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Terrestrial plant roots exude compounds that promote the proliferation of microorganisms, a phenomenon called the rhizosphere effect. However, little is known about the influence of roots of aquatic plants on microbial populations. We compared cultivable bacteria and protozoa from the rhizoplanes ofAzolla filiculoidesLam.,Lemna gibbaL., andRicciocarpus natansL., collected at the Tecocomulco Lagoon (Hidalgo, Mexico). The functional bacterial groups isolated fromA. filiculoides,L. gibbaandR. natans, were macro- and microscopically characterized, and phylogenetically identified using the 16S rDNA gen. About 96% of isolates corresponded to Gram-negative bacteria, and potential N-fixing free-living bacteria (diazotrophic bacteria) were the most abundant. Molecular analysis detected 15 bacterial genera in the rhizoplane ofR. natans, whereasA. filiculoidesandL. gibbaonly yielded five genera, among whichAeromonaswas predominant. Twenty-five genera of flagellated and 20 genera of ciliated protozoa were identified.Bodowas the more abundant flagellated, whereasHalteriawas the most frequent ciliate. All three aquatic plants showed the rhizospheric effects. The most abundant and diverse community of protozoa was found inA. filiculoides, which also had the most abundant bacterial community, but the highest bacterial diversity was found inR. natans. [ABSTRACT FROM PUBLISHER]