Objective: To study Ulva linza (U. linza) as macro-alga exposed to different levels of ionizing radiation from the natural background of radioactivity. Methods: Samples of U. linza were collected from two different habitats at the two terminals of the rocky shore of Abu Qir Bay, Alexandria, Egypt. The western terminal at Abu Qir represented an area of normal background radiation while the eastern terminal at Rosetta represented an area of relatively high background radiation. Distinguishing

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... een the algae grown in areas of relatively high and normal background radiation was investigated by using different indicators. Moreover, the ambient water quality was measured and the concentrations of natural radionuclides ( 238 U, 232 Th and 40 K) bio-accumulated in the tested macro-alga were detected. Results: The algae naturally exposed to radionuclides ( 238 U series, 232 Th series and 40 K) were investigated. Radiation dose rates in U. linza inhabiting in Rosetta and Abu Qir were calculated as 70.12 and 42.67 nGy/h, respectively. Chemical analysis of algal samples demonstrated that total pigment contents were 2.21 and 2.19 mg/g on a fresh weight basis for U. linza inhabiting in Rosetta and Abu Qir, respectively. Fatty acid compositions showed comparable profiles for both algal samples with saturated fatty acids as major component. The results of protein profiles confirmed slight differential expression in protein bands. Sequence-related randomly amplified polymorphic DNA provided evidence that both samples were strongly similar. By using transmission electron microscopy, no obvious ultra structural changes in the examined cells were observed. Conclusions: These experimental results demonstrate that radiation doses are not high enough to cause damage or manifest any significant variation in Ulva tissues.