Question

Name a nutrient that limits productivity in both aquatic and terrestrial ecosystems.

Answer 1

Ans. The major macronutrients including the elements CHONPS.

C is taken up by primary producers (photosynthetic organisms) in form of CO₂. Since the rate of CO₂ fixation during photosynthesis by RuBisCO is very low, around 3 CO₂ molecules per enzyme molecule, atmospheric CO₂ level is generally sufficient for primary productivity (photosynthesis) both for terrestrial (CO₂ gas) as well as aquatic producers (dissolved CO₂, in form of carbonic acid)

# H and O are provided by H₂O, and generally present in excess.

# S is required only for amino acids (cysteine and methionine). S is generally obtained from H2S or FeS, etc. Because of its relatively low requirement, S does not act as the limiting nutrient.

# N is a constituent of both amino acids and nucleic acid- thus required in large quantity. Though atmospheric N₂ constitute around 78%, the photosynthetic organisms generally do not take up N₂ directly from atmosphere. Instead they take up N in form of NH₄⁺ or other forms already present in the medium from decaying biomass or by the symbiotic of N₂-fixing microbes. So, because of plant’s large requirement of N but limited availability in the medium, N acts as the limiting reactant in both the terrestrial and aquatic producers.

# Phosphate is a major constituent of nucleic acids and nucleotides for energy transduction. However, P also acts as a limiting nutrient because of its low availability in the medium because the medium (soil or the aqueous medium but not the atmosphere) acts as the source.

# Conclusion: Both N and P may acts as the limiting nutrient for terrestrial and aquatic producers.