Variation in growth rate, carbon assimilation, and photosynthetic efficiency in response to nitrogen source and concentration in phytoplankton isolated from upper San Francisco Bay

Six species of phytoplankton recently isolated from upper San Francisco Bay were tested for their sensitivity to growth inhibition by ammonium (NH4+ ), and for differences in growth rates according to inorganic nitrogen (N) growth source. The quantum yield of photosystem II (Fv /Fm ) was a sensitive indicator of NH4+ toxicity, manifested by a suppression of Fv /Fm in a dose-dependent manner. Two chlorophytes were the least sensitive to NH4+ inhibition, at concentrations of >3,000 μmoles NH4+  · L-1 , followed by two estuarine diatoms that were sensitive at concentrations >1,000 μmoles NH4+  · L-1 , followed lastly by two freshwater diatoms that were sensitive at concentrations between 200 and 500 μmoles NH4+  · L-1 . At non-inhibiting concentrations of NH4+ , the freshwater diatom species grew fastest, followed by the estuarine diatoms, while the chlorophytes grew slowest. Variations in growth rates with N source did not follow taxonomic divisions. Of the two chlorophytes, one grew significantly faster on nitrate (NO3- ), whereas the other grew significantly faster on NH4+ . All four diatoms tested grew faster on NH4+ compared with NO3- . We showed that in cases where growth rates were faster on NH4+ than they were on NO3- , the difference was not larger for chlorophytes compared with diatoms. This holds true for comparisons across a number of culture investigations suggesting that diatoms as a group will not be at a competitive disadvantage under natural conditions when NH4+ dominates the total N pool and they will also not have a growth advantage when NO3- is dominant, as long as N concentrations are sufficient.