Different patterns of growth and nitrogen uptake in two clones of marineSynechococcus spp.

We grew marineSynechococcus Clones WH7803 and WH8018 at 150µE m^–2 s^–1 in dilute batch cultures with NH₄⁺ as the limiting nutrient. The maximal uptake capacities for NH₄⁺ and NO₃^- were measured in frequent experiments during log and stationary phases of growth. Clone WH7803, originally isolated from oceanic waters, had a specific uptake rate of NH₄⁺ that approximated the maximum (log phase) specific growth rate (ca ~ 0.025 h^–1). NO₃^- uptake was observed only after nitrogen in the media was depleted; the NO₃^– uptake capacity was ca 12% the capacity for NH₄⁺ uptake throughout the nitrogen depleted period. Growth was arrested upon nitrogen depletion, but resumed soon after reinoculation into fresh media, even after 5 d of starvation. Clone WH8018, originally isolated from coastal waters, revealed a five-fold enhancement in the NH₄⁺ uptake rate relative to growth rate at the time of nitrogen depletion. As nitrogen starvation proceeded, this enhancement was reduced. This clone, too, was able to take up NO₃^- once nitrogen in the media was depleted, but only after ca 20 h. Growth continued for a limited period during nitrogen depletion, but nitrogen-starved cells were slow to recover upon reinoculation into fresh media. We speculate that clonal differences may reflect differences in the molecular regulation of nitrogen assimilation.