Light‐limited growth on ammonium vs. nitrate: What is the advantage for marine phytoplankton?

Thalassiosira pseudonana (3H) was grown in semicontinuous culture on either NO3− or NH4+ at nine different photon flux densities (PFDs). At PFDs >29 µEinst m−2 s−1, growth rates (µ) were 8% lower for cells growing on NO3− compared to cells growing on NH4+. At high PFDs NO3−‐grown cells had 21% lower carbon quotas than NH4+‐grown cells. At high PFDs the possibility exists that competition for reductant may reduce µ and carbon quotas for NO3−‐grown cells. At PFDs ≤29 µEinst m−2 s−1 NO3−‐grown cells had equal growth rates, nitrogen quotas, Chl a cell−1, and equal or greater carbon quotas compared to NH4+‐grown cells. At these light‐limited growth rates NO3−‐grown cells used more reductant overall and up to 51% more reductant per unit Chl a than NH4+‐grown cells to store carbon and nitrogen. These results suggest that use of NO3− rather than NH4+ is of little energetic importance in terms of growth rate at low PFDs.