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.

Characterization of Aphanizomenon ovalisporum amidinotransferase involved in cylindrospermopsin synthesis.

An increasing abundance of Aphanizomenon ovalisporum in water bodies from diverse world regions has been reported in the last few years, with the majority of the isolated strains producing the toxin cylindrospermopsin (CYN), leading to a rise in ecological and health risks. The understanding of CYN synthesis is crucial in the control of CYN production. An amidinotransferase (AMDT) seems to be the first enzyme involved in the synthesis of CYN. In this study, we have cloned and overexpressed the aoaA gene from the constitutive CYN producer A. ovalisporum UAM-MAO. The recombinant purified AoaA was characterized, confirming that it is an l-arginine:glycine AMDT. It shows an optimal activity between 32 and 37°C, at pH from 8 to 9. The activity exhibits a mixed (ping-pong/sequential) kinetic mechanism, and is inhibited by the reaction product guanidine acetate (GAA) in a noncompetitive manner. Mg(2+) stimulates AoaA activity while Co(2+) and Mn(2+) inhibit it. AoaA conserves the critical residues of the catalytic site and substrate specificity of AMDTs, as the previously reported AMDT from Cylindrospermopsis raciborskii Cyr. Both proteins can be included in a new group of prokaryotic AMDTs involved in CYN production.

Diversity of and selection acting on cylindrospermopsin cyrB gene adenylation domain sequences in Florida.

Aphanizomenon ovalisporum is the only confirmed cylindrospermopsin producer identified in the United States to date. On the other hand, Cylindrospermopsis raciborskii is a prominent feature of many lakes in Florida and other regions of the United States. To see the variation in cylindrospermopsin cyrB gene adenylation domain sequences and possibly discover new cylindrospermopsin producers, we collected water samples for a 3-year period from 17 different systems in Florida. Positive amplicons were cloned and sequenced, revealing that approximately 92% of sequences were A. ovalisporum-like (>99% identity). Interestingly, 6% of sequences were very similar (>99% identity) to cyrB sequences of C. raciborskii from Australia and of Aphanizomenon sp. from Germany. Neutrality tests suggest that A. ovalisporum-like cyrB adenylation domain sequences are under purifying selection, with abundant low-frequency polymorphisms within the population. On the other hand, when compared between species by codon-based methods, amino acids of CyrB also seem to be under purifying selection, in accordance with the one proposed amino acid thought to be activated by the CyrB adenylation domain.