Abstract: Bacteria can move along pollutant concentration gradients under both liquid and solid phases, facilitating the degradation of contaminants. Migration toward concentrated contaminants along immobilized solid surface is called heptotaxis, which plays a crucial role in the bioremediation of contaminated soils. Soil colloids can influence heptotaxis by affecting the solid-liquid distribution of organic contaminants. In this study, we investigated the effect of soil colloids on bacterial transport mediated by heptotaxis with Pseudomonas fluorescens 5RL (Pf_5RL), a bacterium chemically responsive to naphthalene and its necromass as model bacteria and soil colloids. The results showed that the necromass remained stable in quartz sand, and that neither Pf_5RL nor naphthalene affected its transport behavior, with a maximum breakthrough relative concentration (max C/C₀) of ~0.6. However, the presence of necromass could hinder the transport of Pf_5RL, decreasing the max C/C₀ from 0.97±0.02 to 0.75±0.01. This is primarily attributed to the increased electrostatic attraction which enhanced the attachment of Pf_5RL on the necromass. The presence of naphthalene impeded Pf_5RL transport due to the adsorption of naphthalene on sand, resulting in near-surface chemotaxis and heptotaxis effects, accompanied by a declining max C/C₀ of Pf_5RL from 0.97±0.02 to 0.75±0.04. Under the presence of naphthalene, the necromass further hindered Pf_5RL transport by increasing the adsorption sites for naphthalene, which strengthened near-surface chemotaxis and heptotaxis effects, leading to a further decrease in max C/C₀ of Pf_5RL from 0.75±0.04 to 0.50±0.03. Our results highlight the significant role of necromass in the heptotaxis and chemotactic effects mediated by organic contaminants. It is suggested that necromass may be a crucial factor influencing the in-situ degradation of organic contaminants brought into soils by application of animal manure and reclaimed water.

Key words: bacteria, necromass, heptotaxis, near-surface chemotaxis, transport