Effects of the humic substances of de-inking paper sludge on the antagonism between two compost bacteria and Pythium ultimum

Authors

  • Marie-Hélène Charest,

    1. Département de Phytologie, Faculté des Sciences de l’ Agriculture et de l’ Alimentation, Pavillon Paul-Comtois, Université Laval, Sainte-Foy, Que., Canada G1K 7P4
    2. Recherches en Sciences de la Vie et de la Santé, Pavillon Charles-Eugène Marchand, Université Laval, Sainte-Foy, Que., Canada G1K 7P4
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  • Chantal J. Beauchamp,

    1. Département de Phytologie, Faculté des Sciences de l’ Agriculture et de l’ Alimentation, Pavillon Paul-Comtois, Université Laval, Sainte-Foy, Que., Canada G1K 7P4
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  • Hani Antoun

    Corresponding author
    1. Recherches en Sciences de la Vie et de la Santé, Pavillon Charles-Eugène Marchand, Université Laval, Sainte-Foy, Que., Canada G1K 7P4
      *Corresponding author. Tel.: +1 418 656 2131x3650; fax: +1 418 656 7176. antoun@rsvs.ulaval.ca
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*Corresponding author. Tel.: +1 418 656 2131x3650; fax: +1 418 656 7176. antoun@rsvs.ulaval.ca

Abstract

We investigated the in vitro influence of humic substances (HS) extracted from de-inking paper sludge compost on the inhibition of Pythium ultimum by two compost bacteria, Rhizobium radiobacter (Agrobacterium radiobacter) and Pseudomonas aeruginosa. When low concentrations (5 or 50 mgzl−1) of HS were added to the culture medium, fungal inhibition by R. radiobacter significantly increased (P < 0.01) by 2–3%. In contrast, these low levels of HS had no effect on P. ultimum inhibition by P. aeruginosa. The Fe, chelated by HS, was in part responsible for the decrease of P. ultimum inhibition by the bacteria when increasing amounts of HS were added in the culture medium. The addition of 500 mgl−1 of humic acids isolated from de-inking paper sludge compost or from fossil origin completely eliminated the inhibition of P. ultimum by R. radiobacter. This Fe effect also stimulated growth of R. radiobacter and reduced its siderophore production in a minimal medium supplemented with HS as sole source of Fe. The results showed that HS influence microbial antagonism when added to a culture medium. However, this effect varies with different factors such as the type of bacteria, concentration of HS, molecular weight and Fe content.

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