~ Pergamon 0043-1354(95)00189-1 Wat. Res. Vol. 30, No. 2, pp. 481--485, 1996 Copyright © 1996 ElsevierScienceLtd Printed in Great Britain. All rights reserved 0043-1354/96 $15.00 + 0.00 RESEARCH NOTE BIODEGRADATION OF MICROCYSTIN-LR BY INDIGENOUS MIXED BACTERIAL POPULATIONS I. T. COUSINS*, D. J. BEAL1NG, H. A. JAMESf and A. SUTTON WRc plc, Henley Road, Medmenham, Marlow, Bucks SL7 2HD, England (First received July 1994; accepted in revised form August 1995) Abstract--Microcystin-LR is a potent mammalian toxin which is known to have been responsible for the deaths of domesticated animals, and consequently there is concern as to its environmental fate. In laboratory experiments conducted using low levels (10 #g I-~) of microcystin-LR in reservoir water, primary biodegradation of the toxin was shown to occur in less than 1 week. An experiment designed to ascertain the degree of mineralisation of microcystin-LR, using natural microbial populations under aerobic conditions, showed that the peptide ring appears to be fairly resistant to biodegradation, as the degree of mineralisation was not high. However, as the toxicity of microcystin-LR is critically dependent on the orientation of the peptide ring with respect to the "Adda" (the fl-amino acid which is unique to the blue-green algal toxins) side-chain, and it appears that this side chain is affected during biodegradation, it seems likely that a reduction in toxicity occurs. Key words--blue-green algae, algal toxins, microcystin-LR, biodegradation INTRODUCTION Microcystin-LR is a toxic cyclic heptapeptide which may be produced by some strains of various blue-green algae e.g. Microcystis, Oscillatoria and Anabaena species. Low levels of the toxin may be released into water bodies during the normal growth and turnover of blue-green algal populations and high levels may result during algal blooms. Deaths of domestic animals, following the ingestion of toxic blue-green algal scums from algal blooms, have demonstrated that under certain circumstances very high concentrations of the toxin are produced and can be lethal. Information regarding the persistence of this toxin is necessary to allow decisions to be taken on the use of water bodies for recreational purposes following blue-green algal blooms. Such information is also necessary to allow decisions to be taken on the requirements for water treatment. Although microcystin-LR is the most commonly found microcystin (Carmichael, 1992), many other microcystins are known, and blue-green algae which produce hepatotoxins usually produce a mixture of different microcystins. However, as they are all cyclic heptapeptides containing an unusual B-amino acid (3 - amino - 9 - methoxy - 10 - phenyl - 2,3,8 - trimethyl - deca-4,6-dienoic acid) (Adda), it seems likely that their degradative behaviour would be similar. Recent work (Jones and Orr, 1994) showed that Microcystis aeruginosa persisted for 9 d in a recreational lake before degradation commenced. After this initial lag phase, however, degradation was rapid with 90-95% in 3d. This paper presents the results from experiments designed to study the conditions under which microbial degradation of microcystin-LR occurs. A recent study (Block et al., 1992) showed that indigenous mixed bacterial populations (of sus- pended or attached bacteria) were suitable for biodegradative studies so water and sediment samples were obtained from a reservoir which has experienced algal blooms in the past, for use in laboratory degradation experiments. The first of these experiments consisted of an analytical study to assess the rate of primary biodegradation of microcystin-LR in reservoir water. This was followed by an "ultimate" biodegradation study to establish the extent of mineralisation which occurred, as a high degree of mineralisation would indicate that the degradation products would be of little concern. *Present address: Institute of Environmental and Biological Sciences, Lancaster University, Lancaster LAI 4YQ, England. tAuthor to whom all correspondence should be addressed. METHODS Analytical Analysis for microcystin-LR was carried out using a published analytical method (James and James, 1991). 481