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+ from the culture medium were studied. Pb2+ was assayed by graphite furnace atomic absorption spectrophotometry. M. tenera growth and chlorophyll a content were not affected by 0.5, 1.0, and 6.0 ppm of Pb2+. These treatments doubled 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, total free porphyrin also decreased by 64% at day 7 and by 40% at day 14. The highest biomass lead uptake (7454±565 μg Pb2+/g dry weight) was observed at day 3 with 6.0 ppm of Pb2+ in the culture medium. Uptake coefficient was highest (3723±279 μg 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 why the toxic metal did not affect growth. ALA‐D inhibition and high lead biomass content confirm the importance of this enzyme as a biological indicator for stress. © 2001 John Wiley & Sons, Inc. Environ Toxicol 16: 61–67, 2001

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