Maximum B12 uptake rates were the same for limited and replete cultures, supporting the notion that C. polykrikoides can rapidly assimilate B12 even under low levels while its high affinity constant values suggest that depleted cultures acclimate to lower B12 concentra-tions and were more effective at utilizing this vitamin under sub-Ks conditions. Finally, it is possible that some of the vitamin uptake measured in the C. polykrikoides size class was due to attached bacteria