Phosphorus dietary assimilation and efflux in the marine copepod Acartia erythraea

We examined the metabolism of phosphorus (P), including its dietary assimilation, efflux, and regeneration, in the marine subtropical copepod Acartia erythraea feeding on diverse types of marine phytoplankton. The P assimilation efficiency (AE) ranged between 19 and 78% when the copepods were fed 6 algal diets at the same concentration (1.45 mg C l(-1)). Among the different algal diets, the AEs were not significantly related to the ingestion rate and the food gut passage time of copepods, or the P partitioning in the algal cytoplasm. The P AE decreased ca. 2-fold when the food concentration increased from 0.073 to 3.625 mg C l(-1), but was not influenced by the P quota in diatoms. The P efflux rates in the copepods feeding on diatoms were 0.30 to 0.36 d(-1) over the food concentrations examined; the efflux rate was significantly higher when the copepods ingested diatoms with higher P quotas, suggesting that the P homeostasis in the copepods may possibly be achieved by efflux of P from the body. During the dietary assimilation and efflux periods, most P lost from the copepods was regenerated into the dissolved phase and only a small portion of P was detected in other compartments (mainly in the feces), suggesting that copepods rapidly regenerate particulate P into the surrounding waters. Our study indicated that both P dietary assimilation and efflux can play an important role in maintaining the P stoichiometry in copepods.