Ammonium release by nitrogen sufficient diatoms in response to rapid increases in irradiance

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Abstract
It has been hypothesized that nitrogen-replete diatoms, but not flagellates, may release NO2–, NH4+ or dissolved organic nitrogen (DON) following rapid increases in irradiance (and consequently an increase in cellular electron energy), as might be expected to occur in a vertically well mixed estuarine system. Just as the increase in irradiance leads to an increase in cellular energy, so too would a decrease in temperature, due to the temperature dependency of biosynthetic enzymes. This hypothesis was tested by comparing the response of nitrogen-replete diatoms (Skeletomena costatum, Thalassiosira weissflogii and Chaetoceros sp.) and flagellates (Dunaliella tertiolecta, Pavlova lutheri and Prorocentrum minimum) to rapid increases in irradiance and decreases in temperature. Short-term (2– and NH4+ concentrations were measured in cultures following these experimental shifts, as well as in cultures retained at the growth irradiance. Net rates of NO2– and NH4+ release were calculated from the time course of extracellular nitrogen concentrations. As a fraction of NO3– uptake, NO2– release rates under the increased irradiance increased marginally relative to NO2– release rates under the growth irradiance. Release rates of NH4+ under the increased irradiance increased nearly fivefold over release rates at the growth irradiance, and accounted for 84% of the NO3– uptake rate. In direct contrast to the diatom species, the flagellate species released NO2– under the higher experimental irradiance at rates one half those of the release rates under the growth irradiance, and continued to take up NH4+ under both irradiance conditions. Within the experimental boundaries, these findings have important physiological and ecological implications. The magnitude of the observed nitrogen release represents a significant physiological sink for electrons and, in fact, calculations suggest that up to 62% of the total electrons harvested could be consumed. From an ecological perspective, these findings add to the body of literature which suggests that a significant fraction of the nitrogen that is taken up is ultimately released in dissolved form. More importantly, these data suggest that DON is not the only compound that phytoplankton may release in the aquatic environment.

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