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Nitrogen and Phosphorus are usually the limiting nutrient for plants, especially for algae. Phosphorus is used for DNA, ATP and phospholipids, and Nitrogen is used for pretty much every protein a cell might want to produce. That is, their need for biological processes is not tied specifically to photosynthesis: anything that lives is going to need them, pretty much for anything it might want to do. It would make sense for them to be a limiting nutrient for almost anything that's trying to grow, plant or animal.

Yet for animals the limiting "nutrient" seems to always be energy, ie: food. Why aren't animals limited by lack of nutrients in the same way that plants are? Obviously animals need these nutrients, too. Or to reverse the question, why do plants need so much more phosphorus/nitrogen than animals do?

My best guess is that an animal's digestion of plant material is relatively inefficient energy-wise but relatively efficient nutrient-wise. So for an animal to eat enough food to have sufficient energy to survive, it's probably eaten more than enough Nitrogen and Phosphorus for its needs. But I'm just guessing and I can't find any data that would back up that guess.

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More a thought: I think it is because plants a bound to one place. They have to take up what the soil in which they grow contains. Animals on the other side do not have this limitation and as they either feed on plants (which then have these subtances in them) or other animals they do not have this problem. For a number of animals salts can be a problem though. –  Chris May 22 at 5:33
    
Nice thought, but not all plants are static. Consider algae, which float in the oceans and get carried by currents. They're not exactly swimming against the currents, but neither are the zooplankton which feed on them. Yet I don't think zooplankton is directly nutrient limited in the same way algae clearly is. –  Jay Lemmon May 22 at 20:17
    
Thats right. The question is if algae have this as limitations. –  Chris May 22 at 21:17
    
From wikipedia: "Freshwater algal blooms are the result of an excess of nutrients, particularly some phosphates" (en.wikipedia.org/wiki/Algal_bloom#Freshwater_algal_blooms). So yes, algae are nutrient limited in the same way that terrestrial plants are. –  Jay Lemmon May 22 at 21:35

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Phosphorus

Your suggestion that if we are meeting our calorific requirement we will be getting enough is true for phosphorus. Most foods contain lots of phosphorus. The maximum dietary requirement occurs during adolescent growth, estimated at 1250 mg per day. Assuming a calorie intake of 2500 kcal we can calculate a 2500 kcal equivalent phosphorus content for various foods:

skimmed milk contains 7,400 mg phosphorus per 2500 kcal

roasted chicken breast contains 7,500 mg phosphorus per 2500 kcal

cooked white rice contains 3840 mg per 2500 kcal

(Calculations are based upon values obtained via this site.)

Nitrogen

Our requirement for nitrogen is met by our protein intake: inadequate protein intake manifests as kwashiorkor which is essentially due to a dietary deficiency of essential amino acids. In other words, the only way to achieve a nitrogen-deficient diet is to not eat protein, and this would not be alleviated by any inorganic source of nitrogen, even if we could consume enough of such a N source.

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Thanks! Numbers always help actually determine what's going on :) I suppose fundamentally it's a consequence of thermodynamics: matter is conserved during protein synthesis and digestion, but energy is not. You can't disassemble a protein and get back all (or even most) of the energy required to build it. But you can get back most of the amino acids used to build it. –  Jay Lemmon May 23 at 19:19

Off the top of my head:

  • Plants are photosynthetic so have unlimited access to energy. If an animal had unlimited access to energy it would likely have Nitrogen or Phosphorous or something else as the limiting nutrient.
  • Animals eat plants and/or each other. Whatever animals eat will have to a greater or lesser degree sufficient protein/fat/nucleic acids etc to satisfy minimum nutritional requirements. As noted in Chris' comment there can be some species-specific limitations like availability of salt in a herbivore diet, or the human requirement of an external source of vitamin C.
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Light access can very easily be a limiting agent, though. Mature forest floors, for instance, have little undergrowth for exactly this reason. In situations where there's an algae bloom, the top of the water is essentially opaque. Yet in most ecosystems (certainly temperate/tropical oceans) the total biomass production of producers is not limited by light but by nutrients. Likewise if animals were energy efficient but nutrient inefficient during digestion, it's conceivable that nutrients would be more limiting than energy. –  Jay Lemmon May 22 at 19:56

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