- When microzooplankton graze phytoplankton prey, the consumed carbon is partitioned into particulates, dissolved organic carbon (DOC), and CO2. Allocation of prey carbon to these various fates has important consequences for marine ecosystem function. A 2-stage continuous culture system was used to investigate carbon allocation by microzooplankton consuming phytoplankton grown in chemostats at controlled, nutrient-limited growth rates. The chemical composition of Dunaliella tertiolecta and Thalassiosira pseudonana cells varied with growth rate. When a constant amount of prey carbon was fed to the dinoflagellate Oxyrrhis marina, the carbon transfer efficiency to particulates (CTE) decreased from 27 +/- 5% when fast-growing T. pseudonana cells were the prey to only 3 +/- 2% when slow-growing cells were the prey. DOC did not increase with decreasing CTE, indicating that an increase in CO2 remineralization caused the lower CTE when the slow-growing cells were consumed. A similar pattern was observed when D. tertiolecta was the prey, but CTE was higher: 42 +/- 15% for fast-growing cells, declining to 17 +/- 6% for slow-growing prey cells. The microzooplankter showed greater neutral lipid accumulation when fed D. tertiolecta; however, its neutral lipid content did not necessarily mirror that of its phytoplankton prey and varied substantially across treatments. These findings demonstrate that microzooplankton respond strongly to food qualities of prey cells that are influenced by growth rate. We conclude that a significant and variable portion of primary production is lost from ecosystems because microzooplankton CTE is strongly influenced by the impacts of nutrient limitation on prey growth rates.