Effects of inorganic nitrogen on taxa-specific cyanobacterial growth and nifH expression in a subtropical estuary

The potentially toxic, diazotrophic filamentous cyanobacterium Cylindrospermopsis raciborskii has recently become a common component in the summer phytoplankton in the St. Johns River (SJR) estuary, Florida, where Anabaena spp. historically dominated. Using a microcosm nutrient enrichment experiment, we investigated the ability of C. raciborskii and Anabaena spp. to compete under a range of available NO3- and NH4+ concentrations, to test the hypothesis that C. raciborskii benefits from increased dissolved inorganic nitrogen (DIN) availability. TaqMan quantitative polymerase chain reaction (PCR) probes were designed, tested, and applied to target the nifH gene in one C. raciborskii and two Anabaena spp. strains from the SJR. N limitation prevailed, as shown by increased N-2-fixation rates if no N was added, increased chlorophyll a concentrations when DIN was added, and depletion of added DIN. Anabaena spp. and C. raciborskii showed rapid growth with no DIN additions and were the main taxa responsible for N-2 fixation. Abundances of C. raciborskii increased if NH4+ was added, but nifH was expressed at low levels, suggesting growth was relying on NH4+. Anabaena spp. and C. raciborskii expressed nifH genes when NO3- or NH4+ were present, but expression was higher with NO3-. The narB gene sequence was amplified from Anabaena spp. and C. raciborskii from the SJR, suggesting these taxa are capable of assimilating NO3-. However, even small NO3- additions blocked the growth of Anabaena spp. in the mixed phytoplankton community but not that of C. raciborskii. The results suggest that C. raciborskii in the SJR is a stronger competitor than Anabaena spp. when DIN is present.