Abstract
Up to half of the annual new nitrogen inputs into the Baltic Sea originate from blooms of N₂-fixing cyanobacteria. Estimates of the magnitude of this new nitrogen vary, partially because relatively few studies have investigated short-term changes in N₂-fixation rates in response to environmental changesin situ, including phosphorus availability, one of the major factors limiting N₂ fixation in the system. We examined cyanobacterial N₂ fixation in response to phosphorus amendments over the diel cycle during 2002 and 2003 in the Baltic Sea, when bothNodularia spumigenaandAphanizomenonsp. formed blooms. Phosphorus stimulated N₂ fixation in the open-sea areas in the Northern Baltic Proper and Gulf of Finland during both years. In microcosm experiments, both chlorophyllaconcentration and N₂ fixation were positively related to time (R² = 0.79 and 0.54, respectively) for at least 4.5 d after the P amendment. N₂ fixation was enhanced up to 3-fold within 4.5 d by a single P pulse. N₂ 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 N₂ fixation is not regulated in response to darkness by size modifications of the Fe and MoFe proteins inN. spumigenaor of the Fe protein ofAphanizomenonsp. Capability to fix N₂ at high rates at night allows these cyanobacteria to maximize their utilization of periodic P pulses for subsequent growth.