Stimulation of nitrogen-fixing cyanobacteria in a Baltic Sea plankton community by land-derived organic matter or iron addition (original) (raw)

Willem Stolte1,*, Maija Balode2, Per Carlsson3, Daniel Grzebyk4,9, Sven Janson1, Inga Lips5, Renata Panosso1,6, Clive J. Ward7,8, Edna Granéli1

1Department of Biology and Environmental Sciences, Kalmar University, Kalmar, Sweden
2Institute of Aquatic Ecology, University of Latvia, Riga, Latvia
3Campus Helsingborg, University of Lund, 25108 Lund, Sweden
4Centre de Recherche Marine et Aquaculture, CNRS-IFREMER, 17137 L’Houmeau, France
5Marine Systems Institute, Tallin University of Technology, Tallin, Estonia
6Department of Microbiology and Parasitology, Federal University of Rio Grande do Norte, Brazil
7Division of Environmental and Applied Biology, University of Dundee, Dundee DD1 4HQ, UK
8Integrin Advanced Biosystems, The Marine Resource Centre, Barcaldine, Argyl PA37 1SE, UK
9Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey 08901-8521, USA

ABSTRACT: In the Baltic Sea, floating blooms of nitrogen-fixing cyanobacteria occur yearly during late summer. These blooms can sometimes be limited by iron. Due to extensive foresting around the Baltic Sea, iron is entering the Baltic Sea partly bound to dissolved organic material (DOM) via rivers. An experiment was performed in 300 l laboratory mesocosms to test the hypothesis that riverine high-molecular weight dissolved organic matter (HMWDOM), extracted by tangential flow filtration >1000 Da, stimulates the biomass of nitrogen-fixing cyanobacteria, by increasing the availability of iron. The addition of iron/EDTA and of DOM resulted in 5 to 10 times higher biomass of nitrogen-fixing cyanobacteria. Accordingly, higher primary production and particulate nitrogen concentration at the end of the experiment were observed in those treatments compared to the control. The removal of mesozooplankton grazers did not have a significant effect on the microphytoplankton biomass and species composition. Nodularia spumigena biomass was highest in the treatments receiving DOM, but addition of iron alone had no significant effect on this. N. spumigena was less positively affected by iron addition than Anabaena cf_. inaequalis_, suggesting that N. spumigena is a better competitor for iron. Separate microcosms comparing additions of iron, manganese and cobalt showed that iron was limiting for cyanobacterial biomass development. The results strongly suggest that iron bound to DOM can contribute to the iron demands of nitrogen-fixing cyanobacteria in the Baltic Sea.

KEY WORDS: Cyanobacteria · Nitrogen fixation · Iron · High-molecular weight dissolved organic matter · HMWDOM · Humic acid · Mesocosm experiment