# Biofilm engineering through c-di-GMP tuning boosts bioleaching efficiency and arsenic tolerance in Acidithiobacillus ferrooxidans

## TL;DR

Scientists improved a bacteria's ability to extract metals and tolerate arsenic by tweaking a key signaling molecule, making bioleaching more efficient and sustainable.

## Contribution

A novel synthetic biology strategy using c-di-GMP modulation to simultaneously enhance bioleaching efficiency and heavy metal resistance in Acidithiobacillus ferrooxidans.

## Key findings

- Engineered strains with increased c-di-GMP showed improved bioleaching efficiency and biofilm formation.
- High c-di-GMP strains exhibited enhanced arsenic tolerance through polysaccharide-rich biofilms.
- Transcriptomic analysis confirmed gene expression changes linked to biofilm composition and metal resistance.

## Abstract

Bioleaching offers a sustainable alternative to conventional metallurgy, but its application is limited by low leaching rates, inhibition by heavy metals, and prolonged adaptation. Here, we engineered Acidithiobacillus ferrooxidans, a model bioleaching microorganism ubiquitous in mining environments, by modulating intracellular bis(3′−5′)-cyclic dimeric guanosine monophosphate (c-di-GMP) signaling to enhance biofilm formation, bioleaching efficiency, and arsenic tolerance. Overexpression of diguanylate cyclase genes AFE_1379, AFE_0053, and AFE_1373 produced engineered strains S-222, S-306, and S-651, respectively, with 1.7-, 2.5-, and 5-fold higher intracellular c-di-GMP levels than the control carrying the empty plasmid vector. Under arsenic-free conditions, all engineered strains showed similar growth profiles, but S-306, at intermediate c-di-GMP (306.3 ± 28.1 μg mg−1), formed cytochrome-rich biofilms with low internal resistance and achieved the highest bioleaching efficiency. Under arsenic stress, S-651, at high c-di-GMP (651.4 ± 15.5 μg mg−1), developed polysaccharide-rich biofilms that enhanced arsenic tolerance, scorodite (FeAsO₄·2H₂O) precipitation, and bioleaching performance. Transcriptomic analysis confirmed these strain-specific gene expression patterns. These findings demonstrate that tuning intracellular c-di-GMP enables A. ferrooxidans to reprogram biofilm matrix composition for extracellular electron uptake and heavy-metal resistance, providing a synthetic biology strategy for environmentally friendly bioleaching and tailings recycling.

As a model microorganism for bioleaching, Acidithiobacillus ferrooxidans is limited in leaching efficiency by several key constraints, including slow biofilm formation and susceptibility to environmental heavy metals. Although genetic engineering has been widely used to tackle these challenges, conventional strategies typically focus on modifying one single trait at a time, which significantly restricts their industrial applicability. In this study, we present a novel approach that overcomes this limitation through targeted modulation of the global regulatory molecule c-di-GMP. Engineering this upstream signaling pathway allowed for the tunable enhancement of both bioleaching efficiency and heavy metal resistance, providing an integrated strategy to address multiple bottlenecks simultaneously. This work offers a versatile and practical biotechnology route for diverse scenarios to enhance bioleaching performance and environmental adaptability, which may facilitate the utilization of low-grade ores and mining tailings and ultimately contribute to more sustainable and circular metal production.