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Marine Ecology Progress Series

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MEPS 345:41-50 (2007)  -  DOI:

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

Pia H. Moisander1,*, Hans W. Paerl2, Julianne Dyble3, Kaarina Sivonen4

1Ocean Sciences Department, University of California Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA
2Institute of Marine Sciences, University of North Carolina at Chapel Hill, 3431 Arendell Street, Morehead City, North Carolina 28557, USA
3National Oceanic and Atmospheric Administration, Great Lakes Environmental Research Laboratory, 2205 Commonwealth Boulevard, Ann Arbor, Michigan 48105-2945, USA
4University of Helsinki, Department of Applied Chemistry and Microbiology, PO Box 56, Biocenter Viikki, 00014 Helsinki University, Finland

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 changes in 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 both Nodularia spumigena and Aphanizomenon sp. 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 chlorophyll a concentration 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 in N. spumigena or of the Fe protein of Aphanizomenon sp. Capability to fix N2 at high rates at night allows these cyanobacteria to maximize their utilization of periodic P pulses for subsequent growth.

KEY WORDS: Diel cycle · Nitrogenase regulation · Baltic Sea · Phosphorus limitation · Nodularia spumigena

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Cite this article as: Moisander PH, Paerl HW, Dyble J, Sivonen K (2007) Phosphorus limitation and diel control of nitrogen-fixing cyanobacteria in the Baltic Sea. Mar Ecol Prog Ser 345:41-50.

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