PCR amplification of nirK from spring, summer, fall, and winter biofilms with (CNP) and without (Con) nutrient amendments. Total-community DNA extracts were undiluted (0) or diluted 10−1 (−1) or 10−2 (−2) prior to PCR amplification using the nirK consensus primers. Lanes M, molecular size markers; lane +, positive control (50 ng of Pseudomonas aureofaciens ATCC 13985 DNA template); lane −, negative control (no DNA). The arrows indicate the positions of the 376-bp nirK gene amplicon, and the asterisks indicate the positions of the 500-bp fragments of the molecular size markers.