3.
(^) The importance of nitrogen fixation remains to be fully determined. The difficulty is
that measurement techniques are still developing; diazotrophs are extremely patchy
across the oceans and through time; and the geochemical signatures of nitrogen fixed
in the past are complex to calculate, subject to errors, and hard both to sum across the
globe and to interpret. There are two measurement techniques for bulk diazotrophy in
water samples:
(^1) It turns out that the nif enzyme complex also catalyzes a reaction of the
carbon–carbon triple bond in acetylene, producing ethylene. Thus, if some
acetylene is put in the headspace over a sample to dissolve, the amount of
ethylene generated during an incubation is a measure of the N 2 -fixation
capacity of the phytoplankton, possibly of the gross rate. Gross because,
unlike fixed nitrogen in particulates, none of the ethylene is subsequently
lost to metabolism or excretion.
2 Nitrogen gas labeled with ^15 N 2 can be introduced into the headspace over
a sample and dissolves. Organic matter produced subsequently is labeled
with ^15 N, which can be extracted from filtered particulate matter and
measured with mass spectrometry. Many of the data to date are likely
underestimates because of the slow transfer of ^15 N 2 into solution, so that the
(^15) N
2 available for uptake is less than has been assumed (Mohr et al. 2010,
whose improved method remains to be widely applied). These ^15 N 2
measures are also assumed to be, in a sense, “net”, because some of the
fixed nitrogen can be metabolized and released to the environment, and
NH 4 + from fixation can simply leach from diazotroph cells. Both of these
biases may partly explain why direct rate measurements yield estimated
regional rates much smaller than the “geochemical” estimates. However,
simply the patchiness of diazotroph abundance and activity would have the
same effect.
(^) Rates of diazotrophy peak close to the sea surface (e.g. Grabowski et al. 2008; Fig.
11.30) and tail off to zero by 75 m. While fixation depends upon light energy, the
timing of the actual reactions varies among species. In Trichodesmium it rises during
daylight, peaking at noon then decreases again. Some other types of cyanobacterial
diazotrophs photosynthesize during daylight and fix nitrogen at night. The factors
obviously likely to limit fixation are temperature, irradiance (see Fig. 11.30),
phosphate, and iron. Watkins-Brandt et al. (2011), contradicting Zehr et al. (2007)
have shown that small phosphate additions to water samples from the NPSG do
increase fixation rates as measured with ^15 N 2 . The effect would likely be more
pronounced in the Sargasso Sea, which has less phosphorus relative to fixed nitrogen.