With the continuous development of technology, it is urgent to study the utilization mechanism of inorganic carbon of seagrass by applying seagrass omics technology and stable isotope tracing to deepen the understanding of the carbon sink function of seagrass.

Controlling the source of green tides in the Yellow Sea: NaClO treatment of Ulva attached on Pyropia aquaculture rafts

Green macroalgal blooms have occurred in the Yellow Sea for 13 consecutive years since 2007. However, effective control methods have not yet been successfully developed. Ulva prolifera attached on Pyropia aquaculture rafts was identified as the main source of the blooms, thus, eliminating the attached Ulva from the rafts should be the most efficient method to prevent and control these blooms. In this study, we designed a new molecular marker (rps2-trnL) to identify U. prolifera collected from the aquaculture site. Samples identified as U. prolifera were cultured, and different concentrations of sodium hypochlorite (NaClO) solution (0.2, 0.4, 0.6, 0.8, and 1.0%) were used to kill the alga. The measured chlorophyll content, light quantum yield, and photosynthetic oxygen release rate data from the laboratory experiment suggested that the appropriate concentration of NaClO solution should be 0.6–1.0%. To carry out a small-scale field trial, we developed a device that can spray NaClO on Ulva blades and recycle the residual liquid. Results showed that only the 1.0% NaClO solution could quickly kill the Ulva in outdoor experiments. During the treatment, very little NaClO dripped on the tidal flat, which indicated that the recycling device avoided a potential negative impact of residual solution on the marine ecosystem. By the beginning of November 2019, the Pyropia aquaculture area treated by NaClO was about 13,000 ha, covering about one-third of the total area at the coast of Jiangsu Province. Monitoring by an unmanned aerial vehicle showed that very little attached Ulva was present on the ropes after NaClO treatment, which indicated that it was an effective method to control the green tide. Also, the range of residual active chlorine within seawater during the treatment was only from 0.003 to 0.012 mg/L, which complied with Chinese national standards. However, further monitoring of the formation process of green tides should be conducted in future years to evaluate the effect of NaClO treatment.

Recycling nutrients from soy sauce wastewater to culture value-added Spirulina maxima

Soy sauce is one of the most common condiments in East Asian cuisines. Its manufacturing process generates a large amount of organic-rich soy sauce wastewater, which is difficult to treat through traditional processes. In this study, Spirulina NCU-Sm was used to treat soy sauce wastewater and simultaneously produce value-added biomass. The results showed that NCU-Sm grew well in 100% (undiluted) raw soy sauce wastewater (RSW). The maximum biomass yield of NCU-Sm reached 1.984 g/L, and the removal efficiencies of NH4+-N, TN, COD, chromaticity, and salinity in RSW reached 93.86, 81.76, 84.08, 40.93, and 63.08%, respectively. High COD removal efficiency in 100% RSW proved NCU-Sm grew in mixotrophic mode. pH and light spectrum, two major factors affecting growth of soy sauce wastewater-grown Spirulina NCU-Sm, were further studied. When the initial pH of RSW was adjusted to 8, NCU-Sm biomass yield was increased by 11.11%, and the contents of protein, chlorophyll-a, and carotenoid were increased by 8.69, 37.14, 40.40%, and reached 65.57%, 7.57 and 2.78 mg/g, respectively. The removal efficiencies of NH4+-N, TN, and chromaticity in RSW with pH 8 were improved to 99.34, 88.29, 43.18%, respectively. Red light enhanced the photosynthetic oxygen release efficiency of NCU-Sm and increased the carotenoid content of NCU-Sm by 117.95%, reaching 5.95 mg/g. Blue light was adverse to the growth and reduced the protein content of NCU-Sm but significantly increased its chlorophyll-a content to up to 10.99 mg/g. These results suggest that cultivation of Spirulina in soy sauce wastewater serves dual roles in soy sauce wastewater treatment, as well as value-added algal biomass (such as high-quality protein and natural pigments raw materials) production.

Integrating solar photovoltaic capacitor into algal-bacterial photo-bioelectrochemical system towards all-weather synchronous enhanced antibiotic and nitrogen removal from wastewater

Algal-bacterial photo-bioelectrochemical system (ABPBS) operated with daily light/dark cycle provides a novel approach for sustainable treatment of wastewater through the uses of solar energy and bioenergy. However, it can not be effectively driven when the photosynthetic oxygen supply by algae at the cathode stops at night. In this study, a solar photovoltaic capacitor discharging was integrated into the ABPBS to achieve all-weather synchronous enhanced antibiotic and nitrogen removal from wastewater. The results showed that 3.3 F, 10 F and 100 F capacitor discharge increased the degradation rate of florfenicol (FLO) by 44%, 89%, and 582% at the anode, the removal rate of ammonia nitrogen by 20.4%, 39.8%, and 55.6%, the accumulated nitrate was decreased by 70.0%, 86.5%, and 93.3%, and the accumulated nitrite was decreased by 48.6%, 45.7%, and 87.1% at the biocathode, respectively. There was a significant positive correlation between the capacity of the capacitor and its ability to promote FLO degradation and nitrogen removal. The capacitor discharge promoted the photosynthetic oxygen release and biocatalytic oxygen reduction, and stimulated the growth of functional bacteria capable of degrading complex organics through extracellular electron transfer in the anode and diverse nitrogen removal bacteria in the cathode, but there were structural differences of functional bacteria groups induced by the different capacities of capacitor discharges.

Chronic exposure of soybean plants to nanomolar cadmium reveals specific additional high-affinity targets of cadmium toxicity.

Solving the global environmental and agricultural problem of chronic low-level cadmium (Cd) exposure requires better mechanistic understanding. Here, soybean (Glycine max) plants were exposed to Cd concentrations ranging from 0.5 nM (background concentration, control) to 3 µM. Plants were cultivated hydroponically under non-nodulating conditions for 10 weeks. Toxicity symptoms, net photosynthetic oxygen production and photosynthesis biophysics (chlorophyll fluorescence: Kautsky and OJIP) were measured in young mature leaves. Cd binding to proteins [metalloproteomics by HPLC-inductively coupled plasma (ICP)-MS] and Cd ligands in light-harvesting complex II (LHCII) [X-ray absorption near edge structure (XANES)], and accumulation of elements, chloropyll, and metabolites were determined in leaves after harvest. A distinct threshold concentration of toxicity onset (140 nM) was apparent in strongly decreased growth, the switch-like pattern for nutrient uptake and metal accumulation, and photosynthetic fluorescence parameters such as Φ RE10 (OJIP) and saturation of the net photosynthetic oxygen release rate. XANES analyses of isolated LHCII revealed that Cd was bound to nitrogen or oxygen (and not sulfur) atoms. Nutrient deficiencies caused by inhibited uptake could be due to transporter blockage by Cd ions. The changes in specific fluorescence kinetic parameters indicate electrons not being transferred from PSII to PSI. Inhibition of photosynthesis combined with inhibition of root function could explain why amino acid and carbohydrate metabolism decreased in favour of molecules involved in Cd stress tolerance (e.g. antioxidative system and detoxifying ligands).

Effects of temperature on the photosynthetic performance in mature thalli of the red alga Gelidium amansii (Gelidiaceae)

Gelidium amansii, a commercial marine red alga, yields agar of excellent quality, but thus far its cultivation has been unsuccessful. Effects of temperature, especially high temperature, on the photosynthetic performance of this alga are poorly understood. The responses of net O2 evolution and pigment content in mature thalli exposed to various temperatures of 14, 18, 22, 26, and 30 °C for 2 days were studied. Fast Chl-a fluorescence kinetics and RuBPCase activity of plants exposed to 30 °C for 6, 12, 24, and 48 h were also determined. The results showed that 14–26 °C favored the net O2 evolution of G. amansii, which ranged from 0.50 to 1.53 μmol O2∙g−1∙min−1. High temperature (30 °C) decreases the contents of Chl-a and carotenoids. Phycobiliproteins increased in the thalli treated for 12 h and longer at 30 °C, suggesting that phycobiliproteins might play an important role in alleviation of oxidative damage caused by high temperature. In addition, the carbon assimilation process, light harvesting pigments, PSII reaction centers, and PSII acceptor side of G. amansii were damaged under 30 °C at 12 h; however, the PSII acceptor side was relatively stable during the initial 24 h treatment. At 30 °C both PSII photochemistry and carbon assimilation were negatively affected and impacted the photosynthetic response of G. amansii. RuBPCase activity showed a stronger positive correlation with photosynthetic oxygen release than the photochemistry. The results of this study confirm that the optimal temperature for G. amansii photosynthesis is between 22 °C to 26 °C and high temperatures of 30 °C and above limit G. amansii growth in near-shore waters.

High-concentration nitrogen removal coupling with bioelectric power generation by a self-sustaining algal-bacterial biocathode photo-bioelectrochemical system under daily light/dark cycle

High-concentration nitrogen removal coupled with bioelectric power generation in an algal-bacterial biocathode photo-bioelectrochemical system (PBES) was investigated. The PBES can self-sustaining operation with continuous power output under day/night cycle by alternately using photosynthetic dissolved oxygen and nitrate/nitrite as cathodic electron acceptors. The PBES generated a high maximum power of 110mw/m2 under illumination and relatively lower power of 40mw/m2 under dark. The bioelectricity generation was accompanied by high-concentration nitrogen removal in the algal-bacterial biocathode. The NH4N was removed completely within 120 h while maximum NO3N removal efficiency of 86% and maximum total nitrogen removal efficiency of 83% can be reached after 192 h at initial NH4N concentration of 314 mg/L and NO3N concentration of 330 mg/L. Combined processes of bioelectrochemical reduction and algal-bacterial interactions provided multiple approaches for nitrogen removal in the biocathode, including nitrifying using photosynthetic oxygen, bioelectrochemical denitrification using the cathode as electron donor, heterotrophic denitrification using photosynthetically produced dissolved organic matters as carbon source and algal-bacterial uptake. Accelerated nitrogen removal with simultaneously improved cathode performance was observed at high concentration of nitrogen and phosphate buffer due to enhanced algal activities for photosynthetic oxygen release and enhanced algal-bacterial interactions for nitrogen transformation. Addition of external organic carbon negatively affected nitrification and decreased cathode potential due to oxygen consumption by aerobic carbon oxidation but enhanced denitrification due to continuous release of high concentration of photosynthetically produced dissolved organic matters by alga. The PBEC was demonstrated as an energy-saving approach for high-strengthen nitrogenous wastewater treatment.

More than just CO2 -recycling: corticular photosynthesis as a mechanism to reduce the risk of an energy crisis induced by low oxygen.

Reassimilation of internal CO2 via corticular photosynthesis (PScort ) has an important effect on the carbon economy of trees. However, little is known about its role as a source of O2 supply to the stem parenchyma and its implications in consumption and movement of O2 within trees. PScort of young Populus nigra (black poplar) trees was investigated by combining optical micro-optode measurements with monitoring of stem chlorophyll fluorescence. During times of zero sap flow in spring, stem oxygen concentrations (cO2 ) exhibited large temporal changes. In the sapwood, over 80% of diurnal changes in cO2 could be explained by respiration rates (Rd(mod) ). In the cortex, photosynthetic oxygen release during the day altered this relationship. With daytime illumination, oxygen levels in the cortex steadily increased from subambient and even exhibited a diel period of superoxia of up to 110% (% air sat.). By contrast, in the sapwood, cO2 never reached ambient levels; the diurnal oxygen deficit was up to 25% of air saturation. Our results confirm that PScort is not only a CO2 -recycling mechanism, it is also a mechanism to actively raise the cortical O2 concentration and counteract temporal/spatial hypoxia inside plant stems.

Benthic Oxygen Fluxes Measured by Eddy Covariance in Permeable Gulf of Mexico Shallow-Water Sands

Oxygen fluxes across the sediment–water interface reflect primary production and organic matter degradation in coastal sediments and thus provide data that can be used for assessing ecosystem function, carbon cycling and the response to coastal eutrophication. In this study, the aquatic eddy covariance technique was used to measure seafloor–water column oxygen fluxes at shallow coastal sites with highly permeable sandy sediment in the northeastern Gulf of Mexico for which oxygen flux data currently are lacking. Oxygen fluxes at wave-exposed Gulf sites were compared to those at protected Bay sites over a period of 4 years and covering the four seasons. A total of 17 daytime and 14 nighttime deployments, producing 408 flux measurements (14.5 min each), were conducted. Average annual oxygen release and uptake (mean ± standard error) were 191 ± 66 and −191 ± 45 mmol m−2 day−1 for the Gulf sites and 130 ± 57 and −152 ± 64 mmol m−2 day−1 for the Bay sites. Seasonal variation in oxygen flux was observed, with high rates typically occurring during spring and lower rates during summer. The ratio of average oxygen release to uptake at both sites was close to 1 (Bay: 0.9, Gulf: 1.0). Close responses of the flux to changes in light, temperature, bottom current velocity, and wave action (significant wave height) documented tight physical–biological, benthic–pelagic coupling. The increase of the sedimentary oxygen uptake with increasing temperature corresponded to a Q10 temperature coefficient of 1.4 ± 0.3. An increase in flow velocity resulted in increased oxygen uptake (by a factor of 1–6 for a doubling in flow), which is explained by the enhanced transport of organic matter and electron acceptors into the permeable sediment. Benthic photosynthetic production and oxygen release from the sediment was modulated by light intensity at the temporal scale (minutes) of the flux measurements. The fluxes measured in this study contribute to baseline data in a region with rapid coastal development and can be used in large-scale assessments and estimates of carbon transformations.

Trophic interactions of proteolytic bacteria Proteinivorax tanatarense in an alkaliphilic microbial community

Lythic action of an anaerobic proteolytic bacterium Proteinivorax tanatarense on organisms with different cell wall types was studied. In the absence of photosynthetic oxygen release, this proteolytic was able to grow on intact biomass of cyanobacteria belonging to various systematic groups. Itis probably their usual saprotrophic-satellite responsible for the regulation of abundance of primary producers during the dark phase. Growth also occurred on the biomass of a nonphototrophic gram-negative microorganism-Halomonas campisalis, a common component of alkaliphilic. microbial communities: Comparative analysis of the interaction of the proteolytic with.H. campisalis cells at different physiological states revealed the lytic action to be re- stricted to dead and/or weakened cells, rather than the actively dividing ones. Strict specificity of the action of the proteolytic bacterium on gram-negative microorganisms with no effect on gram-positive ones was shown.

Bark and woody tissue photosynthesis: a means to avoid hypoxia or anoxia in developing stem tissues.

In woody plants, oxygen transport and delivery via the xylem sap are well described, but the contribution of bark and woody tissue photosynthesis to oxygen delivery in stems is poorly understood. Here, we combined stem chlorophyll fluorescence measurements with microsensor quantifications of bark O2 levels and oxygen gas exchange measurements of isolated current-year stem tissues of beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.) to investigate how bark and woody tissue photosynthesis impairs the oxygen status of stems. Measurements were made before bud break, when the axial path of oxygen supply via the xylem sap is impeded. At that time, bark O2 levels showed O2 concentrations below the atmospheric concentration, indicating hypoxic conditions or O2 deficiency within the inner bark, but the values were always far away from anoxic. Under illumination bark and woody tissue photosynthesis rapidly increased internal oxygen concentrations compared with plants in the dark, and thereby counteracted against localised hypoxia. The highest photosynthetic activity and oxygen release rates were found in the outermost cortex tissues. By contrast, rates of woody tissue photosynthesis were considerably lower, due to the high light attenuation of the bark and cortex tissues, as well as resistances in radial oxygen diffusion. Therefore, our results confirm that bark and woody tissue photosynthesis not only play a role in plant carbon economy, but may also be important for preventing low oxygen-limitations of respiration in these dense and metabolically active tissues.

Impact of two different humic substances on selected coccal green algae and cyanobacteria—changes in growth and photosynthetic performance

There is growing evidence to show that dissolved humic substances, HSs, can directly interact with freshwater organisms, such as phototrophic organisms, cladocerans, amphipods and fish. The responses are-at least in part-transcriptionally controlled. These interactions can lead to stress symptoms in the exposed organisms. In phototrophs, stress symptoms include a reduction in photosynthetic oxygen release and antioxidative stress. Besides the direct effects, HSs also cause indirect effects that provoke different physiological adaptations in the phototrophs. The HS-influenced photosynthetic performance and stress response of two different green algae, Pseudokirchneriella subcapitata (Koršikov) Hindák and Monoraphidium braunii (Nägeli in Kützing) Komárková-Legnerová, and two cyanobacterial species, Synechocystis sp. (PCC 6803, Institut Pasteur) and Microcystis aeruginosa (PCC 7806, Institut Pasteur), were tested. Two humic preparations were applied, the synthetic HS1500 and HuminFeed, HF, which had previously been proven effective in bioassays with invertebrates and a water mould. When the algae were grown near light saturation, most of the tested species were positively affected by HSs in growth rate or chlorophyll content. Cell sizes decreased with increasing HS concentrations for all eukaryotic phototrophs, except for the cyanobacteria. After 4 to 5 days of cultivation at the highest HS exposure, there was a decrease in total dry weight due to reduced cell sizes in contrast to an increase in cell numbers. With the exception of Synechocystis, the dry weight per cell ratio decreased with increasing HS concentration. The efficiency of utilizing absorbed light quanta increased with increasing HS concentrations; the maximum quantum yield of photosystem II (ΦPSIImax) was higher in all of the tested species, with the exception of M. aeruginosa, after exposure to HS. The applied humic preparations did not interact directly with PSII, but changed the physiological state of the algae, especially the photosynthetic performance. Neither the green algae nor the cyanobacteria were inhibited in growth or negatively affected in their photosynthetic performance. The exposure to lower concentrations of HS stimulated better growth of the phototrophs. The tested humic preparations obviously did not have the potency to act as xenobiotic stressors; furthermore, there was no sign of herbicide potency.

Pumpkin fruit, seed and oil yield is independent of fruit or seed photosynthesis

SUMMARYSummer pumpkin (Cucurbita pepo L.) is an herbaceous plant with big fruits and oily seeds. Photosynthetic oxygen release was previously proposed to promote storage activity of seeds under internal hypoxia, in particular in oilseeds. Photosynthetic activity or any other function of chlorophylls from pumpkin seeds has never been demonstrated. The aim of the present research was to test whether illumination of pumpkin fruits has any influence on the yield and to measure photosynthetic activity of pumpkin seeds. The fruits grown in the dark were not significantly different from controls exposed to light in regard to fruit weight, size, number and weight of seeds or the content of lipids and pigments. Delayed fluorescence measurement showed no photosynthetic activity in pumpkin seeds in any developmental stage. Fruit wall, on the other hand, had significant photosynthetic activity. It is concluded that chlorophyll in pumpkin seeds does not mediate photosynthesis, which would in turn result in increased pumpkin yield.

Phragmites australis and Quercus robur leaf extracts affect antioxidative system and photosynthesis of Ceratophyllum demersum

During senescence, leaves are deposited on aquatic bodies and decay under water releasing chemical substances that might exert physiological stress to aquatic organisms. Leaf litter alone contributes 30% of the total dissolved organic carbon (DOC) in streams. We investigated the impact of leaves extract from Phragmites australis and Quercus robur on the antioxidative system and photosynthetic rate of the aquatic macrophyte Ceratophyllum demersum exposed for 24 h. Rate of photosynthetic oxygen release and antioxidant enzyme activity (glutathione S-transferases, glutathione reductases and peroxidases) as well as lipid peroxidation in C. demersum were measured. Significant ( P<0.01) elevations of antioxidative enzyme activity in C. demersum which tends to plateau at high DOC concentrations were observed. There was no detectable effect on lipid peroxidation. A significant dose-dependent reduction in photosynthetic oxygen production was measured.

Natural Organic Matter Differently Modulates Growth of Two Closely Related Coccal Green Algal Species (6 pp)

Humic substances (HS) comprise the majority of dead and living organic carbon, including organisms. In the environment, they are considered to be chemically inert or at least refractory. Recent papers, however, show that HS (including natural organic matter-NOM, isolated by reverse osmosis) are natural chemicals which interact with aquatic organisms. They are taken up and cause a variety of stress defense reactions which are well known from man-made chemicals. These reactions include chaperon activation, induction and modulation of biotrans-formation enzymes, or induction of antioxidant defense enzymes. One specific reaction with freshwater plants is the reduction of photosynthetic oxygen release. In this contribution, we compare the susceptibilities (cell yield) of two closely related coccal green algae, Monoraphidium convolutum and M. minutum, towards various NOM isolates. Cultures of M. convolutum and M. minutum were obtained from the algal collection of the Leibniz Institute of Freshwater Ecology and Inland Fisheries, Berlin, and from the Culture Collection of Algae, Göttingen, and maintained in a common medium. The cultures were non-axenic. The algae were exposed to 5 mg L(-1) DOC of each humic material, an environmentally realistic concentration. Cell numbers were counted microscopically in Neugebauer cuvettes in 5 replicates on days 1, 4, 7, 10, 14, and 21. Almost all NOM isolates modulated the growth of the algae. Only the NOM of a Norwegian raised peat bog lake did not reveal any significant effect with M. convolutuim. In general, the results with two algal species are by no means uniform. For instance, Suwannee River NOM causes a decrease in cell density with M. minutum, but temporarily stimulates the growth of M. convolutum. The opposite applies to Aurevann NOM: Growth increase in M. minutum, but a bi-phasic response in M. convolutum. Different responses of both Monoraphidium species must be attributed to intrinsic factors of the algae rather than only to chemical features of the exposed materials, because the exposures were identical with both algal species. The reduction in growth yields can be explained as a herbicide-like mode of action that affects the photosystem II most prevalently. The growth promoting effect remains somewhat obscure. It may be due to (1) an increase in bioavailability of some trace nutrients in the presence of HS, (2) the release of some growth promoting substances by microbial or photochemical processing of the humic materials, and (3) a hormetic effect upon the exposure of HS. Hormesis means stimulation of organisms or metabolic activities when exposed to noxes in low concentrations. However, it is still open to discussion why the growth promotion only applies to one or the other, but not simultaneously to both Monoraphidium species. Exposure of the closely related coccal green algal species to humic material changes their growth characteristics. Since the reactions are not consistent within the two species and the various humic materials, it seems that the less sensitive species is favored by HS exposure. The environmental relevance, however, is subject to future studies.

Relationship between photosynthesis and non-photochemical quenching of chlorophyll fluorescence in two red algae with different carotenoid compositions

Algae are continuously exposed to short-term fluctuations in irradiance. We investigate how two red algae species regulate photosynthetic efficiency to cope with such changes and identify some strategies that differ from higher plants. Two red algae, Gracilaria domingensis and Kappaphycus alvarezii, with antheraxanthin and lutein as major xanthophylls, respectively, reacted to the onset of low light (below Ek) with a substantial decrease of NPQ. This is different from higher plants, but similar to previous observations in, e.g. cyanobacteria where it indicates an increase in the effective absorbance cross-section of Photosystem II (PSII) by state transition. Kinetic studies in continuous light revealed a high susceptibility of PSII to light stress ((1-qP)/NPQ) in K. alvarezii immediately after the sudden onset of high light, followed by a decrease. This was caused by a slower onset of NPQ in K. alvarezii, followed by acclimation. In G. domingensis, susceptibility of PSII to light stress was stable with time, but absolute values of (1-qP)/NPQ were higher than in K. alvarezii. These observations suggest that K. alvarezii may be better adapted to high light levels, but is less well prepared for large sudden changes in irradiation. In K. alvarezii, photosynthesis continued to increase with increasing irradiation when NPQ was saturated. As (1-qP) and NPQ were still balanced in this situation, most likely, processes other than photosynthetic oxygen release are responsible for the increasing net O2 production observed.