Distribution and potential activity of aerobic denitrifying bacteria isolated from sediments of a coastal lagoon in northwest Mexico

Denitrification is the main pathway by which fixed nitrogen is lost in marine systems and consists of the sequential respiration of nitrate to molecular nitrogen by various heterotrophic microorganisms. Since estuarine sediments represent an important niche for denitrification, this study evaluated the distribution and potential activity of aerobic denitrifying bacteria isolated from sediments at 2 depths and in 2 habitats (with Zostera marina grasses and without grasses) along a transect in San Quint & iacute;n Bay (Mexico), as well as their relationship with different environmental parameters. A total of 1,611 bacteria were isolated, of which 85.1% contained one of the denitrifying genes (nirK, nirS or nosZ). Their distribution was heterogeneous in the bay and was mainly influenced by texture, pH, total organic carbon and total nitrogen of the sediments, with a higher abundance of denitrifying bacteria found in surface sediments (66.2%). Denitrifying isolates were classified into 23 species belonging to gamma-Proteobacteria (82.4% of isolates), alpha-Proteobacteria (7.9%), Bacilli (5.7%) and Actinobacteria (4%). Denitrifying activity under aerobic conditions was tested in 7 species of bacteria carrying denitrifying genes: Paracococcus marcusii, Planococcus maritimus, Planococcus rifietoensis, Pseudomonas songnenensis, Psychrobacter alimentarius, Psychrobacter celer and Psychrobacter piscatorii. The results suggest that these sediments present a high abundance of culturable bacteria with nirK, nirS or nosZ genes; however, in most of them it was not possible to verify the denitrifying activity under aerobic conditions, possibly due to the inactivation of these genes or of those involved in nitrate reduction (nas, nar or nap). This study represents the first step towards understanding the ecology of microorganisms involved in aerobic denitrification in San Quintin Bay and in coastal lagoons in general.