BIOSORPTION OF TRIBUTYLTIN AND OTHER ORGANOTIN COMPOUNDS BY CYANOBACTERIA AND MICROALGAE

Biosorption of triorganotin compounds by the cyanobacteria Synechocystis PCC 6803 and Plectonema boryanum and the microalga Chlorella emersonii, incubated in 2-(N-morpholino)ethanesulphonic acid (MES) buffer, pH 5.5, in the presence of 0.5 mm organotin (supplied as chlorides), increased with molecular mass of the organotins, the order being triphenyltin > tributyltin (Bu3SnCl) > tripropyltin greater-than-or-equal-to trimethyltin greater-than-or-equal-to triethyltin. In the butyltin series, monobutyltin biosorption was lowest, although levels of dibutyltin uptake were greater than for Bu3SnCl. Cyanobacterial Bu3SnCl biosorption was complete in 5 min with no subsequent accumulation. In contrast, a second phase of uptake in C. emersonii resulted in an approximate 2.4-fold increase in cellular Bu3SnCl between 5 min and 2 h. The external pH had a marked influence on biosorption of Bu3SnCl by Synechocystis PCC 6803 and P. boryanum, with maximal uptake at pH 5.5 and 6.5, respectively. Effects of pH were less evident in C. emersonii. In all the organisms examined, no inhibition of Bu3SnCl biosorption was observed between 0.05 and 50 mm NaCl. However, an increase in the external NaCl concentration from 50 to 500 mm resulted in an approximate 55-65% reduction in Bu3SnCl uptake. Biosorption increased at increasing Bu3SnCl concentrations (0.25-3.0 mm). Saturation of Bu3SnCl biosorption at the higher concentrations was most evident in the cyanobacteria, although uptake levels were greater in these organisms at less-than-or-equal-to 2 mm Bu3SnCl. Theoretical maximum biosorption levels at complete cell saturation, derived from reciprocal Langmuir plots, were approximately 565, 525 and 1050 nmol Bu3SnCl mg dry weight, for Synechocystis PCC 6803, P. boryanum and C. emersonii, respectively.