COBALT SPECIATION AND BIOAVAILABILITY IN MARINE ORGANISMS

We describe comparative studies, using double-radiolabelling techniques, on the uptake and retention of cobalt species in a simple marine food chain which includes phytoplankton (Dunaliella tertiolecta and Chaetoceros pseudocurvisetum), mixed copepods (mainly Centropages sp.), and fish (the sea perch Serranus scriba). Retention of cobalt from the diet was also studied for the latter 2 organisms. Phytoplankton accumulated more than 60 times as much Co-cobalamine as CoCl2 from the Water, and retained the metal for a significantly longer period (a retention half-time of 4.4 vs 0.6 d). Accumulation of CoCl2 after ingestion of radiolabelled phytoplankton by copepods was not measurable, whereas retention of Co-cobalamine reached 42% of the quantities ingested. Fish accumulated Co-cobalamine 21 times more rapidly from seawater than CoCl2 and retained ingested Co-cobalamine 20 times more efficiently (100%) than ingested CoCl2 (5%). Two thirds of the ingested Co-cobalamine was retained in the fish with a retention half-time of 8 d. The remaining one third of the organic form was retained with a half-time of 54 d, a value which was not significantly different from that of CoCl2 (47 d). The results of other experiments indicated that the Co-cobalamine complex was stable in freshwater and in both 0.22 and 10 mum filtered seawater for at least several weeks. Potential size effects on cobalt intake in the diet are discussed, as are size effects on long-term loss rates of the metal. The application of the measured parameters in a simple biokinetic model shows that preferential accumulation of the Co-cobalamine complex over inorganic cobalt species in the food web could explain the cobalt concentrations measured in marine organisms.