Biofilm formation on polyethylene microplastics and their role as transfer vector of emerging organic pollutants.

Microplastic (MP)-colonizing microorganisms are important links for the potential impacts on environmental, health, and biochemical circulation in various ecosystems but are not yet well understood. In addition, biofilms serve as bioindicators for the evaluation of pollutant effects on ecosystems. This study describes the ability of three polyethylene-type microplastics, white (W-), blue (B-), and fluorescent blue (FB-) MPs, to support microbial colonization of Pseudomonas aeruginosa, the effect of mixed organic contaminants (OCs: amoxicillin, ibuprofen, sertraline, and simazine) on plastic-associated biofilms, and the role of biofilms as transfer vectors of such emerging pollutants. Our results showed that P. aeruginosa had a strong ability to produce biofilms on MPs, although the protein amount of biomass formed on FB-MP was 1.6- and 2.4-fold higher than that on B- and W-MP, respectively. When OCs were present in the culture medium, a decrease in cell viability was observed in the W-MP biofilm (65.0%), although a general impairing effect of OCs on biofilm formation was ruled out. Microbial colonization influenced the ability of MPs to accumulate OCs, which was higher for FB-MP. In particular, the sorption of amoxicillin was lower for all bacterial-colonized MPs than for the bare MPs. Moreover, we analysed oxidative stress production to assess the impact of MPs or MPs/OCs on biofilm development. The exposure of biofilms to OCs induced an adaptive stress response reflected in the upregulation of the katB gene and ROS production, particularly on B- and FB-MP. This study improves our understanding of MP biofilm formation, which modifies the ability of MPs to interact with some organic pollutants. However, such pollutants could hinder microbial colonization through oxidative stress production, and thus, considering the key role of biofilms in biogeochemical cycles or plastic degradation, the co-occurrence of MPs/OCs should be considered to assess the potential risks of MPs in the environment.

Toxicity and adaptation of Dictyosphaerium chlorelloides to extreme chromium contamination.

Metals are often spilled by industries into inland water environments, with adverse consequences. Numerous papers have reported that heavy metals produce massive destruction of algae. Nevertheless, algal populations seem to become tolerant when they have had previous exposures to heavy metals. Because the mechanisms allowing heavy metal tolerance of algae are not yet known, the present study analyzed the effect of hexavalent chromium on growth and photosynthetic performance of Dictyosphaerium chlorelloides, stressing on the adaptation mechanisms to chromium contamination. Growth and photosynthetic performance of algal cells were inhibited by Cr(VI) at 10 mg/L, and the 72-h median inhibition concentration was established as 1.64 and 1.54 mg/L, respectively. However, after further incubation for a three month period in an environment with 25 mg/L of chromium, some rare, chromium-resistant cells occasionally were found. A Luria-Delbrück fluctuation analysis was performed to distinguish between resistant algae arising from rare, spontaneous mutations and resistant algae arising from physiological adaptation and other adaptive mechanisms. Resistant cells arose only by spontaneous mutations before the addition of chromium, with a rate of 1.77 x 10(-6) mutants per cell division. From a practical point of view, the use of both chromium-sensitive and chromium-resistant genotypes could make possible a specific algal biosensor for chromium.