Differential responses of growth, photosynthesis, respiration, and phosphate uptake to copper in copper-tolerant and copper-intolerant strains of Scenedesmus acutus (Chlorophyceae)

Abstract

We detected significant differences in copper toxicity to growth and several physiological processes (e.g., photosynthesis, respiration, and phosphate uptake) in three strains of Scenedesmus acutus f. alternans demonstrating differential sensitivity to copper. The copper-intolerant strain (designated X-72) and the two copper-tolerant strains (designated X-Cu and B-4) were tested in long-term (5-day) growth bioassays and short-term (< 24 h) physiological experiments under both nutrient-sufficient and nutrient-limited conditions. The three strains showed the same pattern of sensitivity to copper for growth and for all physiological processes examined: the copper-intolerant strain, X-72, showed the greatest sensitivity, followed by X-Cu. B-4 was most tolerant. Under nutrient-sufficient conditions, growth was more sensitive to copper than photosynthesis or respiration. Depression of the growth rate to 50% of control occurred at 25, 56, and 175 nmol Cu/106 cells in X-72, X-Cu, and B-4, respectively. Photosynthesis and respiration were inhibited in cultures that were preexposed to algistatic copper dosages for 20 h prior to testing. Both processes were equally inhibited in X-72 (to about 8.5% of controls) and in X-Cu (to about 32% of controls), but respiration was more inhibited than photosynthesis in B-4 (to 47.7 vs. 67.1% of control rates). L-buthionine-S,R-sulfoximine (BSO), a known inhibitor of phytochelatin synthesis, significantly reduced the tolerance of photosynthesis to copper in X-72 but not in X-Cu or B-4. Short-term responses among several physiological processes were also examined under phosphorus and nitrogen limitation. In P-limited cultures, phosphate uptake was depressed by 50% at 3 nmol Cu/106 cells in X-72, and at 5.8 nmol Cu/106 cells in X-Cu and B-4. In N-limited cultures, photosynthesis was more tolerant to copper than in nutrient-sufficient conditions in both X-72 and X-Cu but less tolerant in B-4. Key words: copper toxicity, growth, nutrient limitation, nitrogen, phosphate uptake, photosynthesis, respiration, Scenedesmus.