Surface-Adsorbed Contaminants Mediate the Importance of Chemotaxis and Haptotaxis for Bacterial Transport Through Soils

Abstract

Chemotaxis and haptotaxis are important biological mechanisms that influence microbial movement toward concentrated chemoattractants in mobile liquids and along immobile surfaces, respectively. This study investigated their coupled effect, as induced by naphthalene (10 mg L^-1), on the transport and retention of two pollutant-degrading bacteria, Pseudomonas fluorescens 5RL (Pf5RL) and Pseudomonas stutzeri DQ1 (PsDQ1), in quartz sand and natural soil. The results demonstrated that PsDQ1 was not chemotactic, whereas Pf5RL was chemotactic at 25°C but not at 4°C due to the restricted movement. In a quartz sand column, haptotaxis did not play a role in increasing the transport of Pf5RL as compared with chemotaxis. Compared with a naphthalene-free soil column, Pf5RL broke through naphthalene-presaturated soil columns to reach a stable effluent concentration 0.5 pore volumes earlier due to advective chemotaxis occurring behind the plume front in the bulk solution. Pf5RL also demonstrated greater retention (e.g., a doubled rate of attachment and a one-third smaller breakthrough percentage) due to along-surface haptotaxis and near-surface chemotaxis occurring in less mobile water near the soil surface. However, both chemotaxis and haptotaxis were weakened when Pf5RL co-transported with naphthalene due to reduced adsorption of naphthalene on the soil. This study suggests that surface adsorption of naphthalene can mediate the relative importance of advective chemotaxis (facilitating initial breakthrough), near-surface chemotaxis (increasing bacterial collision), and haptotaxis (increasing bacterial residence time).

Keywords: bacterial transport; chemotaxis; haptotaxis; naphthalene; saturated flow.

Figure 1

Kinetics (A) and equilibrium (B) adsorption of naphthalene on sand and soil at 25°C. Error bars represent the standard deviation of two replicates.

Figure 2

Bioluminescent images of the distribution of Pf5RL in a sand and soil cross-section during co-transport with naphthalene and after elution. Images represent a series of two independent experiments. A: zero time point image of the sand and soil cross-section saturated with naphthalene and injected with Pf5RL. B: 4-h time point of same cross-sectional area shows high levels of bioluminescence being produced by Pf5RL due to exposure to naphthalene. C: image acquired after eluting the cross-section area with background solution to flush out naphthalene. Bioluminescent signal indicates that Pf5RL cells have been retained within the soil interface. D, E, F: images of cross-section area 1, 2, and 3 h respectively after the termination of the background solution flushing event. G: image of cross-section area after re-introduction of nine pore volumes of a naphthalene-free suspension of Pf5RL cells. p/s/cm²/sr: photon/s/cm²/steradian.

Figure 3

Bioluminescent images of the movement of Pf5RL in a 0.2-mm air-dried layer of sterilized chestnut soil colloids uniformly deposited on duplicate glass slides (2.5 cm × 7.6 cm). The soil colloid layer was divided into three parallel strips with strip N0 representing areas with no naphthalene soil adsorption, strip N1 representing an area of low naphthalene soil adsorption (47 mg kg⁻¹), and strip N2 representing an area of high naphthalene soil adsorption (90 mg kg⁻¹). The circle represents the area on the slide where a drop of Pf5RL cell suspension was placed between the boundary line of N0 and N1. Bioluminescent images of the slide were then acquired in the IVIS Spectrum instrument at time points of 1, 10, 60, 120, 300, and 600 min after placement of the Pf5RL cell suspension. After the 600 min time point, a 10 mg L⁻¹ drop of naphthalene was placed in the N0 area and an image was taken at either 610 or 630 min. The resulting lack of bioluminescence within this area of naphthalene addition indicated that the Pf5RL cells did not move into the N0 area of the soil colloid layer.

Figure 4

Conceptual diagram of four input modes of naphthalene and bacteria, which represent their different co-existence situations and different distribution ratios of naphthalene between immobile solid phase and mobile liquid phase within the columns during transport process and allow evaluation of the relative importance of chemotaxis and haptotaxis to bacterial transport. (A) Input Mode 1: injection of bacteria only (10 pore volumes), (B) Input Mode 2: co-injection of bacteria and naphthalene (10 pore volumes), (C) Input Mode 3: injection of naphthalene (20 pore volumes) immediately followed by injection of bacteria (10 pore volumes), (D) Input Mode 4: injection of naphthalene (20 pore volumes) followed by a flush with background solution (15 pore volumes) and then injection of bacteria (10 pore volumes). K_d is the partition coefficient of naphthalene between the solid phase and aqueous phase. The ‘“High”, “medium” and “low” refer to general’ concentration gradients of naphthalene in solid and liquid phases.

Figure 5

Transport of bromide (Br⁻¹) (A) and naphthalene through sand (B) and soil (C) columns during injection of bacteria. Input modes are described in Figure 4. Mode 2, co-injection of bacteria and naphthalene; Mode 3, injection of naphthalene immediately followed by injection of bacteria. Error bars represent the standard deviation of two replicates.

Figure 6

Transport of chemotactic Pf5RL through soil at 25°C (A), sand at 25°C (B), soil at 4°C (C), and sand at 4°C (D) under different input modes. Input Mode 1, injection of bacteria only; Input Mode 2, co-injection of bacteria and naphthalene; Input Mode 3, injection of naphthalene immediately followed by injection of bacteria; Input Mode 4, injection of naphthalene followed by a flush with background solution and then injection of bacteria. Error bars represent the standard deviation of two replicates.