Quantitative Ultrastructural Changes Associated with Lead-coupled Luxury Phosphate Uptake and Polyphosphate Utilization

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 shortterm (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. Polyphosphate is a ubiquitous long-chained linear polymer present in a wide variety of plants and animals, although it is most often found in bacteria and unicellular algae (Harold 1966; Sicko 1974). In 1975, Crang and Jensen reported the incorporation of titanium into polyphosphate bodies of a bluegreen alga, Anacystis nidulans during the ~polyphosphate overplus" phenomenon which has been described by many authors (Voelz et at. 1966; Jensen and Sicko 1974; Sicko-Goad and Jensen 1976). Subsequent studies revealed that a wide variety of metals (Pb, Mg, Zn, Cd, St, Co, Hg, Ni, Cu) were sequestered in polyphosphate bodies in a variety of algae, including blue-greens, greens, and diatoms, both in cultures and in natural assemblages (Sicko-Goad and Stoermer 1979; Baxter and Jensen 1980; Jensen et al. t982a, !982b; Stoermer et al. 1980). Polyphosphate functions primarily as a phosphorus reserve (Sicko 1974). It can be formed under several distinct nutritional conditions: 1) Restoration of phosphate following a phosphate deficiency (Jensen and Sicko 1974; Sicko-Goad and Jensen 1976); 2) Nutrient imbalance other than phosphorus (Lawry and Jensen 1979; Smith et at. 1954; Spitznagel and Sharp 1959; Voelz et al. 1966); and 3) Disturbance of nucleic acid metabolism (Harold 1966). The fate and ecological significance of polyphosphate is not well understood. If polyphosphate is metabolized under conditions of limiting phosphorus then, phosphorus limitation should result in polyphosphate degradation and liberation of sequestered metals. Experiments were conducted to determine both the time sequence and consequences of polyphosphate degradation in three cultured algae and the effects of lead exposure at the time of luxury phosphate uptake. The results of these experiments are presented in this paper. Materials and Methods Three algae were selected for study: Cyclote![a meneghiniana (Bacf l lar iophyceae) , Scenedesmus quadricauda (Chlorophy618 L. Sicko-Goad and D. Lazinsky S,..v..on )[ "., . .e )