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We investigated the occurrence and activity of anaerobic ammonia oxidation (anammox) bacteria in sandy and muddy sand sediments of the southern North Sea. The presence of anammox bacteria was established through the detection of specific phosphocholine-monoether ladderane lipids, 16 S r RNA gene, and hydrazine synthase ( hzsA) genes. Anammox activity was measured in intact sediment cores ( in situ rate) and in sediment slurries (potential rate) as the rate of N2 evolution from 15 N-labeled substrates and compared to the transcriptional activity of genes of anammox bacteria. The contribution of anammox to N2 production ranged between 0% and 29%. The potential rate of anammox agreed well with the abundance of anammox bacteria 16S r RNA and hzsA gene copies and the transcriptional activity of the anammox bacteria 16S r RNA gene. We found a higher abundance and activity of anammox bacteria in sediments with higher organic carbon content and also higher activity in summer than in winter. The abundance of anammox bacteria and their potential anammox rates were similar to those reported for other marine coastal sediments, suggesting that potentially they are important contributors to the nitrogen cycle in sandy sediments of shallow continental shelf areas. [ABSTRACT FROM AUTHOR]