Toxicity effect of the Iraqi crude oil on the growth of two Cyanobacteria strains Microcystis flos-aquae (Wittr.) Kircher (unicellular) and Nostoc carneum Agardh (filamentous) was studied basing on it is biomass expressed as chlorophyll-a (μg/ml). Growth rate and doubling time of the cells were also calculated accordingly in present or absent of the crude oil. Also, Microscopical examination was observed to detect the morphological changes. Both species appeared different responses towards oil toxic influences with different concentrations and time of exposure. Growth was decreased gradually with Microcystis with addition of 10, 30, 50 and 70 mg/l of the crude oil, slightly toxic effects were observed with Nostoc in comparison to the control. Different significant correlations (P ≤ 0.05) were observed between growth of the cyanobacteria and the oil concentrations. Furthermore, results demonstrated that Microcystis was considered to be more sensitive than Nostoc towards crude oil concentrations.
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