Variation in growth rate, carbon assimilation, and photosynthetic efficiency in response to nitrogen source and concentration in phytoplankton isolated from upper San Francisco Bay
Berg, GM; Driscoll, S; Hayashi, K; Ross, M; Kudela, R
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.
