Uptake of ¹³ C and ¹⁵ N (ammonium, nitrate and urea) by Microcystis in Lake Kasumigaura

Abstract

The uptake rate of carbon and nitrogen (ammonium, nitrate and urea) by the Microcystis predominating among phytoplankton was investigated in the summer of 1984 in Takahamaira Bay of Lake Kasumigaura. The V_max values of Microcystis for nitrate (0.025–0.046 h ⁻¹ ) and ammonium (0.15–0.17 h ⁻¹ ) were considerably higher than other natural phytoplankton. The ammonium, nitrate and urea uptake by Microcystis was light dependent and was not inhibited with nigh light intensity. The K ₁ values were far lower than the I_k values. The carbon uptake was not influenced by nitrogen enrichment. Microcystis accelerated the uptake rate by changing V_max / K_s value when nitrogen versus carbon content in cells declined. Nitrate was scarcely existent in Takahamairi Bay during the summer, when Microcystis usually used ammonium as the nitrogen source. However, the standing stock of ammonium in the water was far lower than the daily ammonium uptake rates. Therefore, the ammonium in this water had to be supplied because of its rapid turn-over time (−0.7–2.6 h).