Responses to arsenic stress by the Andean benthic-extremophile cyanobacteria Rivularia halophila

Microorganisms living in naturally arsenic (As)-enriched environments exposed to extreme conditions play an active role in As mobility by influencing chemical speciation or inducing dissolution of As-bearing phases. In this work, the cyanobacterium Rivularia halophila was evaluated in its capability to tolerate, accumulate and biotransform As species. R. halophila was isolated from a hypersaline and high-altitude Andean Lake characterized by high geogenic As content in the sediments and water. Cultures of R. halophila were exposed to concentrations of As(V) and As(III) up to 15,000 and 200 ppm, respectively. Results showed that R. halophila effectively tolerated high As doses (IC50: 8500 and 70 ppm for As(V) and As(III) respectively), responding with a gradual decrease in growth and chlorophyll content. Cell damage and reactive oxygen species (ROS) only significantly increased at 5000 ppm of As(V) and 20 ppm of As(III), demonstrating an extreme adaptation to both As species. Moreover, As accumulation was significantly higher in As(III) treatment (i.e., 9.500 ppm) than in As(V) (i.e., 480 ppm). The main representative As-bearing phases in the biomass fractions were determined using micro-focused X-ray photoelectron spectroscopy. Three main As-bearing phases were identified: arsenate, arsenite and As(III)-S (mostly associated with sulfur). The redox evolution of As-bearing phases in the biomass fractions, coupled with increase of bulk As content and ROS activity, suggest that active bioaccumulation and transformation of As by R halophila occurs during environmental stress conditions.