Toxic cyanobacteria in reservoirs in northeastern Brazil : detection using a molecular method

Cyanobacterial blooms are a frequent occurrence in northeastern Brazil and constitute a serious public health problem. Using the polymerase chain reaction (PCR) method, eleven environmental samples with cyanobacteria from seven reservoirs were used to determine the presence of the gene involved in microcystin biosynthesis (mcyB). Two sets of oligonucleotide primers were designed from the sequencing of Brazilian populations of microcystin producing cyanobacteria (mcyB-F/R and mcyB-F/R-A). The presence of the mcyB gene involved in microcystin biosynthesis was found in all samples, indicating the potential of this gene for producing the toxin. The PCR method proved sensitive and appropriate for the detection of potential producers of microcystins in environmental samples. Its ability to reveal potentially toxic cyanobacteria demonstrates that it can be a valuable tool in the monitoring of blooms.


Introduction
Cyanobacterial blooms in public water supply reservoirs have become a matter of ever increasing concern throughout the world due to their potential for producing toxins.Water scarce semiarid northeast reservoirs, coupled with cultural eutrophication, have given rise to a recurring bloom formation of toxin-producing cyanobacterial species (Bouvy et al., 2000;Chellappa et al., 2000;2008a, b;Costa et al., 2006;Panosso et al., 2007).The infrequent rainfall pattern, its heterogeneous distribution, punctuated by prolonged and intense dry periods, have all caused drought polygon in water storage in reservoirs of this region (Almeida et al., 2009).The occurrence of cyanobacterial blooms is therefore due systems and the high cost of the current technology used for their removal.
The aim of the present study was to evaluate the existence of microcystin-producing genotypes using the molecular marker for the mcyB gene in public water supply reservoirs.

Material and Methods
Eleven environmental samples were collected with a 25 μm mesh plankton net from seven public water supply reservoirs in northeastern Brazil (Figure 1, Table 1).The cyanobacterial bloom-forming was determined from morphological characteristics used for the identification of taxa.
DNA was extracted from fresh cells of the environmental samples following the procedure described by Rogers and Bendich (1985).Polymerase chain reaction (PCR) was performed in a GeneAmp PCR System 9700 thermocycler (Applied Biosystems, Foster City, CA, USA) using pure Taq Ready-To-Go PCR Beads kits (Amersham, Piscataway, NJ, USA) with 20 μM of each oligonucleotide (mcyB-F/R and mcyB-F/R-A).For PCR amplification, the following cycling parameters were used: 94 ºC for 2 minutes, 35 cycles at 94 ºC for 10 seconds; 50 ºC for 20 seconds, and 72 ºC for 1 minute, followed by a final extension at 72 ºC for 5 minute.The oligonucleotide primers were designed using sequences from Brazilian populations of microcystinproducing Microcystis.All primers were synthesised by IDT (Medley, FL, USA).
As a positive control, amplification reactions with oligonucleotide primers were used for the phycocyanin operon (PC) (Neilan et al., 1995) following the procedure described by Baker et al. (2001).Amplification products were visualised by electrophoresis on 0.7% agarose gels stained with ethidium bromide (0.2 μg.mL -1 ) in 1X TBE running buffer and recorded using the Electrophoresis Documentation and Analysis System 290 (EDAS 290) and maximum acceptable level of microcystin of 1 μg.L -1 in drinking water (Regulation MS N.518/2004).
Microcystin production depends on the existence of genes that codify microcystin synthetase.Microcystin is a hepatotoxin produced by a non-ribosomal pathway through a polyketide synthase/non-ribosomal peptide synthetase (PKS/NRPS) complex known as microcystin synthetase, which is responsible for the incorporation of amino acids in the peptide chain (Kleinkauf and von Döhren, 1996;Dittmann et al., 1997).The mcy gene cluster is responsible for toxin production, coding for microcystin synthetase (Dittmann et al., 1997;Tillett et al., 2000).Some cyanobacteria are unable to produce toxins, but all those that have genes associated to microcystin biosynthesis should be viewed as potential toxin producers (Ouellette et al., 2006).Kurmayer et al. (2002Kurmayer et al. ( , 2004) ) found that a few strains of cyanobacteria in nature have no microcystin production, but contain all the genes for microcystin synthetase.The authors speculate that these strains had lost the ability to synthesise microcystin due to gene inactivation and that another small peptide could functionally substitute this toxin.
A large number of authors have used microcystin synthetase genes (mcy) as molecular markers for microcystinproducing cyanobacteria (Baker et al., 2002;Nonneman and Zimba, 2002;Bittencourt-Oliveira, 2003;Hisbergues et al., 2003), for, in other studies, the presence of microcystin has been correlated, almost without exception, to the presence of mcy genes (Kurmayer and Kutzenberger, 2003;Via-Ordorika et al., 2004;Dittmann and Börner, 2005).There are no morphological differences between toxic and non-toxic cyanobacteria.Thus, the early detection of toxic cyanobacteria in water resources is advisable in order to avoid contamination and public health problems.Predicting toxic blooms is quite important in view of both their increasing occurrence in large water supply

Discussion
Microcystis aeruginosa, M. novacekii, M. panniformis, Planktothrix agardhii and species from the genus Anabaena are frequent in bodies of water in Brazil.However, not all these organisms or populations necessarily have a potential for microcystin production.The presence of microcystin synthetase genes does not necessarily indicate the production of microcystins, but it does indicate the potential for their production.Recent studies have demonstrated that some strains have no capacity for toxin production because they do not have the entire mcy gene cluster (Christiansen et al., 2008).
The evaluation of the genetic potential for microcystin production can be determined by investigating the presence of mcy genes, for microscopic analysis does not allow the distinction of microcystin-producing from non-microcystinproducing organisms.Studies using ELISA (Bittencourt-Oliveira, 2003;Hisbergues et al., 2003;Vaitomaa et al., 2003), MALDI-TOF MS (Kurmayer et al., 2002;Saker et al., 2005), HPLC (Nonneman and Zimba, 2002;Ouahid et al., (Kodak, Melville, NY, USA).In all reactions sets, a control treatment (without DNA) was included (data not shown).All tests were performed in duplicate.The reproducibility band was tested with DNA from different extractions using the same methodology and conditions.Fragments obtained from the primers mcyB-F/R and mcyB-F/R-A resulted in sizes with approximately 570 and 315 bp, respectively.
The mcyB gene involved in microcystin biosynthesis was found in all samples.The oligonucleotide primer mcyB-F/R-A revealed a higher number of unexpected bands (Table 2 and Figure 2).2005) and bioassays with mice (Baker et al., 2002;Pan et al., 2002) have confirmed the production of microcystin in cyanobacteria with mcy genes.However, recent studies report the presence of amplified fragments in samples in which microcystins were not detected through highperformance liquid chromatography (HPLC) (Saker et al., 2007;Ouahid et al., 2005;Mankiewicz-Boczek et al., 2006).There are a number of interpretations for this, such as microcystin-producing genotypes that are not expressed (Kurmayer and Kutzenberger, 2003;Bittencourt-Oliveira, 2003), or the toxin is present but at concentrations below the HPLC detection limit.
The occurrence of these inactive microcystin genotypes is rare and poorly understood, but this should not be a significant limitation to the applicability of molecular detection.In previous studies, the presence of microcystin has been correlated, almost without exception, to the presence of mcy genes (Kurmayer and Kutzenberger, 2003;Via-Ordorika et al., 2004;Dittmann and Börner, 2005).
The presence of non-specific bands in the samples using mcyB-F/R-A did not impede the correct reading of the results obtained, since the size of the expected marker was previously known.Among the oligonucleotide primers tested, mcyB-F/R proved to be more specific than mcyB-F/R-A, as it did not exhibit non-specific bands.
Molecular analysis confirms the need for the monitoring of water quality in public water supply reservoirs in Brazil due to the occurrence of genotypes indicating toxicity.The PCR method was sensitive and appropriate for the detection of potential microcystin-producing cyanobacteria in environmental samples.The method could be used either alone or in conjunction with other techniques, such as the screening of samples to be sent for quantitative toxicological analysis using HPLC, thereby reducing monitoring cost and time.The usefulness of this molecular method in the investigation of potentially toxic cyanobacteria in reservoirs suggests that PCR could be a valuable tool in the monitoring of blooms.

Table 1 .
Sampling locations and dates used.