MICROBIAL BIOFILMS AND CATABOLIC PLASMID HARBORING DEGRADATIVE FLUORESCENT PSEUDOMONADS IN SCOTS PINE MYCORRHIZOSPHERES DEVELOPED ON PETROLEUM-CONTAMINATED SOIL

Cellular interactions and catabolic activities of mycorrhizal root ass ociated non-sporulating bacteria were investigated in a simplified phy toremediation simulation involving a woody plant species. Mycorrhizal Scots pine (Pinus sylvestris) seedlings pre-colonised by Suillus bovin us or Paxillus involutus were grown in forest humus containing microco sms amended with petroleum hydrocarbon (PHC) contaminated soil. Fungal hyphae of both species, emanating from mycorrhizal roots, colonised t he PHC contaminated soil over a 16-week period and dense long-lived pa tches of S. bovinus hyphae formed on the PHC contaminated soil. Transm ission electron microscopy revealed a microbial biofilm at the PHC soi l-fungal interface composed of differentiated pseudoparenchymous patch hyphae supporting a morphologically diverse bacterial population. Cer tain non-sporulating bacterial isolates closely associated with the S. bovinus patch hyphae or P. involutus 'web' hyphae from the PHC soil h arboured similar sized mega-plasmids (approx. 150 kb). Isolates of Pse udomonas fluorescens from the 'patch' mycorrhizospheres represented di fferent biovars, displayed similar REP-PCR genomic fingerprints, grew on e.g. m-toluate and m-xylene as sole carbon sources, cleaved catecho l, and harboured plasmid-borne catabolic marker genes, xylE and xylMA, involved in degradation of mono-aromatics. The plasmids were transmis sible in vitro, and Pseudomonas putida transconjugants retained a simi lar catabolic profile. The identification of microbial biofilms contai ning catabolic bacteria in the external mycorrhizosphere is discussed in relation to both phytoremediation mechanisms and normal efficient n utrient mobilisation from highly lignin-rich forest soils. (C) 1998 Fe deration of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.