Assimilation efficiency (AE) of metals from ingested food is critical for understanding trace metal accumulat~on and trophic transfer in aquatic animals. Most recent measurements of metal AEs have been on aquatic invertebrates, whereas relatively few studes have examined metal assimilation in fishes. In this study we determined the AEs of Cd, Cr and Znin 2 fishes (pelagic glassy Ambassis urotaenia, Ambassidae, and the intertidal mudskipper Penophthalmus cantonensis, Gobiidae) feeding on 2 zooplankton prey (brine shrimp k t e m i a larvae and copepods). Zooplankton were radiolabeled either by feeding on radiolabeled phytoplankton or by direct exposure to radiotracers in the dissolved phase. Fishes were then fed with radiolabeled zooplankton prey for < l h., and the retention of ingested metals in the fishes was followed for 2 d. The measured AEs of Cd, Cr and Zn were 14 to 33, 4 to 12, and 5 to 17% in glassy fish, and 10 to 26, 4 to 19, and 11 to 31 % in mudskipper, respectively. Routes of radiolabeling in copepod prey did not affect metal AEs in either mudskipper or glassy, whereas metal AEs differed by up to 10-fold in glassy fish feeding on Artemia larvae labeled from different routes. There was little difference m the gut passage time of metals for different food types and metals or between fishes. AE was not significantly related to metal gut passage time or metal distribution in the soft tissues of zooplankton prey, for each metal. However, AE in mudskippers was significantly correlated with metal distribution in the prey's soft tissues when all 3 metals were considered. Our study demonstrated that marine fishes can appreciably assimilate trace metals, and trophic transfer should be considered as a source for metal accumulation in fishes.
Bacterial utilization of dissolved organic matter plays an important role in marine carbon cycling. In this study, the response of bacterioplankton to a gradient of carbon (glucose) addition was investigated experimentally in a subtropical coastal environment in the absence of top-down control by viruses and flagellates. Bacterial abundance and production were stimulated by glucose addition corresponding to a gradient of glucose. Differences in the extent of stimulation suggested different bacterial life strategies under different nutrient conditions. Bacterial community diversity as revealed by denaturing gradient gel electrophoresis (DGGE) showed a unimodal productivity-diversity (number of DGGE bands) relationship after 3-day incubation. DNA fingerprinting profiling and cluster analysis showed clear and gradual changes in bacterial community structure along the gradient of glucose concentrations, reflecting the competition for carbon supply among bacterial groups. Sequencing analysis of the DGGE bands disclosed the relative abundance of seven bacterial genotypes in the Alteromonadaceae and Roseovarius that gradually decreased with the glucose enrichment while two Vibrio genotypes showed the reverse increasing trend. This suggested that Vibrio was a more successful opportunist at high carbon availability.
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