Cyanobacteria and bacteria co-occurrence in a wastewater treatment plant: absence of allelopathic effects.

Fifty-one cyanobacteria strains, belonging to Phormidium autumnale, Planktothrix mougeotii, Limnotrix sp. and Synechocystis sp. species, were isolated during a twelve months period, from a Wastewater Treatment Plant (WWTP) located in the north of Portugal. Isolated strains and environmental samples were analyzed by Polymerase Chain Reaction (PCR) technique to evaluate the presence of the genes linked to the biosynthesis of the cyanotoxins, microcystin (MC), nodularin (NOD) and cylindrospermopsin (CYN). Even though none of the isolated strains contained the genes responsible for these toxins production, the use of PCR in the water samples detected the mcyE gene, in the biological treatment tank, and the result of its sequence revealed 92% of maximum identity with the microcystin gene. ELISA assay confirmed the presence of the toxin with values of 0.60 μg MC-LR eq/l. The effects of extracts of the cyanobacteria strains on bacteria isolated from the WWTP, revealed that none of the tested strains inhibited or reduced the bacterial growth. This may indicate that those prokaryote communities may have co-evolved in a way that success of both cyanobacteria and bacteria was achieved.