Phytoplankton uptake of ammonium and urea during growth on oxidized forms of nitrogen

Uptake capabilities for ammonium (NH 4+ ) and urea by diatoms ( Thalassiosira pseudonana and Skeletonema costatum ) growing on oxidized forms of nitrogen were studied in short-term uptake experiments. Even when nutrient-saturated, an enhanced uptake capability not coupled with the growth rate was present for NH 4+ and urea. No such enhanced uptake ability was seen for NO 2− or NO 3− under either nutrient-saturated or nutrient-depleted conditions. The presence of NH 4+ decreased the enhanced ability to take up urea, but the urea uptake rate in 5 min incubations remained greater than the growth rate even when NH 4+ was present.