Zhongxing Wu

The role of microcystins in maintaining colonies of bloom-forming Microcystis spp.

Microcystis is a cosmopolitan genus of cyanobacteria and occurs in many different forms. Large surface blooms of the cyanobacterium are well known in eutrophic lakes throughout the globe. We evaluated the role of microcystins (MCs) in promoting and maintaining bloom-forming cell aggregates at environmentally relevant MC concentrations (0.25-10 µg l(-1)). MCs significantly enhanced Microcystis colony sizes. Colonial diameters in microcystin-RR (MC-RR)-treated cultures (at 1 µg l(-1)) were significantly larger than control colonies, by factors of 1.5, 2.6 and 2.7 in Microcystis wesenbergii DC-M1, M. ichthyoblabe TH-M1 and Microcystis sp. FACHB1027 respectively. Depletion of extracellular MC concentrations caused Microcystis colony size to decrease, suggesting that released MCs are intimately involved in the maintenance of Microcystis colonial size. MC-RR exposure did not influence Microcystis growth rate, but did significantly increase the production of extracellular polysaccharides (EPS). In addition, MC-RR exposure appeared to trigger upregulation of certain parts of four polysaccharide biosynthesis-related genes: capD, csaB, tagH and epsL. These results strongly indicate that induction of polysaccharides by MC-RR was the major mechanism through which MCs enhanced colony formation in Microcystis spp. Cellular release of MCs, therefore, may play a key role in the persistence of algal colonies and the dominance of Microcystis.

Allelopathic mechanism of pyrogallol to Microcystis aeruginosa PCC7806 (Cyanobacteria): From views of gene expression and antioxidant system

Pyrogallol is a potent allelochemical on Microcystis aeruginosa, but its allelopathic mechanism is not fully known. In order to explore this mechanism, gene expressions for prx, mcyB, psbA, recA, grpE, fabZ under pyrogallol stress were studied, and activities of the main antioxidant enzymes were also measured. The results showed that expression of grpE and recA showed no significant change under pyrogallol stress, while psbA and mcyB were up-regulated at 4 mg L(-1). Both prx and fabZ were up-regulated even under exposure to 1 mg L(-1) pyrogallol concentration. The activities of superoxide dismutase (SOD) and catalase (CAT) were enhanced under pyrogallol stress. Levels of malodialdehyde (MDA) at 2 and 4 mg L(-1) pyrogallol were significantly higher than those of the controls. It was concluded that oxidant damage is an important mechanism for the allelopathic effect of pyrogallol on M. aeruginosa.

Physiological Comparison between Colonial and Unicellular forms of Microcystis Aeruginosa Kütz. (Cyanobacteria)

Z-X. Wu and L-R. Song. 2007. Physiological comparison between colonial and unicellular forms of Microcystis aeruginosa Kütz. (Cyanobacteria). Phycologia 47: 98–104. DOI: 10.2216/07–36.1 In order to gain insight into the bloom sustainment of colonial Microcystis aeruginosa Kütz., physiological characterizations were undertaken in this study. Compared with unicellular Microcystis, colonial Microcystis phenotypes exhibited a higher maximum photosynthetic rate (Pm), a higher maximum electron transfer rate (ETRmax), higher phycocyanin content, and a higher affinity for inorganic carbon (K0.5 DIC ≤ 8.4 ± 0.7 µM) during the growth period monitored in this study. This suggests that photosynthetic efficiency is a dominant physiological adaptation found in colonial Microcystis, thus promoting bloom sustainment. In addition, the high content of soluble and total carbohydrates in colonial Microcystis suggests that this phenotype may possess a higher ability to tolerate enhanced stress conditions when compared to unicellular (noncolonial) phenotypes. Therefore, high photosynthetic activities and high tolerance abilities may explain the bloom sustainment of colonial Microcystis in eutrophic lakes.

Non-microcystin producing Microcystis wesenbergii (Komárek) Komárek (Cyanobacteria) representing a main waterbloom-forming species in Chinese waters

It is well known that several morphospecies of Microcystis, such as Microcystis aeruginosa (Kützing) Lemmermann and Microcystis viridis (A. Brown) Lemmermann can produce hepatotoxic microcystins. However, previous studies gave contradictory conclusions about microcystin production of Microcystis wesenbergii (Komárek) Komárek. In the present study, ten Microcystis morphospecies were identified in waterblooms of seven Chinese waterbodies, and Microcystis wesenbergii was shown as the dominant species in these waters. More than 250 single colonies of M. wesenbergii were chosen, under morphological identification, to examine whether M. wesenbergii produce hepatotoxic microcystin by using multiplex PCR for molecular detection of a region (mcyA) of microcystin synthesis genes, and chemical analyses of microcystin content by ELISA and HPLC for 21 isolated strains of M. wesenbergii from these waters were also performed. Both molecular and chemical methods demonstrated that M. wesenbergii from Chinese waters did not produce microcystin.

Phenotype and temperature affect the affinity for dissolved inorganic carbon in a cyanobacterium Microcystis

The cyanobacterium Microcystis is the most common bloom-forming species in eutrophicated water bodies. Known eco-physiological advantages of this organism help it to compete effectively with other algae and cyanobacteria; however, little is known about the physiological characteristics competence of colonial Microcystis. In the present study, carbonic anhydrase (CA) activity, the affinity for dissolved inorganic carbon (DIC), and the transcription of CA genes were examined in unicellular and colonial Microcystis strains. In comparison with unicellular strains, colonial Microcystis exhibited dramatically higher inorganic carbon affinity at 25 and 35°C, but no significant differences were observed at 15°C. The relative transcription levels of the CA genes icfA1, icfA2, ecaA, and ecaB in all colonial Microcystis were significantly higher than those in unicellular Microcystis at 25 and 35°C. In addition, CA activities of Microcystis increased with temperature, but no significant difference was observed between the unicellular and colonial Microcystis. These results suggest that temperature and phenotypes probably play important roles in the utilization of DIC and trigger the expression of CA genes of Microcystis. The present results may indicate that the capacity for utilizing inorganic carbon plays a role in the persistence or/and succession of different Microcystis blooms.

Identification and characterization of the chromium(VI) responding protein from a newly isolated Ochrobactrum anthropi CTS-325

A Gram-negative, chromium(VI) tolerant and reductive strain CTS-325, isolated from a Chinese chromate plant, was identified as Ochrobactrum anthropi based on its biochemical properties and 16S rDNA sequence analysis. It was able to tolerate up to 10 mmol/L Cr(VI) and completely reduce 1 mmol/L Cr(VI) to Cr(III) within 48 h. When the strain CTS-325 was induced with Cr(VI), a protein increased significantly in the whole cell proteins. Liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis revealed that this protein was a superoxide dismutase (SOD) homology. The measured superoxide dismutase activity was 2694 U/mg after three steps of purification. The SOD catalyzes the dismutation of the superoxide anion (O2*-) into hydrogen peroxide and molecular oxygen. This protein is considered to be one of the most important anti-oxidative enzymes for O. anthropi as it allows the bacterium to survive high oxygen stress environments, such as the environment produced during the reduction process of Cr(VI).

Combined effects of carbon and phosphorus levels on the invasive cyanobacterium, Cylindrospermopsis raciborskii

Wu Z., Zeng B., Li R. and Song L. 2012. Combined effects of carbon and phosphorus levels on the invasive cyanobacterium, Cylindrospermopsis raciborskii. Phycologia 51: 144–150. DOI: 10.2216.10.87.1 The invasive cyanobacterium, Cylindrospermopsis raciborskii, was cultured semicontinuously for 18 days at different carbon and phosphorus levels. Growth and photosynthesis were measured to study the combined effects of CO2 and phosphorus availabilities. The results showed that enrichment of CO2 or inorganic phosphorus (Pi) levels in the culture medium significantly increased specific growth rate (μ), light-saturated photosynthesis (Pm) and photosynthetic efficiency (α), but tended to decrease dark respiratory rate (Rd), saturating irradiance for photosynthesis (Ek) and the light compensation point (Ec). However, photosynthetic responses to inorganic carbon concentration in C. raciborskii were affected by interactions between CO2 and Pi. At high Pi concentration, elevated CO2 significantly increased the inorganic carbon-saturated photosynthesis rate (Vmax) and substrate concentration giving half-maximal photosynthetic rate [K0.5 (DIC)], whereas at low Pi levels, elevated CO2 slightly decreased the values of Vmax and K0.5 (DIC). Additionally, enrichment with Pi significantly increased K0.5 (DIC) and Vmax regardless of the CO2 level, suggesting that Pi plays a role in photosynthetic carbon fixation in C. raciborskii. Furthermore, when C. raciborskii was batch cultured in different carbon and phosphorus concentrations for 72 h, the uptake rate of nitrogen increased with enrichment of both CO2 and Pi, while the uptake rate of phosphorus was enhanced only by increases in Pi concentration. The results suggest that the availability of CO2 and phosphorus have combined and independent effects on photosynthesis and nutrient uptake in C. raciborskii, reflecting that changes in environmental factors could influence the dominance of this invasive cyanobacterium. These combined effects need to be considered when generalizing this species response to changing CO2 levels.

First report of microcystin production in Microcystis smithii Komárek and Anagnostidis (Cyanobacteria) from a water bloom in Eastern China

A water bloom sample collected from Lake Dishui in Shanghai was characterized. The morphological identification showed that Micorcystis wesenbergii and Micorcystis smithii were the main component of the bloom. Five strains of M. smithii were successfully isolated. Their 16S rRNA gene sequences based phylogenetic tree showed that the five strains of M. smithii intermixed with strains of other morphospecies in Microcystis. A fragment of mcy gene encoding for microcystin synthetase was detected in one of the five M. smithii strains (CHAB 2183), indicating its potential of microcystin production. High performance liquid chromatography analysis confirmed M. smithii CHAB 2183 to produce microcystin-RR as 1550 microg per gram dry weight cells. The present investigation, for the first time, reported the isolated strains of M. smithii and microcystin production from M. smithii.

Responses of Synechocystis sp. PCC 6803 (cyanobacterium) photosystem II to pyrene stress

In order to explore the mechanism of acute toxicity for pyrene to cyanobacterial organisms, the responses of Synechocystis sp. PCC 6803 photosystem II (PS II) under pyrene stress were studied. The results showed there was no significant difference about the oxygen evolution under 0.125 mg/L pyrene stress when compared with control, but it was significantly lower than control at 0.625 mg/L pyrene. Polyphasic chlorophyll-a fluorescence transients in cells of Synechocystis sp. PCC 6803 exhibited a typical increase including O, J, I, and P phases. Fluorescence yield at phases J, I and P declined slightly at 0.125 and 0.625 mg/L pyrene, and significantly lower than control at 3.125 mg/L. According to the parameters deviated from JIP-test, no modification was induced by pyrene both at the donor side and at the acceptor side of PS II, and the reaction centre of PS II is the primary damaging target. Based on the expressing of four key genes (psbA, psbB, psbC and psbO) of PS II, only psbA showed significant difference at 3.125 mg/L pyrene when compared with control.

Use of chlorophyll a fluorescence to elucidate the toxicity target of N-phenyl-2-naphthylamine on photosynthetic system of Cylindrospermopsis raciborskii (Cyanobacteria)

Abstract: In order to investigate the toxicity targets of the secondary metabolite from aquatic plants, N-phenyl-2-naphthylamine, on Cylindrospermopsis raciborskii photosynthetic system II (PSII), we determined chlorophyll a (Chl a), specific growth rate (μ−d), photosynthetic rapid light curves and polyphasic Chl a fluorescence transients following exposure to N-phenyl-2-naphthylamine. The results showed significant decreases in Chl a following 48 h at high concentration (0.50 mg l−1) exposure to N-phenyl-2-naphthylamine. The PSII electron transfer rate and Chl a fluorescence transients appeared to be affected after the cells were exposed to concentrations of 0.50, 1.00 and 2.00 mg l−1. Exposure to higher concentration of N-phenyl-2-naphthylamine (> 0.50 mg l−1) mainly depressed the density of reaction centers (RC/CS0) and quantum energy flux ratios (ϕP0 and ϕE0) but increased the absorption flux per RC (ABS/RC), dissipated energy flux per RC (DI0/RC) and the average redox state of QA−/QA in the time span from 0 to tFmax (Sm/tFmax). However, exposure to low concentration (≤ 0.5 mg l−1) mainly inhibited the trapping energy fluxes per reaction center (TR0/RC), electron transport energy fluxes per reaction center (ET0/RC) and energy flux ratio (ψ0). This suggested that the donor and the acceptor sides of PSII were targets of N-phenyl-2-naphthylamine in C. raciborskii. Additionally, our results indicated that the toxicity target at low concentrations of N-phenyl-2-naphthylamine was at each step of electron transport between QA− and the Cyt b6/f complex; whereas, the reduction of QA was the potential target at high concentrations. We suggest that N-phenyl-2-naphthylamine may be a potential allelochemical to control harmful cyanobacterial blooms.