Nature 420, 403–407 (2002).

In this Letter, the conversion to SI units led to several errors. On page 404, left column, lines 16 and 17, the values should read 10–15 and 30–40 nanolitres per litre. On page 405, right column, lines 3 and 4, the values should read 360 microlitres per litre and 36.0–38.8 nanolitres per litre. On page 407, left column, lines 3 and 4, the values should read 560 microlitres per litre, and 46.4 to 55.5 nanolitres per litre. The conclusions of the paper are not affected.