12 well polystyrene plates

12-well polystyrene plates (NUNC, Roskilde, Denmark) were cast with Muller-Hinton (MH) agar (Oxoid). Briefly, 7.6 g MH agar powder was dissolved in 200 mL water, autoclaved, and poured into each well of the 12-well plates and allowed to solidify.

Thiomonas spp. CB1, CB2 and CB3 were isolated from the Carnoulès AMD (Gard, France) (Bryan et al., 2009; Arsène‐Ploetze et al., 2010). They were grown in vitro on modified 126 medium (m126) as previously described (Bryan et al., 2009). As(III) was added at the concentration required from a sterile stock of 667 mM obtained with NaAsO₂ salts (Prolabo, Fontenay‐sous‐bois, France). Biofilm cell cultures were performed in 12‐well polystyrene plates (Nunc) for crystal violet detection assays, 6‐well plates (Nunc) for RNA‐seq experiments and glass‐bottom 12‐well plates (InVitroScientific) for confocal microscopy analysis; 1 or 2.75 ml of m126 (in the case of 12‐well and 6‐well plates respectively) supplemented or not with As(III) was inoculated with exponential cell culture to a final concentration of 1%. The plates were then incubated under stagnant conditions at 30°C, and the medium was renewed after 24 h to prevent nutrient depletion.