What Limits Nitrogen Fixation

Numerous factors, physical, chemical and biotic, can affect the extent of N2 fixation in an ecosystem (Capone, 1988 Howarth et at., 1988a,b Karl et al, 2002). Many factors which bear on nitrogenase activity are interdependent such as light, temperature, oxygen and turbulence. Indeed, a variety of different factors may limit the growth and activity of diazotrophs in various areas of the world s oceans at different times of the year. 

One key factor for tropical diazotrophs may be water temperature. For example, the distribution of Trkhodesmium spp. is roughly limited by the 20°C isotherm, and other planktonic cyanobacteria are likewise primarily tropical or subtropical in distribution. MetabolicaUy active populations of Trkhodesmium have been observed at 18.3°C in the North Atlantic (McCarthy and Carpenter, 1979), but activity was low, and substantial growth is typically not seen until water temperature exceeds 20°C (Carpenter, 1983a,b). Moreover, water temperature co-varies inversely with surface nutrient concentrations (Kamykowski and Zentara, 1986). Indeed, in previous studies we have used sea surface temperature as a proxy for oligotrophic waters in order to estimate the areal range of Trkhodesmium (Capone et al., 2005). 

but measurable N2 fixation rates were recently reported by Needoba et al. (2007), from the inter-convergence zone, 200 km off the coast of Northern California, where sea surface temperatures were 19°C. In addition, they reported the temporal dynamics of mRNA abundance for nijH down to 80 m, where temperatures ranged 14-19°C, and expression was dominated by a unicellular Group A phylotype, and to a lesser extent by Group B. Similarly, HoU et al. (2007) reported N2 fixation in the upper water column of warm core and cold core eddies at temperatures at or below 19C in the Leeuwin Current off the west coast of Australia. 

As for most enzyme mediated reactions, increases in temperature often increase activity in a characteristic fashion. The response of an enzyme to temperature increases, often evaluated as the Qjq parameter (i.e., the increase in activity over a 10°C range), varies among and within enzyme systems and can show compensation. Staal et al. (2003) have evaluated Qjo responses for N2 fixation on short time scales in a range of cyanobacteria including Trkhodesmium, which exhibited a Qio of 1.12 for dark N2 fixation over the temperature range of 20—35°C, and a much 

Natural populations of Trichodesmium which are often found in the upper layers of the euphotic zone (see Chapter 16 by Karl et al, this volume), appear to be adapted to high light with a relatively shallow compensation depth (typically 100-200 imol quanta s ) for photosynthesis (Carpenter, 1983a,b LaRoche and Breitbarth, 2005). Several early studies considered the light-photosynthesis relationships o Trichodesmium (e.g., Lewis et al, 1988 Li et al, 1980). Half saturation (4) constants for photosynthesis are reported to be about 300 pmol quanta m s (based on results from four studies, LaRoche and Breitbarth, 2005). See LaRoche and Breitbarth (2005) for a recent comprehensive summary of observed physiological and photosynthetic parameters for Trichodesmium. 

---

Then there are wider questions about the possible limiting roles of various trace elements in key biogeochemical processes Do trace elements other than iron limit phytoplankton growth and primary production Is the composition of phytoplankton assemblages controlled by trace elements Are various processes in the cycle of nitrogen limited by trace elements (e.g., N2 fixation by iron N2O reduction by copper) What are the links between trace elements and the reduced sulfur cycle in surface seawater ... 

---