Changes in Growth, Photosynthesis Performance, Pigments, and Toxin Contents of Bloom-Forming Cyanobacteria after Exposure to Macroalgal Allelochemicals.

Macroalgae can directly restrict the growth of various phytoplankton species by releasing allelopathic compounds; therefore, considerable attention should be paid to the allelopathic potential of these organisms against harmful and bloom-forming cyanobacteria. The main aim of this study was to demonstrate for the first time the allelopathic activity of Ulva intestinalis on the growth, the fluorescence parameters: the maximum PSII quantum efficiency (Fv/Fm) and the effective quantum yield of PSII photochemistry (ΦPSII), the chlorophyll a (Chl a) and carotenoid (Car) content, and the microcystin-LR (MC-LR) and phenol content of three bloom-forming cyanobacteria, Aphanizomenon sp., Nodularia spumigena, and Nostoc sp. We found both negative and positive allelopathic effects of U. intestinalis on tested cyanobacteria. The study clearly showed that the addition of the filtrate of U. intestinalis significantly inhibited growth, decreased pigment content and Fv/Fm and ΦPSII values of N. spumigena and Nostoc sp., and stimulated Aphanizomenon sp. The addition of different concentrations of aqueous extract also stimulated the cyanobacterial growth. It was also shown that the addition of extract obtained from U. intestinalis caused a significant decrease in the MC-LR content in Nostoc sp. cells. Moreover, it the phenol content in N. spumigena cells was increased. On the other hand, the cell-specific phenol content for Aphanizomenon sp. decreased due to the addition of the filtrate. In this work, we demonstrated that the allelopathic effect of U. intestinalis depends on the target species' identity as well as the type of allelopathic method used. The study of the allelopathic Baltic macroalgae may help to identify their possible role as a significant biological factor influencing harmful cyanobacterial blooms in brackish ecosystems.

Influence of Glycine and Arginine on Cylindrospermopsin Production and aoa Gene Expression in Aphanizomenon ovalisporum.

Arginine (Arg) and glycine (Gly) seem to be the only substrates accepted by the amidinotransferase that catalyze the first step of the synthesis pathway of the cyanotoxin cylindrospermopsin (CYN), leading to guanidinoacetate (GAA). Here, the effect of these amino acids on the production of CYN in cultures of the cylindrospermopsin-producing strain, Aphanizomenon ovalisporum UAM-MAO, has been studied. Arg clearly increased CYN content, the increment appearing triphasic along the culture. On the contrary, Gly caused a decrease of CYN, observable from the first day on. Interestingly, the transcript of the gene ntcA, key in nitrogen metabolism control, was also enhanced in the presence of Arg and/or Gly, the trend of the transcript oscillations being like that of aoa/cyr. The inhibitory effect of Gly in CYN production seems not to result from diminishing the activity of genes considered involved in CYN synthesis, since Gly, as Arg, enhance the transcription of genes aoaA-C and cyrJ. On the other hand, culture growth is affected by Arg and Gly in a similar way to CYN production, with Arg stimulating and Gly impairing it. Taken together, our data show that the influence of both Arg and Gly on CYN changes seems not to be due to a specific effect on the first step of CYN synthesis; it rather appears to be the result of changes in the physiological cell status.

Physiological Responses of Aphanizomenon flos-aquae Under the Stress of Sagittaria sagittifolia Extract.

The algal growth and physiological characters of Aphanizomenon flos-aquae were studied under the stress of Sagittaria sagittifolia extract. The results showed that the growth of A. flos-aquae was significantly inhibited by S. sagittifolia extract. The exopolysaccharide (EPS), total soluble protein, intracellular phosphorus (o-PO4-P) contents and malondialdehyde (MDA) contents in A. flos-aquae cells increased significantly. These results suggested that A. flos-aquae can adapt to stress by increasing its normal metabolic activity. The algal cellular antioxidant enzymes, superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD), were triggered to different degrees when exposed to S. sagittifolia extract. The MDA contents and activities of SOD, CAT and POD in algal cells suggested that oxidative damage induced by S. sagittifolia extract via the oxidation of ROS (O2·-) might be an important factor responsible for the inhibition of the growth of A. flos-aquae. In addition, SOD may be an important site for the inhibition of S. sagittifolia extract on A. flos-aquae cells. These results indicate that S. sagittifolia may be a good candidate for controlling A. flos-aquae blooms.

The response and detoxification strategies of three freshwater phytoplankton species, Aphanizomenon flos-aquae, Pediastrum simplex, and Synedra acus, to cadmium.

The response and detoxification mechanisms of three freshwater phytoplankton species (the cyanobacterium Aphanizomenon flos-aquae, the green alga Pediastrum simplex, and the diatom Synedra acus) to cadmium (Cd) were investigated. The cell growth of each species was measured over 10 days, and chlorophyll a fluorescence, Cd bioaccumulation (including surface-adsorbed and intracellular Cd), and phytochelatin (PC) synthesis were determined after 96-h exposures. The growth of the three phytoplankton species was significantly inhibited when Cd concentrations were ≥5 mg L(-1). Compared with P. simplex, greater growth inhibition in S. acus and A. flos-aquae occurred. The changes in chlorophyll fluorescence parameters including the maximal quantum yield of PSII (Fv/Fm) and relative variable fluorescence of the J point (Vj) demonstrated that the increase in Cd concentration damaged PSII in all three species. After 96-h exposures, the accumulation of surface-adsorbed Cd and intracellular Cd increased significantly in all three species, with the increase of Cd concentrations in the media; total cadmium accumulation was 245, 658, and 1670 times greater than that of the control in A. flos-aquae, P. simplex, and S. acus, respectively, after exposure to 10 mg L(-1). Total thiols exhibited a similar trend to that of Cd accumulation. PC3 was found in A. flos-aquae and P. simplex in all Cd treatments. Glutathione (GSH) and PC2 were also produced in response to exposure to high concentrations of Cd. PC4 was only discovered at exposure concentrations of 10 mg L(-1) Cd and only in S. acus. The intracellular Cd/PCs ratio increased in all three phytoplankton with an increase in Cd concentrations, and a linear relationship between the ratio and the growth inhibition rates was observed with P. simplex and S. acus. Our results have demonstrated that metal detoxification mechanisms were dependent on the species. This study suggested that the variance of metal detoxification strategies, such as cadmium accumulation and PCs, might be an explanation why algal species have different sensitivity to Cd at various levels.

Assessment of growth rate, chlorophyll a fluorescence, lipid peroxidation and antioxidant enzyme activity in Aphanizomenon flos-aquae, Pediastrum simplex and Synedra acus exposed to cadmium.

In this study, the effects of cadmium on the cyanobacterium Aphanizomenon flos-aquae, the green alga Pediastrum simplex and the diatom Synedra acus was evaluated on the basis of growth rate, chlorophyll a fluorescence, lipid peroxidation and antioxidant enzyme activity. The EC50 values (effective concentration inducing 50 % of growth inhibition) of cadmium in A. flos-aquae, P. simplex and S. acus were 1.18 ± 0.044, 4.32 ± 0.068 and 3.7 ± 0.055 mg/L, respectively. The results suggested that cadmium stress decreases growth rate and chlorophyll a concentration. The normalized chlorophyll a fluorescence transients significantly increased at cadmium concentrations of 5.0, 10.0 and 20.0 mg/L, but slightly decreased at concentrations of 0.2, 0.5 and 1.0 mg/L. The chlorophyll fluorescence parameters showed considerable variation among the three species, while lipid peroxidation and antioxidant enzyme activities showed a significant increase. Our results demonstrated that blockage of electron transport on the acceptor side of photosystem II is the mechanism responsible for cadmium toxicity in freshwater microalgae, and that the tolerance of the three species to cadmium was in the order green alga P. simplex > diatom S. acus > cyanobacterium A. flos-aquae.

Phosphorus-limited growth dynamics in two Baltic Sea cyanobacteria, Nodularia sp. and Aphanizomenon sp.

Rates of carbon (C) specific growth and nitrogen (N2) fixation were monitored in cultures of Baltic Sea Nodularia and Aphanizomenon exposed to gradual limitation by inorganic phosphorus (P). Both cyanobacteria responded by decreased cellular P content followed by lowered rates of growth and N2 fixation. C-specific growth and cellular N content changed faster in Aphanizomenon both when inorganic P was lowered as well as during reintroduction of P. Aphanizomenon also showed a more rapid increase in N-specific N2 fixation associated with increased C-specific growth. When ambient concentrations of inorganic P declined, both cyanobacteria displayed higher rates of alkaline phosphatase (APase) activity. Lower substrate half-saturation constants (KM) and higher Vmax : KM ratio of the APase enzyme associated with Nodularia suggest a higher affinity for dissolved organic P (DOP) substrate than Aphanizomenon. Aphanizomenon, which appears more sensitive to changes in ambient dissolved inorganic P, may be adapted to environments with elevated concentrations of P or repeated intrusions of nutrient-rich water. Nodularia on the other hand, with its higher tolerance to increased P starvation may have an ecological advantage in stratified surface waters of the Baltic Sea during periods of low P availability.