Quantitative ultrastructural changes associated with lead-coupled luxury phosphate uptake and polyphosphate utilization

Abstract

Quantitative electron microscopy (stereology) was used to assess the ultrastructural response of three algae representative of the classes Chlorophyceae, Cyanophyceae, and Bacillariophyceae to lead-coupled polyphosphate degradation. The organisms were exposed to a culture medium concentration of 20 ppb Pb for 3 hr at the time of luxury phosphate uptake and subsequently transferred to phosphorus and lead-free medium. A differential sensitivity was observed as follows: Plectonema > Scenedesmus > Cyclotella . In Plectonema and Scenedesmus , detrimental cytological changes were observed when the polyphosphate relative volume dropped below 0.5%, which was approximately the P-starvation level of polyphosphate. Few significant ultrastructural changes were observed in Cyclotella after one week in P-deficient medium. At this time, the relative volume of polyphosphate was still 1.5%. Although a few significant ultrastructural changes occurred with phosphate deprivation, the greatest numbers of changes occurred in cells that had been exposed to a short-term (3 hr) low level of Pb. Changes in the relative volume of polyphosphate in all three organisms suggest that Plectonema and Scenedesmus have higher phosphate nutrient requirements than Cyclotella . The ecological implications of metal sequestering by polyphosphate are discussed.