Summer blooms of nitrogen-fixing filamentous cyanobacteria are recurrent phenomena in the Baltic Sea. Salinity, varying from 0 to10 PSU in the surface layer of the Baltic Sea, is among the major factors affecting the basin-scale distribution of various bloom-forming cyanobacterial species. The effects of salinity on the growth rate and cellular carbon, nitrogen, and phosphorus ratios of two major cyanobacterial species that form dense blooms in the Baltic, Aphanizomenon sp. (strain KAC 15) and Nodularia spumigena (strain HEM), were studied. Cells were grown under N2-fixing conditions in a salinity gradient from 0 to 10 PSU. The growth rates of the species showed contrasting responses to salinity. For Aphanizomenon sp. the maximum growth rates (0.28–0.31 d–1) were observed at salinities of 0–2 PSU, while for N. spumigena the maximum growth rate occurred at 8–10 PSU (0.14–0.16 d–1). The latter species did not tolerate low salinities (< 2 PSU). The observed differences in salinity tolerances constrain the distribution patterns of these two species during cyanobacterial blooms, Aphanizomenon sp. being more abundant in the coastal and less saline areas. The variations in growth rates were largely reflected in cellular N : P and C : P ratios, which varied two-fold, and in C : Chla ratios with 5-fold variability. Cellular C : N ratios were rather constant at all salinities and close to the Redfield ratio for Aphanizomenon sp. (on average 5.9 g g–1) and above the Redfield ratio for N. spumigena (on average 8.0 g g–1). The relatively higher N : P and lower C : N ratio showed a higher need of N for Aphanizomenon sp. than for N. spumigena. This is partly explained by the higher abundance of N-rich phycobilin pigments in Aphanizomenon sp. as indicated by fluorescence measurements. The observed differences in pigmentation indicate species-specific strategies in light harvesting.
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