||p.8199 right column 3rd paragraph:"The cellular abundances of N and P are most often discussed in terms of the N:P ratio. However, carbon is by far the dominant element in biomass (not counting H and O, derived from water). It typically outweighs nitrogen by a factor of about seven, because of its central structural role in most organic molecules, and is thus the most logical normalizing factor for considering nutrient ratios. Relative to carbon, the ~30 essential nutrient elements display a range of plasticities in their cellular requirements (ref 15 BNID 111705). Nitrogen tends to be the least plastic, varying in cellular N:C molar ratios from ~1/5 to 1/10, a factor of two (primary source). The cellular P:C ratio is more variable, due to the ability of organisms to substitute phosphorus compounds such as phospholipids (ref 16), and ranges from ~1/50 to 1/500, or by about a factor of 10. Iron is even more variable, given the ability to substitute with other elements, such that Fe:C varies from about 1/1,000 to 1/1,000,000, or three orders of magnitude (BNID 111708)."