Nitrogen uptake pathways

Diacetyl, and its reduction products, acetoin and 2,3-butanediol, are also derived from acetaldehyde (Fig 8D.7), providing additional NADH oxidation steps. Diacetyl, which is formed by the decarboxylation of a-acetolactate, is regulated by valine and threonine availability (Dufour 1989). When assimilable nitrogen is low, valine synthesis is activated. This leads to the formation of a-acetolactate, which can be then transformed into diacetyl via spontaneous oxidative decarboxylation. Because valine uptake is suppressed by threonine, sufficient nitrogen availability represses the formation of diacetyl. Moreover, the final concentration of diacetyl is determined by its possible stepwise reduction to acetoin and 2,3-butanediol, both steps being dependent on NADH availability. Branched-chain aldehydes are formed via the Ehrlich pathway (Fig 8D.7) from precursors formed by combination of acetaldehyde with pyruvic acid and a-ketobutyrate (Fig 8D.7). 

A second pathway is by the uptake up dissolved inorganic nitrogen (DIN) by the symbiont. The inorganic nitrogen may itself derive from recycling of animal wastes which are then retained by the symbionts in exchange for photosynthate released back to the host (Muscatine et al., 1989 Piniak and Lipschultz, 2004 Wang and... 

Nitrate is a major source of inorganic nitrogen for plants, fungi, and many species of bacteria. Animals cannot assimilate nitrate. Assimilation involves three pathway-specific steps uptake followed by reduction to nitrite, and further reduction to ammonia which is then metabolized into central pathways. 

This could have important atmospheric consequences. Because the primary pathway for removal of inorganic nitrate (nitric acid or ammonium nitrate) from the atmosphere is by wet (i.e., uptake by water droplets) or dry deposition, followed by rainout/wash off to the ground, this photochemical reductimi of NOa" provides a mechanism to recycle nitrate back to the gas phase as active nitrogen oxides (HONO, NO2, or NO). These observations are finally similar to the renoxificatimi processes on dust discussed above. 

---