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 little known. In this study, we used the kinetic modeling approach to examine the relative contribution of the aqueous phase, bacteria, and phytoplankton, as sources for Cd, Cr, and Zn accumulation by the slipper limpet Crepidula 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% for Zn) was comparable to AEs of metals from ingested bacteria (21 to 42% for Cd, 35 to 47% for Zn), whereas the AEs of Cr from ingested phytoplankton (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 animal's guts. The limpets filtered the bacteria at rates (17 1 h^-1 g^-1) 1.3 to 1.9× 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 × 10⁵ for Cd, 1 to 7 × 10⁵ for Cr, and 1.5 to 7 × 10⁵ 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 bacteria as a potentially important source of metal accumulation in this filter-feeding mollusc.