Effects of light on microalgae concentrations and selenium uptake in bivalves exposed to selenium-amended sediments

The purpose of the current study was to evaluate the role of visible light during trophic transfer of the essential metalloid, selenium (Se) from sediments by way of the microalgae Phaeodactylum tricornutum to bivalves. Initial experiments compared uptake of Se with algal cell number after deposit-feeding (Tellina deltoidalis) and filter-feeding (Soletellina alba) bivalves were exposed for 12 days under 16:8 h light:dark and 24-hour dark regimes to sediments amended with Se (17 mg/kg dry weight). Comparisons of filter-feeding with deposit-feeding organisms indicated > 2-fold higher body burdens in the deposit-feeding organisms after 12 days. Darkness failed to significantly and consistently diminish algal cell densities. However, general trends indicated a decrease of pelagic algal numbers in +Se/dark treatments during the 12-day exposure. To determine whether time-dependent patterns were present in the uptake of Se by the deposit-feeding species, a second experiment was conducted using Se-laden sediments under light and dark conditions. Algal cell counts were measured and Se concentrations determined in animals sampled at days 0, 3, 6, 9, and 12. Deposit-feeding animals maintained in the dark accumulated, Se more rapidly than animals on light to dark cycles until 12 days at which time no significant differences were observed between treatments. Concentrations of Se in tissues of deposit-feeding bivalves were not directly related to algal cell density in lighted or dark chambers. These results indicate that pelagic microalgae may only play a key role in Se uptake during early hours of exposure, and feeding behavior may be a more important factor in deposite-feeding bivalves.