Variation in growth rate, carbon assimilation, and photosynthetic efficiency in response to nitrogen source and concentration in phytoplankton isolated from upper San Francisco Bay. Issue 3 (2nd May 2017) - Berg, Gry Mine
Abstract : 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.