Eleven seaweeds were collected from seven different sites—one from the backwaters of Muttukadu, Chennai and six regions along the coast, Tamil Nadu, India and tested for antibacterial activity against the plant pathogenic bacterium, Xanthomonas oryzae pv. oryzae. Unsaponified fractions of red and green seaweeds exhibited maximum antibacterial activity followed by petroleum ether extracts, lipophilic fractions, diethyl ether extracts, saponified fractions, chloroform extracts and methanol extracts. However, methanol extracts of brown seaweeds showed the highest antibacterial activity followed by lipophilic fractions, unsaponified fractions, ethanol extracts, saponified fractions, chloroform:methanol (2:1 v/v) extracts and chloroform extracts. The antibacterial potential of the seaweeds was in the following order: Enteromorpha flexuosa > Sargassum wightii > Turbinaria conoides > Padina boergesenii > Gracilaria edulis > G. blodgettii > Hypnea valentiae ≥H. musciformis > Spyridia insignis > Chnoospora minima > Ulva lactuca. Seaweeds collected from the backwaters of Muttukadu possessed higher antibacterial potential than the same species collected from coastal waters. Significant seasonal differences in antibacterial activity of seaweeds were found in specimens collected at seven different localities.
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