Baosheng Qiu

Effects of CO2 enrichment on the bloom-forming cyanobacterium Microcystis aeruginosa (Cyanophyceae): Physiological responses and relationships with the availability of dissolved inorganic carbon

Microcystis aeruginosa Kütz. 7820 was cultured at 350 and 700 μL·L−1 CO2 to assess the impacts of doubled atmospheric CO2 concentration on this bloom‐forming cyanobacterium. Doubling of CO2 concentration in the airflow enhanced its growth by 52%–77%, with pH values decreased and dissolved inorganic carbon (DIC) increased in the medium. Photosynthetic efficiencies and dark respiratory rates expressed per unit chl a tended to increase with the doubling of CO2. However, saturating irradiances for photosynthesis and light‐saturated photosynthetic rates normalized to cell number tended to decrease with the increase of DIC in the medium. Doubling of CO2 concentration in the airflow had less effect on DIC‐saturated photosynthetic rates and apparent photosynthetic affinities for DIC. In the exponential phase, CO2 and HCO3− levels in the medium were higher than those required to saturate photosynthesis. Cultures with surface aeration were DIC limited in the stationary phase. The rate of CO2 dissolution into the liquid increased proportionally when CO2 in air was raised from 350 to 700 μL·L−1, thus increasing the availability of DIC in the medium and enhancing the rate of photosynthesis. Doubled CO2 could enhance CO2 dissolution, lower pH values, and influence the ionization fractions of various DIC species even when the photosynthesis was not DIC limited. Consequently, HCO3− concentrations in cultures were significantly higher than in controls, and the photosynthetic energy cost for the operation of CO2 concentrating mechanism might decrease.

Use of chlorophyll fluorescence as a tool for determination of herbicide toxic effect: Review

Chlorophyll fluorescence-based methods have been used for many years to study the effect of environmental factors (including herbicides) on plants, and, with the development of very sensitive fluorometers, these methods have become very useful in ecotoxicological studies. This review provides a summary of the research done over the last 10 years concerning the study, by using chlorophyll fluorescence, of the herbicide toxic effects. Although numerous parameters are available, it is striking to notice that the majority of the studies done in the recent years used only a limited number of parameters, thus restraining in some way the interpretation of the obtained results. Furthermore, the use of the maximal photosystem II quantum yield to evaluate toxic effect of herbicides is probably not the best parameter due to its relatively low sensitivity. One should take the advantage of using other parameters such as the operational PSII quantum yield and the non-photochemical quenching since they integrate the entire physiological state of the plant and therefore should be more sensitive. We also recommend the systematic determination of EC50 values in future studies since such determination will permit a more accurate comparison between published reports.

PHOTOSYNTHETIC ADAPTATION OF A BLOOM-FORMING CYANOBACTERIUM MICROCYSTIS AERUGINOSA (CYANOPHYCEAE) TO PROLONGED UV-B EXPOSURE1

The bloom‐forming cyanobacterium Microcystis aeruginosa Kütz 854 was cultured with 1.05 W·m−2 UV‐B for 3 h every day, and its growth, pigments, and photosynthesis were investigated. The specific growth rates represented by chl a concentration and OD750 were inhibited 8% and 9% by UV‐B exposure, respectively. Six days of UV‐B treatment significantly reduced cellular contents of phycocyanin and allophycocyanin by 32% and 62%, respectively, and markedly increased the carotenoid content by 27%, but had little effect on the chl a content. The initial values of optimal photosynthetic efficiency for UV‐B treated samples were, respectively, 52%, 87%, and 93% of controls on days 4, 7, and 10 of growth. The light‐saturated photosynthetic rates at day 6 were significantly lower than controls grown without UV‐B. The probability of electron transfer beyond QA decreased during UV‐B exposure, and this indicated that the acceptor side of PSII was one of main damage sites. The adaptation of M. aeruginosa 854 to UV‐B radiation could be observed from light‐saturated photosynthetic rates on day 13 and diurnal changes of chl fluorescence during the late growth phase. When both exposed to higher UV‐B, samples cultured under 1.05 W·m−2 UV‐B for 9 days recovered faster than controls. It is suggested that M. aeruginosa 854 had at least three adaptive strategies to cope with the enhanced UV‐B: increasing the synthesis of carotenoids to counteract reactive oxidants caused by UV‐B exposure, degrading phycocyanin and allophycocyanin to avoid further damage to DNA and reaction centers, and enhancing the repair of UV‐B induced damage to the photosynthetic apparatus.

Differential sensitivity of five cyanobacterial strains to ammonium toxicity and its inhibitory mechanism on the photosynthesis of rice-field cyanobacterium Ge–Xian–Mi (Nostoc)

Effects of two fertilizers, NH(4)Cl and KCl, on the growth of the edible cyanobacterium Ge-Xian-Mi (Nostoc) and four other cyanobacterial strains were compared at pH 8.3+/-0.2 and 25 degrees C. Their growth was decreased by at least 65% at 10 mmol L(-1) NH(4)Cl but no inhibitory effect was observed at the same level of KCl. Meanwhile, the strains exhibited a great variation of sensitivity to NH(4)(+) toxicity in the order: Ge-Xian-Mi>Anabaena azotica FACHB 118>Microcystis aeruginosa FACHB 905>M. aeruginosa FACHB 315>Synechococcus FACHB 805. The 96-h EC(50) value for relative growth rate with regard to NH(4)(+) for Ge-Xian-Mi was 1.105 mmol L(-1), which was much less than the NH(4)(+) concentration in many agricultural soils (2-20 mmol L(-1)). This indicated that the use of ammonium as nitrogen fertilizer was responsible for the reduced resource of Ge-Xian-Mi in the paddy field. After 96 h exposure to 1 mmol L(-1) NH(4)Cl, the photosynthetic rate, F(v)/F(m) value, saturating irradiance for photosynthesis and PSII activity of Ge-Xian-Mi colonies were remarkably decreased. The chlorophyll synthesis of Ge-Xian-Mi was more sensitive to NH(4)(+) toxicity than phycobiliproteins. Thus, the functional absorption cross section of Ge-Xian-Mi PSII was increased markedly at NH(4)Cl levels >or=1 mmol L(-1) and the electron transport on the acceptor side of PSII was significantly accelerated by NH(4)Cl addition >or=3 mmol L(-1). Dark respiration of Ge-Xian-Mi was significantly increased by 246% and 384% at 5 and 10 mmol L(-1) NH(4)Cl, respectively. The rapid fluorescence rise kinetics indicated that the oxygen-evolving complex of PSII was the inhibitory site of NH(4)(+).

New insights into iron acquisition by cyanobacteria: an essential role for ExbB-ExbD complex in inorganic iron uptake

Cyanobacteria are globally important primary producers that have an exceptionally large iron requirement for photosynthesis. In many aquatic ecosystems, the levels of dissolved iron are so low and some of the chemical species so unreactive that growth of cyanobacteria is impaired. Pathways of iron uptake through cyanobacterial membranes are now being elucidated, but the molecular details are still largely unknown. Here we report that the non-siderophore-producing cyanobacterium Synechocystis sp. PCC 6803 contains three exbB-exbD gene clusters that are obligatorily required for growth and are involved in iron acquisition. The three exbB-exbDs are redundant, but single and double mutants have reduced rates of iron uptake compared with wild-type cells, and the triple mutant appeared to be lethal. Short-term measurements in chemically well-defined medium show that iron uptake by Synechocystis depends on inorganic iron (Fe′) concentration and ExbB-ExbD complexes are essentially required for the Fe′ transport process. Although transport of iron bound to a model siderophore, ferrioxamine B, is also reduced in the exbB-exbD mutants, the rate of uptake at similar total [Fe] is about 800-fold slower than Fe′, suggesting that hydroxamate siderophore iron uptake may be less ecologically relevant than free iron. These results provide the first evidence that ExbB-ExbD is involved in inorganic iron uptake and is an essential part of the iron acquisition pathway in cyanobacteria. The involvement of an ExbB-ExbD system for inorganic iron uptake may allow cyanobacteria to more tightly maintain iron homeostasis, particularly in variable environments where iron concentrations range from limiting to sufficient.

Sll1263, a Unique Cation Diffusion Facilitator Protein that Promotes Iron Uptake in the Cyanobacterium Synechocystis sp. Strain PCC 6803

Cyanobacteria are known to survive in iron-deficient environments, but the ways in which they acquire Fe and acclimate are not completely understood. Here we report a novel gene sll1263 that is required for Synechocystis sp. strain PCC 6803 to grow under iron-deficient conditions. sll1263 encodes a putative cation diffusion facilitator protein (CDF) that shows 50% amino acid similarity with ferrous iron efflux protein (FieF) of heterotrophic bacteria. In bacteria, the gene product is involved in metal export from the cell, but in Synechocystis sll1263 plays a role in iron uptake. The results show that expression of sll1263 was induced by iron-deficient conditions and its inactivation significantly decreased the growth rate of an sll1263(-) mutant. Other genes known to be required for Fe acquisition were also strongly up-regulated in the mutant even in the presence of high Fe. Overexpression of sll1263 increased growth under iron deficiency but reduced growth under high-iron stress, suggesting that the gene product was involved in iron uptake rather than detoxification. Expression of FieF in the sll1263(-) mutant was unable to rescue the Fe-deficient phenotype, but Sll1263 completely restored it. Measurements of cellular iron content and the iron uptake rate showed that they were significantly less in the sll1263(-) mutant than in the wild type, consistent with a role for sll1263 in iron uptake. We hypothesize that the low-iron habitats and high-iron requirements of cyanobacteria may be the reason why cyanobacterial CDF protein functions in Fe uptake and not efflux as in non-photosynthetic bacteria.

EFFECTS OF POTASSIUM ON THE PHOTOSYNTHETIC RECOVERY OF THE TERRESTRIAL CYANOBACTERIUM, NOSTOC FLAGELLIFORME (CYANOPHYCEAE) DURING REHYDRATION1

Effects of potassium on the photosynthetic recovery of Nostoc flagelliforme (Berk. & Curtis) Bornet & Flahault were investigated to determine its exact role during rehydration. Potassium enhanced recovery of the ability to reduce the primary quinone‐type acceptor (QA) and plastoquinone (PQ) pool and the area over the fluorescence rise curve was increased by 127%. The proportions of closed PSII reaction centers at phases J and I and the net rate of closure of PSII reaction centers were decreased by, respectively, 19%, 8%, and 23% with the addition of potassium, due to changes in the ability of PSII for multiple turnovers needed to reduce the PQ pool. Potassium significantly enhanced the probability of electron transfer beyond QA and the recovery of electron transport flux per PSII reaction center. Electron transport from water to methyl viologen for samples rehydrated in K+‐free BG11 medium was 54% of those with the addition of potassium. However, electron flow from water to p‐benzoquinone and from reduced 2,6‐dichlorophenol‐indophenol to methyl viologen showed little change with the addition of potassium. The fast phase and slow phase of millisecond delayed light emission and the ATP content for samples rehydrated in K+‐free BG11 medium were, respectively, 71.6%, 50.7%, and 77.1% of those with the addition of potassium. These suggested that potassium affected electron transfer from PQ to plastocyanin through the cytochrome b6f complex and the proton motive force across the thylakoid membranes, probably reflecting its role in charge balance during H+ transport by the cytochrome b6f complex.

Photosynthetic recovery of Nostoc flagelliforme (Cyanophyceae) upon rehydration after 2 years and 8 years dry storage

Liu Y., Yu L., Ke W., Gao X. and Qiu B. 2010. Photosynthetic recovery of Nostoc flagelliforme (Cyanophyceae) upon rehydration after 2 years and 8 years dry storage. Phycologia 49: 429–437. DOI: 10.2216/09-01.1 The photosynthetic recovery of Nostoc flagelliforme (Berkeley & Curtis) Bornet & Flahault upon rehydration was investigated with the 2-year and 8-year dried samples. It was found that the 2-year sample absorbed water much faster compared with the 8-year sample during 3–24-h rehydration. The 2-year sample recovered its photosynthesis and respiration fully upon 24-h rehydration. However, the 8-year sample almost lost its ability to recover dark respiration, net photosynthetic activity, maximal quantum yield and photosystem II (PSII) electron transport activity. The F645/F685 ratio of the 8-year sample increased significantly from 1.70 to 3.09 during 24-h rehydration, which suggested damage to the coupling between phycobiliprotein and the PSII reaction centers. The 8-year sample retained PSI activity during rehydration, although it had almost lost PSII activity. The PSI electron transport activity of the 8-year sample was comparable to that of the 2-year sample. The up-regulation of psbD and petB transcripts in the 2-year sample was the most rapid during rehydration and then in the order: rbcL, rbcX and psaA; psbA2 and psaB; psbA1. For the 8-year sample, the initial transcript abundances of rbcL, rbcX, psbA1, psbA2, psbD, petB, psaB and psaA were, respectively, 56%, 17%, 4%, 10%, 16%, 4%, 36% and 7% of the 2-year sample. The production rate of intracellular reactive oxygen species (ROS) and ion leakage of the 8-year sample were significantly higher than those of the 2-year sample. When the 2-year and 8-year samples were taken together, a good linear correlation could be established between the production of intracellular ROS and ion leakage.

Inactivation of Ca2+/H+ Exchanger in Synechocystis sp Strain PCC 6803 Promotes Cyanobacterial Calcification by Upregulating CO2-Concentrating Mechanisms

ABSTRACT Cyanobacteria are important players in the global carbon cycle, accounting for approximately 25% of global CO2 fixation. Their CO2-concentrating mechanisms (CCMs) are thought to play a key role in cyanobacterial calcification, but the mechanisms are not completely understood. In Synechocystis sp. strain PCC 6803, a single Ca2+/H+ exchanger (Slr1336) controls the Ca2+/H+ exchange reaction. We knocked out the exchanger and investigated the effects on cyanobacterial calcification and CCMs. Inactivation of slr1336 significantly increased the calcification rate and decreased the zeta potential, indicating a relatively stronger Ca2+-binding ability. Some genes encoding CCM-related components showed increased expression levels, including the cmpA gene, which encodes the Ca2+-dependent HCO3 − transporter BCT1. The transcript level of cmpA in the mutant was 30 times that in wild type. A Western blot analysis further confirmed that protein levels of CmpA were higher in the mutant than the wild type. Measurements of inorganic carbon fluxes and O2 evolution proved that both the net HCO3 − uptake rate and the BCT1 transporter supported photosynthetic rate in the slr1336 mutant were significantly higher than in the wild type. This would cause the mutant cells to liberate more OH− ions out of the cell and stimulate CaCO3 precipitation in the microenvironment. We conclude that the mutation of the Ca2+/H+ exchanger in Synechocystis promoted the cyanobacterial calcification process by upregulating CCMs, especially the BCT1 HCO3 − transporter. These results shed new light on the mechanism by which CCM-facilitated photosynthesis promotes cyanobacterial calcification.

The CO2‐concentrating mechanism in the bloom‐forming cyanobacterium Microcystis aeruginosa (Cyanophyceae) and effects of UVB radiation on its operation1

The bloom‐forming cyanobacterium Microcystis aeruginosa (Kütz.) Kütz. 854 was cultured with 1.05 W · m−2 ultraviolet‐B radiation (UVBR) for 3 h every day, and the CO2‐concentrating mechanism (CCM) within this species as well as effects of UVBR on its operation were investigated. Microcystis aeruginosa 854 possessed at least three inorganic carbon transport systems and could utilize external HCO3− and CO2 for its photosynthesis. The maximum photosynthetic rate was approximately the same, but the apparent affinity for dissolved inorganic carbon was significantly decreased from 74.7 μmol · L−1 in the control to 34.7 μmol · L−1 in UVBR‐treated cells. At 150 μmol · L−1 KHCO3 and pH 8.0, Na+‐dependent HCO3− transport contributed 43.4%–40.2% to the photosynthesis in the control and 34.5%–31.9% in UVBR‐treated cells. However, the contribution of Na+‐independent HCO3− transport increased from 8.7% in the control to 18.3% in UVBR‐treated cells. The contribution of CO2‐uptake systems showed little difference: 47.9%–51.0% in the control and 49.8%–47.2% in UVBR‐treated cells. Thus, the rate of total inorganic carbon uptake was only marginally affected, although UVBR had a differential effect on various inorganic carbon transporters. However, the number of carboxysomes in UVBR‐treated cells was significantly decreased compared to that in the control.