# Boundary conditions for exploiting the cooperation of Aminobacter niigataensis MSH1 with Piscinibacter sp. K169 to support 2,6-dichlorobenzamide biodegradation in sand filters for drinking water treatment: role of cell density and organic carbon

## TL;DR

This study explores how two bacteria work together to break down a harmful chemical in drinking water treatment systems, without needing extra carbon sources.

## Contribution

The paper is the first to examine boundary conditions for bacterial mutualism in bioaugmentation for water treatment.

## Key findings

- Cooperation between Aminobacter niigataensis MSH1 and Piscinibacter sp. K169 supports BAM degradation without added carbon.
- The mutualistic interaction remains effective at low cell densities, addressing bioaugmentation challenges.
- Organic carbon on sand, not acetate, drives the cooperative interaction between the two bacterial strains.

## Abstract

Aminobacter niigataensis MSH1 mineralizes the groundwater micropollutant 2,6-dichlorobenzamide (BAM) and is a candidate for bioaugmentation of sand filters in drinking water treatment plants (DWTP) to avert BAM-contamination. Piscinibacter sp. K169 is a sand filter isolate that improves MSH1-mediated BAM mineralization through a cooperative interaction, and co-inoculation of MSH1 with K169 is proposed as a strategy to support bioaugmentation with MSH1. In this study, boundaries regarding the initial population size and the supply of organic carbon resources determining the interaction between MSH1 and K169 in sand filter microcosms were explored. The cooperative interaction was only disturbed when initial cell densities of one of the two partners were 104 cells/mL or lower. Supplying acetate as a carbon source appeared redundant for supporting BAM mineralization. Instead, the organic carbon present on the sand drove the cooperative interaction between K169 and MSH1 as the effect of K169 on BAM mineralization disappeared, and none of the two strains showed growth in sand devoid from organic carbon. These findings highlight the feasibility of K169-assisted bioaugmentation with MSH1 under realistic field conditions, as it requires no supplementary organic carbon and remains effective, even at relatively low inoculum densities, thereby addressing key challenges in bioaugmentation strategies.

Bioaugmentation of sand filters exploited in drinking water treatment, with the BAM catabolic strain Aminobacter niigataensis MSH1, has previously been successful during the first 1–2 weeks, where after BAM degradation deteriorated together with the loss of MSH1 cell density and cell activity. Bacterial isolates obtained from sand filters can support BAM degradation activity by MSH1 involving mutualistic interactions which resulted in the proposition of a novel bioaugmentation approach involving the co-inoculation of “support” bacteria that are adapted to the target environment. This paper focuses on understanding the boundary conditions required for sustaining the mutualistic interaction between MSH1 and such a “supportive” sand filter isolate in sand microcosm, showing that the interaction could be maintained when using relatively low cell densities and with no additional carbon supplemented. To the best of our knowledge, this paper is the first study to examine the boundary conditions of a bacterial mutualistic interaction, particularly in a bioaugmentation context of water treatment.