Spatial and temporal variations related to hydric seasonality in abundance and diversity of denitrifier communities were examined in sediments taken from two sites differing in nitrate concentration along a stream Doñana National Park during a 3-year study. We found a positive relationship between the relative abundance of denitrifiers, determined as narG, napA, nirK, nirS and nosZ denitrification genes, and sediment nitrate content, with similar spatial and seasonal variations. However, we did not find association between denitrification activity and the community structure of denitrifiers. Because nosZ showed the strongest correlation with the content of nitrate in sediments, we used this gene as a molecular marker to construct eight genomic libraries. Analysis of these genomic libraries revealed that diversity of the nosZ-bearing communities was higher in the site with higher nitrate content. Regardless of nitrate concentration in the sediments, the Bradyrhizobiaceae and Rhodocyclaceae were the most abundant families. On the contrary, Rhizobiaceae was exclusively present in sediments with higher nitrate content. Results showed that differences in sediment nitrate concentration affect the composition and diversityof nosZ-bearing communities. résumé du livre : Metagenomics continues to be one of the most dynamic scientific fields due largely to the development of new and cheaper sequencing technologies. The diversity of habitats explored with metagenomics and other meta-omics techniques has increased exponentially in recent years. The resulting cascade of data has led to a new range of methodological problems and solutions. In this collection of reviews, expert authors describe the cutting-edge and emerging conceptual and methodological tools being employed to deal with current issues in metagenomics. Topics covered include the integration of ecology and metagenomics; the organization, classification, analysis and interpretation of the vast amount of data; the new statistical and bioinformatic techniques; sample extraction and processing techniques; and various applications of metagenomics in specific areas. The volume is essential reading for researchers and students commencing projects in this field, for researchers active in metagenomics areas, and for educators interested in the latest developments. The volume is also of value to anyone involved in biotechnology, bioremediation, biodegradation and environmental microbiology.