Citrate and malonate increase microbial activity and alter microbial community composition in uncontaminated and diesel-contaminated soil microcosms

Abstract

Abstract. Petroleum hydrocarbons (PHCs) are among the most prevalent sources of environmental contamination. It has been hypothesized that plant root exudation of low molecular weight organic acid anions (carboxylates) may aid degradation of PHCs by stimulating heterotrophic microbial activity. To test their potential implication for bioremediation, we applied two commonly exuded carboxylates (citrate and malonate) to uncontaminated and diesel-contaminated microcosms (10 000 mg kg −1 ; aged 40 days) and determined their impact on the microbial community and PHC degradation. Every 48 h for 18 days, soil received 5 µmol g −1 of (i) citrate, (ii) malonate, (iii) citrate + malonate or (iv) water. Microbial activity was measured daily as the flux of CO 2 . After 18 days, changes in the microbial community were assessed by a community-level physiological profile (CLPP) and 16S rRNA bacterial community profiles determined by denaturing gradient gel electrophoresis (DGGE). Saturated PHCs remaining in the soil were assessed by gas chromatography-mass spectrometry (GC-MS). Cumulative soil respiration increased 4-to 6-fold with the addition of carboxylates, while diesel contamination resulted in a small, but similar, increase across all carboxylate treatments. The addition of carboxylates resulted in distinct changes to the microbial community in both contaminated and uncontaminated soils but only a small increase in the biodegradation of saturated PHCs as measured by the n-C17 : pristane biomarker. We conclude that while the addition of citrate and malonate had little direct effect on the biodegradation of saturated hydrocarbons present in diesel, their effect on the microbial community leads us to suggest further studies using a variety of soils and organic acids, and linked to in situ studies of plants, to investigate the role of carboxylates in microbial community dynamics.

The carboxylate composition of rhizosheath and root exudates from twelve species of grassland and crop legumes with special reference to the occurrence of citramalate

“…The exudation rate for C. arietinum (C) would allow accumulation of the carboxylate amounts present in the rhizosheath in <1 h. This rapid exudation could perhaps be a response to fast microbial degradation, as found for citrate and malonate (Martin et al 2016), or losses through soil sorption (Suriyagoda et al 2016). However, these factors would have had relatively little impact in the river sand used in our experiment, as soil sorption and microbial degradation are minimal (Ryan et al 2012).…”

“…However, there are many other interactions that involve root exudates in the rhizosphere. These include, but are not limited to, chemotaxis (essential for legume nodulation) (Currier and Strobel 1976), as well as stimulation or inhibition of fungal and microbial communities, including root pathogens (Barber and Lynch 1977;Martin et al 2016;Přikryl and Vančura 1980;Rovira 1959;Rudrappa et al 2008;Weisskopf et al 2006). These interactions with root exudates may be of equal importance to nutrient acquisition as the preservation of the root itself is vital for efficient nutrient uptake.…”

Low Light Availability Alters Root Exudation and Reduces Putative Beneficial Microorganisms in Seagrass Roots

“…One of the most important ways in which microbial composition can be regulated from the host level in terrestrial plants is via the exudation of compounds from plant roots. Exudates can shape communities that are specific to the plant species or even plant genotype by selectively stimulating or inhibiting particular microbial populations ( Haichar et al, 2012 ; Chaparro et al, 2014 ; Alegria Terrazas et al, 2016 ; Kawasaki et al, 2016 ; Martin et al, 2016 ). This root exudate–microbe interaction has been demonstrated to some extent for seagrass species.…”

Bacterial Isolate Inhabiting Spitsbergen Soil Modifies the Physiological Response of Phaseolus coccineus in Control Conditions and under Exogenous Application of Methyl Jasmonate and Copper Excess

“…LMWOAs may be crucial for achieving an appropriate concentration of metals and non-metals in plant tissues by their involvement in the sequestration or translocation of elements [56]. Much more evidence is published for the contribution of LMWOAs in increasing metal tolerance [21,39,40] than their role in inoculation [57] or in the environment of J [5]. LMWOAs can be produced in high quantities by plants, especially in roots and seeds, but also in low quantities by soil microorganisms, which means that these acids, as root exudates, are involved in plant–microorganism interactions [58].…”