Physiological ecology of cadmium uptake and accumulation in phytoplankton and culture Pacific oysters

Pacific oyster ( Crassostrea gigas ) aquaculture is an important shellfish industry in British Columbia, with regular export to Asia and the USA.   Since 1999, however, several shipments of BC oysters have been rejected in Hong Kong, as a result of cadmium (Cd) levels in the organisms being higher than maximum tolerable limits (> 2 ppm by wet weight).   In potential foreign markets such as the EU, the Cd tolerance levels in the oysters are even lower (< 1 ppm).   In order for the BC shellfish growers to be competitive, re-establish markets in Hong Kong and initiate sales in the EU, this crucial Cd issue needs to be addressed.   The situation is difficult to resolve because, at present, BC oyster growers have no information on the origin or cause of high Cd levels sometimes detected in their product.   To resolve this issue, it is imperative to determine the main inputs of Cd in waters of the aquaculture farms, including their temporal and spatial variation, as well as identify possible Cd pathways from seawater to the oysters.  

The high levels of Cd in oysters since 1999 may be due to either higher concentrations of Cd in the waters of the BC coast, or to major variations in the pathways of Cd from seawater to the oysters, associated with changes in phytoplankton community structure in the BC waters in the last five years.   Sporadic historical data suggest that the concentration of seawater Cd along the BC coast has not changed dramatically in the past 30 years.   More recent data indicate great variability in the concentrations of Cd in the waters of the Straight of Georgia, and no systematic trends on Cd levels in the waters of the BC coast.   The emerging picture suggests that the high Cd levels measured sometimes in BC Pacific oysters since 1999 may not be due to dramatic changes in inputs of Cd into BC waters, but instead, may simply reflect changes in transfer pathways of Cd from seawater to the oysters.   The uptake of Cd by phytoplankton and subsequent transfer to filter feeders is highly likely, although remains to be tested in BC oyster aquaculture.

Phytoplankton are known to accumulate high concentrations of metals intracellularly.   Moreover, phytoplankton species differ greatly in the degree to which they are able to exclude toxic metals.   Therefore, the concentrations of Cd in phytoplankton are highly dependent on the species.   For example, diatoms ( Bacillariophycea e) have been demonstrated to accumulate relatively less Cd than other phytoplankton taxa such as Prymnesiophyceae or Chlorophyceae , at identical concentrations of dissolved Cd.   Variations in the response of different phytoplankton classes to toxic metals are not completely surprising because species vary in the types and amount of metal-binding complexes they produce to detoxify the metal.   If phytoplankton are indeed the most important vector of Cd into the oysters, changes in phytoplankton community structure throughout the year may have a major impact in the variability of Cd levels in the oysters.

The main objective of this project is to provide management tools and options to BC oyster growers to avoid systematic harvest of oysters with high levels of Cd.   The first objective of this project is to examine the physiological ecology of Cd assimilation by various phytoplankton species naturally abundant in BC waters.   We are presently enumerating and identifying phytoplankton species bimonthly at two BC oyster farms during one annual cycle.   The dominant species are then isolated in the laboratory, in order to determine their intracellular Cd levels under relevant Cd concentrations.   The second objective is to investigate the differential accumulation of Cd in the oysters as a result of changes in phytoplankton community structure throughout the year.   These experiments will establish which phytoplankton species account for the highest transfer of Cd from seawater into the oysters.   In the future, oyster growers will be able to monitor blooms of these relevant phytoplankton species in the field, and may be able to avoid harvesting oysters during or following these phytoplankton events.   In addition, these experiments will provide information on the ability of different oyster organs/tissues to accumulate Cd, and may allow appropriate design of Cd depuration practices prior to shipment of the oysters.

last updated October 28, 2004