Contribution of dietary bacteria to metal accumulation in the slipper limpet

Recent studies have shown that dietary ingestion can be an important source for metal accumulation in aquatic animals. Whether bacteria can contribute significantly to the overall metal accumulation in these animals remains l ittle known. In this study, we used the kinetic modeling approach to examin e the relative contribution of the aqueous phase, bacteria, and phytoplankt on, as sources for Cd, Cr, and Zn accumulation by the slipper limpet Crepid ula onyx, a suspension feeder that can capture bacteria efficiently. Metal uptake rate constants measured in C. onyx were 0.200, 1.232, and 1.294 1 g( -1) d(-1) for Cd, Cr, and Zn, respectively. The assimilation efficiency (AE ) of metals from ingested phytoplankton (11 to 44% for Cd, and 31 to 41 % f or Zn) was comparable to AES of metals from ingested bacteria (21 to 42 % f or Cd, 35 to 47 % for Zn), whereas the AEs of Cr from ingested phytoplankto n (10 to 22 %) were lower than from ingested bacteria (44 to 59 %). The AES of Cr from different phytoplankton and bacterial diets were related to its distribution in the cytoplasm of cells and its passage time across the ani mal's guts. The limpets filtered the bacteria at rates (17 1 h(-1) g(-1)) 1 .3 to 1.9x lower than the filtration rate on different phytoplankton diets (22 to 33 1 h(-1) g-(1)). The estimated average bioconcentration factors of metals by 2 bacterial strains were 2 to 6 x 10(5) for Cd, 1 to 7 x 10(5) f or Cr, and 1.5 to 7 x 10(5) for Zn. The kinetic model predicted that uptake from dietary phase dominated metal accumulation in the slipper limpets (87 % for Cd, 72% for Cr, and 83 % for Zn). Ingestion of bacteria contributed 23 % for Cd, 27 % for Cr, and 17 % for Zn accumulation in the limpets under typical conditions for the limpets. Our study therefore highlights bacteri a as a potentially important source of metal accumulation in this filter-fe eding mollusc.