Salinity effects on growth, photosynthetic parameters, and nitrogenase activity in estuarine planktonic cyanobacteria

Salinity has been suggested as being a controlling factor for blooms of N-2-fixing cyanobacteria in estuaries. We tested the effect of salinity on the growth, N-2 fixation, and photosynthetic activities of estuarine and freshwater isolates of heterocystous bloom-forming cyanobacteria. Anabaena aphanizomenoides and Anabaenopsis sp. were isolated from the Neuse River Estuary, North Carolina, and Cylindrospermopsis raciborskii from Lakes Dora and Griffin, central Florida. Salinity tolerance of these cyanobacteria was compared with that of two Nodularia strains from the Baltic Sea, We measured growth rates, N2 fixation (nitrogenase activity), and CO2 fixation at salinities between 0 and 20 g L-1 NaCl. We also examined photosynthesis-irradiance relationships in response to salinity. Anabaenopsis maintained similar growth rates in the full range of salinities from 2 to 20 g L-1 NaCl. Anabaena grew at up to 15 g L-1, but the maximum salinity 20 g L-1 NaCl was inhibitory. The upper limit for salinity tolerance of Cylindrospermopsis was 4 g L-1 NaCl. Nodularia spp. maintained similar growth rates in the full range of salinities from 0 to 20 g L-1. Between 0 and 10 g L-1, the growth rate of Nodularia spumigena was slower than that of the Neuse Estuary strains. In most strains, the sensitivity of nitrogenase activity and CO2 fixation to salinity appeared similar. Anabaenopsis, Anabaena, and the two Nodularia strains rapidly responded to NaCl by increasing their maximum photosynthetic rates (P-max). Overall, both Neuse River Estuary and Baltic Sea strains showed an ability to acclimate to salt stress over short-(24 h) and long-term (several days to weeks) exposures. The study suggested that direct effect of salinity (as NaCl in these experiments) on cyanobacterial physiology does not alone explain the low frequency and magnitude of blooms of N-2-fixing cyanobacteria in estuaries.