Marisol Goñi-Urriza

The plastisphere critically modulates interactions among microplastics (MPs), biofilms, and trace metals under environmentally realistic conditions, governing the interfacial reactivity of plastisphere-coated microplastics. Here, we investigated cobalt (Co) adsorption onto large polypropylene (PP) primary microplastics (~4 mm) exposed for 28 days in a flow-through riverine mesocosm across a gradient of Co concentrations (0–60 µg·L⁻¹). A multi-technique analytical approach was employed, combining inductively coupled plasma mass spectrometry (ICP-MS), laser ablation ICP-MS (LA-ICP-MS), quantitative PCR (qPCR), and scanning electron microscopy (SEM). Cobalt accumulation increased linearly with time, consistent with apparent first-order dependence on aqueous Co concentration under constant exposure, reaching 38 ± 3 mg·kg⁻¹ after 28 days at 60 µg·L⁻¹, with no saturation observed. The concentration-dependent Co accumulation was well described by an empirical power-law model (Q = KC₀ⁿ), with the empirical coefficient K increasing linearly over time, reflecting the progressive increase in Co accumulation at a given aqueous concentration. Microbial colonization developed rapidly on MPs, with 16S and 18S rRNA gene copy numbers stabilizing after 14 days, while surface-normalized Co signals increased sharply after day 21, indicating a time-dependent modification of biofilm properties influencing Co retention. SEM confirmed complex microbial structures, including diatom-like cells. No cobalt adsorption was observed under sterile conditions, confirming the key role of biofilm presence. These findings highlight the dynamic role of the plastisphere as a chemically and biologically active interface under environmentally realistic riverine conditions, with implications for contaminant fate, bioavailability, and risk assessment in freshwater systems affected by plastic pollution.

In search of a Cr(VI) removal bioprocess, within a circular economy perspective, we combined two autochthonous bacterial consortia obtained from industrial residues and an agricultural waste (rice husk of Oryza sativa). After characterizing the rice husk (rh) properties, the bacterial growth and biofilm formation, we studied the taxonomical profile of the bacterial consortia (C55, C33, and C21, the last is a combination 2:1 of the previous ones). By using SEM analysis, we confirmed the presence of a smooth layer of exopolysaccharides (EPS) over the rice husk surface. Total bacterial biodiversity, assessed through MiSeq analysis, showed that the addition of Cr(VI) stimulated the growth of two main populations ( Cellulosimicrobium and Siminovitchia ) while inhibiting Peptoniphilus , Cutibacterium and Rhodococcus . We also evaluated chromium depletion, in batch experiments, with particular emphasis on its sorption to the agroresidue and its biotransformation by the biomass. When exposed to 200 mg.L ^-1 Cr(VI), the system containing both rh and the bacterial consortium completely removed Cr(VI) from the effluent with half-times between 53 and 67 h. The combination of Cr(VI) biosorption by rh and its biotransformation to Cr(III) by bacterial consortia confirmed an effective synergy capable of removing 92% of Cr(VI). This work supports the potential of an innovative bioprocess based on bacterial biofilms grown on rh for the mitigation of Cr(VI) contamination.

Two bacterial consortia (C55 and C33), obtained from an industrial residue contaminated with hexavalent chromium (Cr(VI)), were used to study the behavior of their mixture for depleting this ion in liquid media. In the absence of Cr(VI), C55 showed a greater growth rate than C33, while the latter exhibited biofilm formation. In the presence of this ion, C55 showed resistance up to 800 mg·L ⁻¹ and an ability to diminish up to 400 mg·L ⁻¹ of the Cr(VI) from the medium, while for C33, these concentrations were 400 and 200 mg·L ⁻¹ , respectively. Bacterial synergism between these consortia was evaluated using different compound ratios (C55:C33 ratios of 1:1, 1:2, and 2:1), growing at 50, 100, and 200 mg·L ⁻¹ Cr(VI). The best half‐lives of Cr(VI) decrease were 16, 31, and 98 h, respectively, for the 1:1 mixture. The ability of C33 and the mixed consortia to form biofilms was verified. MiSeq sequencing revealed 4 major populations for C55 (in a total of 14) and 3 for C33 (8), most of which were common. After an isolation process, 2 bacterial strains were obtained from C55 and 4 from C33. Three of these strains (QRePLB33E, similar to Oceanobacillus profundus ; QRePLB33G, to Shouchella clausii ; and QRePLB55C, to Cellulosimicrobium funkei ) showed resistance to Cr(VI) and the ability to remove 100% of it at least up to 300 mg·L ⁻¹ . Thus, synergism between different bacterial consortia obtained from the same site is possible and can improve, by complementing their capacities, both the growth rate and the ability to diminish the xenobiotic from the medium.

Biofilms play a crucial role in biogeochemical cycles, making them essential to the functionality and stability of aquatic ecosystems. Their functioning is mainly driven by interactions between microorganisms that ensure the cycling of major elements. Evaluating the impact of stressors, such as metals, on these interactions is challenging. This study examined the effects of cobalt (Co) on the microeukaryotic community and their relationships with prokaryotes within biofilms grown in the presence of several Co concentrations (background concentrations, 0.1, 0.5, and 1 µM) for 28 days and 35 days after the end of Co injection. A previous work has suggested that Cyanobacteria were highly sensitive to Co leading to a reduction of the photosynthetic potential of the biofilm. In this study, the major primary producers, namely Bacillariophyceae, were also found to be sensitive to Co. The direct consequence of this sensitivity was an impairment of the autotroph-heterotroph coupling and the dominance of prokaryotic taxa in microbe-microbe interactions. The biofilms co-occurrence networks were then smaller, less connected but more centralized at 1 µM Co. Keystones, that were half affiliated with microalgae in the absence of Co, were mostly prokaryotes. As such, prokaryotes were found to control resource production and element cycling in Co-stressed biofilms. These changes in co-occurrence network organization and microbial community composition demonstrated a cascade of effects related to contamination in rivers. Our results further highlighted the strategy biofilms adopt to counteract the impairment of autotroph-heterotroph coupling, and to maintain their functions and ecosystemic roles in contaminated environments.

1. Although cobalt (Co) is widely used in the transition to low-carbon energy technologies, its environmental impact remains almost unknown. This study examines Co impacts on the prokaryotic communities of river biofilms to assess their potential use as bioindicators of Co contamination. To that end, biofilms were grown on blank glass slides placed in artificial streams enriched with Co (0.1, 0.5 and 1 µM Co) for 28 days and prokaryotic abundance and diversity were analyzed using DNA-metabarcoding every 7 days. The resilience of the prokaryotic community was investigated after a further 35 days without Co contamination. Prokaryotic communities were impacted by 0.5 and 1 µM Co from the beginning of the biofilm colonization. Although biofilms reached similar biomasses regardless of Co concentration, control biofilms were dominated by Cyanobacteria and Planctomycetes while Bacteroidetes dominated Co contaminated biofilms. Potential functional redundancy was observed with the implementation of carbon fixation alternatives by non-photosynthetic prokaryotes in biofilms subjected to high Co concentrations. No structural resilience of the biofilms was observed after 35 days without Co contamination. The use of prokaryotic community response measured using molecular approaches appears to be a promising and cost-effective approach for assessing changes in water quality due to metals. Graphical Synopsis: Few knowledge is available about Co ecotoxicity in freshwaters. This study assess the potential of prokaryotic communities developing in freshwater biofilms to be used as bioindicator of Co contamination.