Re: vast confusion regarding nitrogen

"Cows get their nitrogen from grass."  True.  Grass is a very dilute source 
of protein, cattle are farily amazing machines in that they are able to 
turn a poor nitrogen source that we have no desire or intention of eating 
(grass) into something useful to us (beef).  I won't comment on grain fed 

"Intestinal bacteria of cows fix nitrogen."  False, if by "fixing" you mean 
the reduction of N2 to NH3.  Cows and other ruminants can use urea as a 
nitrogen source.  They can't use N2 as a nitrogen source, even with their 
full compliment of intestinal bacteria.

Our use of the word "fixation" is sometimes inconsistent with respect to 
nitrogen metabolism.  Some people refer to fixation as the incorporation 
of any inorganic nitrogen into organic molecules (in which case most 
organisms can fix some nitrogen) and others mean the functionalization of 
N2 to a more chemically accessible form (usually ammonia) when they say 
fixation.  In the latter sense, an organism needs nitrogenase, a reducing 
environment and a lot of energy to fix nitrogen.  In the first sense, it 
is as easy as a transamination reaction to make glutamine from glutamate 
and NH3, which every organism can do.  Fish are *not* net nitrogen 
fixers, even in the most relaxed definition of the word.  

"Cyanobacteria and other N2 fixing bacteria are at the bottom of the food 
chain."  More or less true, they are certainly the point of entry of Most 
fixed nitrogen into the biological nitrogen cycle.  The remainder is 
primarily generated by electrical discharges (lightning) in the atmosphere.

"Lightning strikes make NH3."  False.  No way in hell are you going to get 
appreciable NH3 from a lighting strike in an atmosphere that is ~20% 
oxygen.  You do get *nitrogen oxides*.  The same things that you get 
from internal combustion engines operating in an oxidizing environment, 
by very similar chemistry.  The NOx then falls to the ground in rain and 
is chemically accessible.  They are functionalized and can be reduced to 
ammonia with enough reducing equivalents.

"Lightning strikes made amino acids on the prebiotic earth."  Who knows?  
Geologists cannot agree about the composition of the atmosphere on the 
prebiotic earth.  Some believe it was strongly reducing (lots of 
hydrogen) in which case lightning strikes would have made NH3 from N2 and 
H2.  Some believe it was neither strongly reducing nor oxidizing, in 
which case it is not clear what lightning strikes would have made.  
Probably not very much NH3.  The people who want to "facilitate" 
prebiotic chemistry tend to "believe" the prebiotic atmosphere was 
strongly reducing, geologists tend to believe it was dominated by CO2, N2 
and H2O, in which case it was more or less redox neutral.

"All the protein on the planet... cyanobacteria and legumes... millions 
of years ago."  Partially true.  Most of the nitrogen fixation on the 
earth is biological, only a few percent comes from atmospheric electrical 
discharges.  Cyanobacteria and rhizobia account for much of the 
biological nitrogen fixation.  Millions of years ago?  No, the halflife 
of reduced nitrogen in the biosphere is much less than a million years.