Effects of nitrogen source, concentration, and irradiance on growth rates of two diatoms endemic to northern San Francisco Bay

Impacts on growth rates from exposure to ammonium (NH4+) and nitrate (NO3-), at non-limiting concentrations, in combination with irradiances varying from 25 to 600 µmol photons m-2 s-1 were investigated in the pelagic diatom Thalassiosira weisflogii and the benthic diatom Entomoneis paludosa recently isolated from Suisun Bay in northern San Francisco Bay. Growth rates were higher in T. weisflogii (0.76 ± 0.3 d-1) compared with E. paludosa (0.58 ± 0.2 d-1) across all irradiances and nitrogen (N) treatments. Differences in growth rates with N source were regulated by irradiance in both diatoms and were greatest at the intermediate irradiance due to saturation of rates at 85 µmol photons m-2 s-1 when growing on NH4+ and at 200 µmol photons m-2 s-1 when growing on NO3-. Notable physiological differences between these 2 diatoms included a larger range in the quantum yield of photosystem II (Fv/Fm) and in chlorophyll a per cell as a function of irradiance in T. weissflogii compared with E. paludosa. In addition, a negative interaction of high NH4+ concentration (1000 µmol l-1) and irradiance (≥200 µmol photons m-2 s-1) was observed on growth rates in E. paludosa that was not evident in T. weissflogii. Differences in physiological parameters of these diatoms are discussed in relation to their distributions and frequency of occurrence in Suisun Bay.