Phosphorus limitation and diel control of nitrogen-fixing cyanobacteria in the Baltic Sea

Abstract

Up to half of the annual new nitrogen inputs into the Baltic Sea originate from blooms of N 2 -fixing cyanobacteria. Estimates of the magnitude of this new nitrogen vary, partially because relatively few studies have investigated short-term changes in N 2 -fixation rates in response to environmental changes in situ, including phosphorus availability, one of the major factors limiting N 2 fixation in the system. We examined cyanobacterial N 2 fixation in response to phosphorus amendments over the diel cycle during 2002 and 2003 in the Baltic Sea, when both Nodularia spumigena and Aphanizomenon sp. formed blooms. Phosphorus stimulated N 2 fixation in the open-sea areas in the Northern Baltic Proper and Gulf of Finland during both years. In microcosm experiments, both chlorophyll a concentration and N 2 fixation were positively related to time (R 2 = 0.79 and 0.54, respectively) for at least 4.5 d after the P amendment. N 2 fixation was enhanced up to 3-fold within 4.5 d by a single P pulse. N 2 fixation continued in the dark at 16 to 61% of maximum rates during the day, and there were no consistent changes in nitrogenase enzyme abundance in response to darkness. Immunoblotting showed that N 2 fixation is not regulated in response to darkness by size modifications of the Fe and MoFe proteins in N. spumigena or of the Fe protein of Aphanizomenon sp. Capability to fix N 2 at high rates at night allows these cyanobacteria to maximize their utilization of periodic P pulses for subsequent growth.