In waste stabilisation pond systems the interactions between algae and bacteria have an important ecological effect and appeared to play a key role in the self-purification process. The aim of the present study is to evaluate the interactions between two axenic cyanobacteria: Synechocystis sp. and Pseudanabaena sp. with heterotrophic bacteria and some pathogenic bacteria: E. coli, Salmonella sp. and non-O1 V. cholerae. The results obtained showed that Synechocystis sp. (planktonic cyanobacterium) and Pseudanabaena sp. (benthic cyanobacterium) stimulated the growth and the survival of heterotrophic bacteria and non-O1 V. cholerae and reduced the survival of E. coli and Salmonella. Blooms of these cyanobacteria during hot periods which are a particularity in stabilisation ponds of Marrakech, could explain the dynamics of bacteria studied in this sewage treatment process. The presence of heterotrophic bacteria, showing relative high densities during hot periods, could be considered as an important bioticfactor which led to the cyanobacteria blooms in the Marrakech ponds which function under an arid Mediterranean climate.

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