Biodegradation of s-triazine herbicides by soil microorganisms and their application for in situ bioremediation

Moderately persistent s-triazine compounds are widely used in agriculture for the control of various weeds. In this review, microbial degradation of chloro- and methylthio-s-triazines and a recent attempt to utilize isolated simazine- degrading bacteria for in situ bioremediation are described. As microorganisms capable of degrading chloro-s-triazines, atrazine-degrading Pseudomonas sp. strain ADP has been studied most extensively. Dechlorination of the s-triazine ring by the strain is mediated by atrazine chlorohydrolase (AtzA) and the subsequent degradation is associated with AtzBCDEF. A novel simazine-degrading beta-proteobacterium strain CDB21 isolated by the author's group also possessed entire-set of genes of these enzymes (atzABCDEF). However, while atzA is located between ORF30 and atzB in strain ADP, ORF30 fused with atzB in strain CDB21. This indicates that localization of atzA in CDB21 differs from that in the pseudomonad, meaning that genetic variations regarding the degradation genes exist among microorganisms capable of degrading chloro-s-triazines. It is known that methylthio-s-triazines can be transformed to the corresponding hydroxy analogues via sulfur oxidation in aerobic and flooded soil. Recently, two types of bacteria that engage the process were isolated. One is Bacillus cereus strain JUN7 that can degrade methylthio-s-triazines to give the methylsulfinyl and hydroxy analogues under nutrient-rich conditions such as those with the Luria-Bertani medium. The other is Rhodococcus sp. strain FJ1117YT that is able to degrade methylthio-s-triazines supplied as the sole sulfur source. These strains were unable to degrade chloro-s-triazines. No gene encoding AtzA or TrzN (triazine hydrolase) was found in these strains, suggesting that they may have another metabolic system. Using a special charcoal material enriched with a simazine-degrading bacterial consortium involving strain CDB21, in situ bioremediation study was conducted in a golf course, where simazine is routinely applied for preservation of turf. As a result, the material was effective for preventing penetration of simazine into subsoils and aquatic environments nearby for approximately 2 years. This result suggests that use of adequate habitat materials is preferable to maintain population and activity of inoculant bacteria applied to soil environment.