Conversion of acetylene reduction rates to nitrogen fixation rates in natural populations of blue-green algae
✍ Scribed by R.B. Peterson; R.H. Burris
- Publisher
- Elsevier Science
- Year
- 1976
- Tongue
- English
- Weight
- 336 KB
- Volume
- 73
- Category
- Article
- ISSN
- 0003-2697
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✦ Synopsis
Twelve experiments are reported on naturally occurring groups of N,-fixing blue-green algae in which reduction of acetylene and 15N, were determined simultaneously. Simple averaging of the C,H.JN, reduction rates yielded a conversion factor of 4.4. Linear and log transform regression analyses yielded values of 4.8 and 4.2, respectively. Reports estimating nitrogen inputs based on acetylene reduction determinations are evaluated.
The first reports by Dilworth (1) and Schollhorn and Burt-is (2) of catalytic reduction of acetylene by nitrogenase provided the basis for the widely-used acetylene reduction assay for N, fixation. The assay is simple, inexpensive, and highly sensitive, and hence has been particularly useful in field studies to estimate biological input of fixed N into the ecosystem (3).
As six electrons are required for reduction of N2 to 2 NH, and two electrons to reduce acetylene to ethylene, it often has been suggested that acetylene reduction rates should be divided by three to give N2 fixation rates. The CfH2/N2 conversion factor has been measured in a number of systems and has been found to be quite variable; experimentally observed values have ranged from 2 to 25 (4). Deviations from the expected value would be expected if acetylene and/or N, were supplied at less than saturating levels. When N2 is the substrate, there is a substantial electron flow to H+, and H, is produced: such evolution of H, is almost completely inhibited at substrate levels of acetylene. This diversion of electrons to H+ raises the conversion factor above 3 and it commonly is closer to 4 (5,6).
Investigators are strongly urged to employ an experimentally derived conversion factor obtained from parallel determination of acetylene and 15N, reductions rather than to rely on a theoretical factor. We present here data on conversion factors obtained by parallel determinations of acetylene and 15N, reduction by naturally occurring populations of N,-fixing bluegreen algae. The results suggest a conversion factor close to 4:l for this system. 404