THE IMPACT OF INORGANIC TIN ON THE PLANKTONIC CYANOBACTERIUM SYNECHOCYSTIS-AQUATILIS - THE EFFECT OF PH AND HUMIC-ACID

The influence of inorganic tin compounds on the unicellular cyanobacte rium Synechocystis aquatilis was studied, and its dependence on changi ng pH of the surrounding medium and the presence of humic acid. Both S n(II) and Sn(IV), used as chlorides (at the concentrations 1-l0 mg lit re(-1)), inhibit the growth and chlorophyll a content of the cyanobact erium cultures, but only under alkaline conditions. Generally, the obs erved tin toxicity increased with increase of metal concentration, tim e of exposure and pH value of the medium (in the range 7-9.8). Sn(II) seems to be more toxic than Sn(IV). At the lowest studied metal concen tration (1 mg litre(-1)), Sn(II) caused a 36 and 40% decrease in growt h and chi a content, respectively, after 96 h exposure at pH 9.8, whil e Sn(IV) caused even a slight increase of both physiological parameter s (hermetic effect). Similar increases in growth and chi a content wer e also observed at a high Sn (II) and Sn(IV) concentration (10 mg litr e(-1)), but only in cultures exposed to metal at pH 7. At high pH (9.8 ), 10 mg litre(-1) of Sn(II) and Sn(IV) significantly suppressed both the growth of the cyanobacterium (by 54.2 and 26.1%, respectively) and the chi a content in cultures (by 58.2 and 24%, respectively). Humic acid reduced the toxicity of tin towards the cyanobacterium. The obser ved effects of pH and complexing ligand on the inorganic tin toxicity are discussed in the context of changing, chemical metal speciation an d bioavailability. (C) 1997 Elsevier Science Ltd.