In temperate Atlantic waters (18.8 to 20.1°C), biological nitrogen fixation has been<br/>demonstrated by 2 independent measurements: 15N-N2 incorporation and nifH identification in the<br/>DNA and expressed messenger RNA (mRNA). At 2 stations in the western English Channel, bulk<br/>waters were incubated with 15N-N2. At the high levels of particulate nitrogen (?11.5 ?mol N l–1),<br/>absolute fixation rates of 18.9 ± 0.01 and 20.0 nmol N l–1d–1 were determined. While a caveat must<br/>accompany the magnitude of the rates presented due to the limited number of data, the presence and<br/>activity of diazotrophic organisms in these waters is of ecological significance and may affect current<br/>attitudes to nitrogen and carbon budgets. In particular, our estimate of the rate of N fixation<br/>(0.35 mmol N m–2 d–1) is comparable to that of denitrification rates in UK shelf seas. Molecular analysis<br/>identified a diversity of expressed nifH genes, and 21 different prokaryotic nifH transcripts were<br/>identified
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