Phosphate and ATP uptake and growth kinetics in axenic cultures of the cyanobacterium Synechococcus CCMP 1334

Abstract

Phosphorus potentially limits the growth and productivity of the unicellular cyanobacterium Synechococcus in many oligotrophic oceans. Both dissolved inorganic phosphorus (DIP) and dissolved organic phosphorus (DOP) could be important P sources in these regimes. We compared DIP and adenosine tri-phosphate (ATP) uptake and growth kinetics using axenic semi-continuous cultures of Synechococcus CCMP 1334 grown on both P substrates, under both P-limited and P-replete conditions. Molar C:total P ratios and N:total P ratios were 417–431 and 65–68, respectively, under severely P-limited growth conditions for both P sources, but were near Redfield values at >1.0 µM P as either DIP or ATP, and were inversely proportional to growth rates. Synechococcus grown on ATP as the sole P-source have the same maximum growth rates, lower half-saturation constants for growth, and similar cellular P quotas compared with cultures grown on DIP. DI33P and 33P-ATP uptake experiments confirmed that both can be used by Synechococcus, with relative uptake rates depending on prior growth history. DIP-limited cultures assimilated ATP faster than DIP, while DIP-replete cultures took up DIP more rapidly. In contrast, for Synechococcus grown under ATP-limited conditions, there is no priority for the utilization of ATP or DIP, but ATP-replete cultures have a greater ability to use DIP. Like other phytoplankton, kinetic constants for Synechococcus DIP uptake and cellular P quotas, C:P and N:P ratios of DIP-grown cells differ when total cellular and intracellular P pools are compared, suggesting a need to re-evaluate these parameters to account for abiotic phosphate scavenging onto cell surfaces. DIP half-saturation constants for growth of this Synechococcus strain would not support observed natural growth rates at the nanomolar DIP concentrations found in most oligotrophic regimes, and hence populations in these areas probably rely primarily on DOP.