Lead toxicity in cyanobacterial porphyrin metabolism

The effect of Pb2+ on growth, tetrapyrrole photosynthetic pigment content, total free porphyrin, and 5-aminolevulinate dehydratase (ALA-D) activity of a cyanobacterium, Microchaete tenera, and its ability to sequester Pb2+ fr om the culture medium were studied. Pb2+ was assayed by graphite furnace at omic absorption spectrophotometry. M. tenera growth and chlorophyll a conte nt were not affected by 0.5, 1.0, and 6.0 ppm of Pb2+. These treatments dou bled the protein content and increased the phycobiliprotein content by four times after 7 days. The ALA-D activity decreased in all concentrations by 63% at day 7 and by 34% at day 14. As a consequence of ALA-D inhibition, to tal free porphyrin also decreased by 64% at day 7 and by 40% at day 14. The highest biomass lead uptake (7454 +/- 565 mug Pb2+/g dry weight) was obser ved at day 3 with 6.0 ppm of Pb2+ in the culture medium. Uptake coefficient was highest (3723 +/- 279 mug Pb2+ g (-) (1) dry weight/ppm of applied Pb2 + ) with 1.0 ppm after 3 days. The increase in protein and antenna pigments on day 7 was probably a response to stress conditions and could explain wh y the toxic metal did not affect growth. ALA-D inhibition and high lead bio mass content confirm the importance of this enzyme as a biological indicato r for stress. (C) 2001 by John Wiley & Sons, Inc.