N2 assimilation

Dore et al. (2002) were the first to report euphotic zone depth-integrated measurements of the relative contributions of smaU (<10 pm) and large diazotrophs to total N2 fixation in the North Pacific trades biome. Based on N2 assimilation rate measurements from HOT cruises conducted in November 2000... 

Chloroplast ferredoxin is a small water soluble protein M W 000) containing an Fe-S center [245]. Its midpoint potential ( — 0.42 V [246]) is suitable for acting as an electron acceptor from the PSI Fe-S secondary acceptors (Centers A and B) and as a donor for a variety of functions on the thylakoid membrane surface and in the stroma. Due to its hydrophylicity and its abundance in the stromal space, ferredoxin is generally considered as a diffusable reductant not only for photosynthetic non-cyclic and cyclic electron flow, but also for such processes as nitrite and sulphite reduction, fatty acid desaturation, N2 assimilation and regulation of the Calvin cycle enzyme through the thioredoxin system [245]. Its possible role in cyclic electron flow around PSI has already been discussed. The mobility of ferredoxin along the membrane plane could be an essential feature of this electron transfer process the actual electron acceptor for this function and the pathway of electron to plastoquinone is, however, still undefined. 

Costs associated with the membrane transport of either the intermediates or the final products of NO3" or N2 assimilation to the xylem stream are largely unknown. A frequently quoted cost for transport of this nature is 1 ATP/mole-cule transported across a single membrane (Atkins et al, 1979 Layzell et ai. 1979). Assuming this value to be correct, one still has to speculate as to the number of membrane barriers likely to be crossed ... 

It obviously was difficult to establish details of the N2 fixation process with intact organisms, as the ammonia fixed was rapidly assimilated into other compounds. So a search was initiated in several laboratories for a cell-free preparation that would fix N2. 

Nitrogen uptake that results in the formation of new biomolecules is termed an assimilation process, such as assimilatory nitrogen reduction. The processes that result in the release of DIN into seawater are referred to as dissimilations, such as dissimi-latory nitrogen reduction. An example of the latter is denitrification, in which nitrate and nitrite obtained from seawater serve as electron acceptors to enable the oxidation of organic matter. This causes the nitrate and nitrite to be transformed into reduced species, such as N2O and N2, which are released back into seawater. 

Figure 24-1 The nitrogen cycle. Conversion of N2 (oxidation state 0) to NH4+ by nitrogen-fixing bacteria, assimilation of NH4+ by other organisms, decay of organic matter, oxidation of NH4+ by the nitrifying bacteria Nitrosomas and Nitro-bacter, reduction of N03 and N02 back to NH4+, and release of nitrogen as N2 by denitrifying bacteria are all part of this complex cycle.1...

Ammonia is oxidized in nature to nitrate via several intermediates in the process of nitrification. Nitrate may be reduced to nitrite by either a dissimilatory or an assimilatory process. Nitrite may be assimilated into the cell via reduction to ammonia, or it may be reduced by microorganisms to N20 and N2 in denitrification. A major part of the total nitrogen in this pathway is lost to the atmosphere. However, in turn, atmospheric dinitrogen is converted to ammonia by various bacteria in nitrogen fixation. 

Oxidized forms of DIN (NC>3 and NO2-) must also be converted into ammonia by either nitrate or nitrate reductases before being fixed into organic matter. Similarly, there is a nitrogenase reductase reaction involved in the fixation of N2. The range of fractionation observed with NC>3 assimilation is similar to that observed with NH4+, with more fractionation (by diatoms) occurring with higher ambient NO3 concentrations (Wada and Hattori, 1978). Active transport of NC>3 has been observed by marine diatoms however, details of the membrane-bound enzyme involved with this reaction remain unclear (Falkowski, 1975 Packard, 1979). 

Figure 10.5 Major processes involved in the biogeochemical cycling of N in estuaries and the coastal ocean (1) biological N2 fixation (2) ammonia assimilation (3) nitrification (4) assimilatory NC>3 reduction (5) ammonification or N remineralization (6) ammonium oxidation (speculative at this time) (7) denitrification and dissimilatory NO3 reduction to NH4+ and (8) assimilation of dissolved organic nitrogen (DON). (Modified from Libes, 1992.)...

Several comprehensive studies of N assimilation in the North Pacific trades biome have been conducted over the past several decades. Gundersen and his colleagues (1974, 1976) were the first to estabhsh N2 fixation as a source of new N to the open ocean ecosystem, and concluded that it was a more important source of fixed N than wet deposition from the atmosphere (see Case Studies section). They also made measurements of the rates of nitrification, denitrification and assimilatory nitrate-reduction. These latter experiments involved the addition of fairly high concentrations of exogenous N substrates (NH4 , N02, NOa ) and extended incubations (days to months), so the rates reported must be viewed as potential rates at best. 

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