Cadmium chelation by frustulins: a novel metal tolerance mechanism in Nitzschia palea (Kützing) W. Smith.

The ubiquity of diatom distribution, species richness, short generation time, and specific sensitivity to several environmental stressors such as metals, make diatoms particularly useful for scientific studies. Anthropogenic activities have increased the concentration of metals in air, soil and water. Due to their toxicity and persistent character, the effects of metals on organisms have been extensively studied. In this work, the association of cadmium to different extracellular molecules of Nitzschia palea cells was investigated. Cells were grown in the absence and presence (0.2 mg l(-1)) of cadmium in Chu no. 10 medium. Extracellular polysaccharides were extracted, and subsamples were used for polysaccharide and Cd determination. The frustules were broken mechanically under liquid nitrogen and the intracellular and frustule fractions separated. Frustulins, a protein family found on the outmost frustule layer, constituting a protection coating to environmental stress, were extracted. In each fraction proteins were quantified by the BCA method and separated by gel electrophoresis (SDS-PAGE). Cadmium associated to each fraction was quantified by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analysis. Exposure of Nitzschia palea to cadmium decreased extracellular polysaccharides by 52.8 % and increased 6 times the amount of frustulins. Cadmium was mostly retained extracellularly: 85.4 % was bound to the frustulin fraction, and 11.1 % to polysaccharides. The ability of Nitzschia palea to increase the production of frustulins due to the presence of Cd, the extracellular location of this frustulin coating and the ability of these proteins to bind Cd, suggests a new cellular defense mechanism to metals unknown until now.