In the modern ocean, nitrogen fixation (converting N2 to bioavailable NH4+) and denitrification (converting NO3- to N2) are roughly balanced. What would happen to marine primary productivity if denitrification rates doubled without a corresponding increase in nitrogen fixation?
AProductivity would increase because more nitrogen would be available
BProductivity would decrease in nitrogen-limited regions because the ocean's inventory of bioavailable nitrogen would decline as more NO3- is converted to unavailable N2, eventually limiting phytoplankton growth
CNo effect, because phosphorus limits marine productivity
Much of the ocean is nitrogen-limited: phytoplankton growth depends on the supply of bioavailable nitrogen (NH4+, NO3-). If denitrification removes nitrogen faster than fixation replaces it, the bioavailable nitrogen pool shrinks, reducing productivity. Over geological time, this imbalance would trigger a negative feedback: reduced productivity means less organic matter export, less oxygen consumption in subsurface waters, and less denitrification (which requires suboxic conditions) -- eventually restoring balance. This feedback coupling illustrates the self-regulating nature of biogeochemical cycles.
Question 2 True / False
The Redfield ratio (C:N:P = 106:16:1) is a fixed biological constant that applies to all marine organisms.
TTrue
FFalse
Answer: False
The Redfield ratio represents the average stoichiometry of marine phytoplankton and dissolved nutrients, but individual species deviate significantly. Diatoms, coccolithophores, and cyanobacteria have different C:N:P ratios depending on nutrient availability, growth rate, and taxonomy. Under phosphorus limitation, organisms accumulate more C and N per P; under nitrogen limitation, C:N increases. The Redfield ratio is a remarkably useful average that emerges from community-level averaging in the deep ocean, but it is not a biological law.
Question 3 Short Answer
Explain why the iron and sulfur cycles are tightly coupled in marine sediments.
Think about your answer, then reveal below.
Model answer: In anoxic marine sediments, microbial sulfate reduction produces H2S, which reacts with reactive iron minerals (iron oxyhydroxides) to form iron sulfide minerals (FeS, eventually pyrite FeS2). This coupling means that iron availability limits how much sulfide is trapped in sediments (vs escaping to the water column), while sulfide production controls the redox state of iron. In the geological record, the ratio of reactive iron to total iron and the degree of pyritization are proxies for water column redox conditions. When reactive iron is exhausted and excess H2S accumulates in the water column (euxinia), the coupled Fe-S system records this as high degrees of pyritization and distinctive iron speciation patterns.
Iron and sulfur are coupled through their redox chemistry: iron oxyhydroxides are the primary sink for microbially produced sulfide, and the balance between iron supply and sulfide production determines the redox character of the depositional environment.