Expression of the nitrogen stress response gene ntcA reveals nitrogen-sufficient Synechococcus populations in the oligotrophic northern Red Sea

Determining the nitrogen (N) status of phytoplankton is important for understanding primary production and N cycling in marine ecosystems. We assayed transcript levels of the N regulatory gene ntcA to assess the physiological N status of Synechococcus populations exposed to different N regimes in the meso- to oligotrophic Gulf of Aqaba, Red Sea. Synechococcus populations were N sufficient even in low-N environments when the ratio of dissolved nitrogen to phosphorus indicated that overall phytoplankton biomass was constrained by N. Ammonium supported Synechococcus N requirements under most conditions, but during a massive spring bloom in April 2000 alternative N sources were utilized. Evidence from ntcA clone libraries indicates changes in the genotypic makeup of Synechococcus populations under different N regimes, suggesting that the Synechococcus genotypes present in N-poor waters were those adapted for life in these environments. Thus, the success of Synechococcus in the open oceans is likely to be at least partially due to the selection of genotypes suited to life under prevailing N conditions rather than to prolonged manifestation of the N stress response, mediated by ntcA, in less well-adapted genotypes.