Denitrification in the rhizosphere of the two seagrasses Thalassia hemprichii (Ehrenb.) Aschers and Halodule uninervis (Forsk.) Aschers

Abstract

This work examined the denitrification in the rhizosphere (rhizome–root complex with attached sediment) and rhizoplane (rhizome–root complex) of the two seagrass species Thalassia hemprichii and Halodule uninervis, as well as that in the nonrhizosphere sediment and seawater, by the acetylene blockage technique. The samples were collected from a seagrass bed in Nanwan Bay, Kenting National Park, Taiwan. Most of the rhizoplane and rhizosphere samples (25 out of 31 for T. hemprichii and 26 out of 29 for H. uninervis) exhibited detectable levels of denitrifying (N 2O-producing) activity within 12 h of aerobic or anaerobic incubation (0.3 to 2.2 μmol N 2O·g wet wt −1·h −1); however, this was undetected in the nonrhizosphere sediment and seawater samples. Insufficiency of nitrate or organic matter would be apt to restrict the magnitude of in situ denitrification in the seagrass rhizosphere including rhizoplane. Most-probable-number values of denitrifying bacteria in the rhizosphere and rhizoplane (29 out of 32 counting values between 10 3 and 10 4 cells·g wet wt −1) in nearly all cases were significantly greater than those in the nonrhizosphere sediment (seven out of ten counting values lower than 10 3 cells·g wet wt −1) and seawater (<10 1 cells·ml −1). Eighty-five strains of denitrifying bacteria in total were isolated from the rhizosphere, rhizoplane and nonrhizosphere samples. Only 35 strains among them did not lose their denitrifying ability after subculture, the majority of which were motile, slightly to moderately halophilic, Gram-negative rods.