Competition and Facilitation between Unicellular Nitrogen-Fixing Cyanobacteria and Non-Nitrogen-Fixing Phytoplankton Species

• Published in: Limnology and oceanography Vol. 52; no. 5; pp. 2233 - 2248
• Main Authors: Nona S. R. Agawin, Sophie Rabouille, Marcel J. W. Veldhuis, Lidewij Servatius, Suzanne Hol, Harriët M. J. van Overzee, Huisman, Jef
• Format: Journal Article
• Language: English
• Published: Waco, TX The American Society of Limnology and Oceanography 01-09-2007; American Society of Limnology and Oceanography; Association for the Sciences of Limnology and Oceanography
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Biological competition; Chlorella; Cyanobacteria; Cyanophyta; Cyanothece; Ecological competition; Environmental Sciences; Fundamental and applied biological sciences. Psychology; General aspects; Luminous intensity; Marine; Modeling; Nitrates; Nitrogen; Nitrogen fixation; Phytoplankton; Population density; Synechococcus bacillaris; Synecology; Transmission lines; ASW, USA, Florida, Miami; Competition; Facilitation; Aquatic environment; Plankton; Cyanobacteria; Nitrogen fixation; Bacteria; Phytoplankton
• ISSN: 0024-3590; 1939-5590
• DOI: 10.4319/lo.2007.52.5.2233

Recent discoveries show that small unicellular nitrogen-fixing cyanobacteria are more widespread than previously thought and can make major contributions to the nitrogen budget of the oceans. We combined theory and experiments to investigate competition for nitrogen and light between these small unicellular diazotrophs and other phytoplankton species. We developed a competition model that incorporates several physiological processes, including the light dependence of nitrogen fixation, the switch between nitrate assimilation and nitrogen fixation, and the release of fixed nitrogen. Model predictions were tested in nitrogen-limited and light-limited chemostat experiments using the unicellular nitrogen-fixing cyanobacterium Cyanothece sp. Miami BG 043511, the picocyanobacterium Synechococcus bacillaris CCMP 1333, and the small green alga Chlorella_cf sp. CCMP 1227. Parameter values of the species were estimated by calibration of the model in monoculture experiments. The model predictions were subsequently tested in a series of competition experiments at different nitrate levels. The model predictions were generally in good agreement with observed population dynamics. As predicted, in experiments with high nitrate input concentrations, the species with lowest critical light intensity (S. bacillaris) competitively excluded the other species. At low nitrate input concentration, nitrogen release by Cyanothece enabled stable coexistence of Cyanothece and S. bacillaris. More specifically, model simulations predicted that fixed nitrogen release by Cyanothece enabled S. bacillaris to become four times more abundant in the species mixture than it would have been in monoculture. This intricate interplay between competition and facilitation is likely to be a major determinant of the relative abundances of unicellular nitrogen-fixing cyanobacteria and non-nitrogen-fixing phytoplankton species in the oligotrophic ocean.