Growth, composition and metal removal potential of a Phormidium bigranulatum-dominated mat at elevated levels of cadmium

Abstract

Prompted by the fact that interaction of metals with cyanobacterial mats has been little studied, the present study evaluates the response of a cyanobacterial mat, dominated by Phormidium bigranulatum, to elevated concentrations of Cd2+ in the medium. The mat failed to grow at 7μM of Cd2+ when the metal as also the mat inoculum were simultaneously added to the medium right in the beginning of the experiment due to marked sensitivity of P. bigranulatum, the main constituent of the mat, to high concentrations of Cd2+. However, the mat previously grown in Cd2+-free medium for a time period of 1–4 weeks grew successfully when exposed to media containing very high concentrations of Cd2+. Four-week-old mat could grow at 250μM of Cd2+, which has been found toxic to many cyanobacteria and algae by previous researchers. Greater tolerance of older mats to Cd2+ may be due to greater proportion of exopolysaccharides, which are well known to sequester metal ions extracellularly, in them. Whereas the relative proportion of P. bigranulatum declined at high concentrations of the test metal that of green algae increased due most likely to their tolerance to Cd2+. Air bubbles were seen entrapped in the mat due obviously to photosynthetic activity. Elevated concentrations of Cd2+ reduced the number of air bubbles in the mat. Decline in number of air bubbles at high concentrations of metal ions was more prominent in the case of younger mat than in the older one. The present study also evaluated changes in species composition of mats of different age that were subsequently grown in Cd2+ enriched culture medium. Younger mats showed change in species composition at very low concentrations of Cd2+, but older mats showed little changes even at very high concentrations of the test metal. Hence older mats more strongly resisted to changes in its species composition than the younger ones upon exposure to high concentrations of Cd2+. The growing mat successfully removed Cd2+ from the medium, which was greater at lower concentrations of Cd2+ in the external environment.