The effect that the hydrodynamic conditions under which biofilms are formed has on their persistence is still unknown. This study assessed the behaviour of Pseudomonas fluorescens biofilms, formed on stainless steel under different shear stress ( ${\mathrm{\tau }}_{\mathrm{w}}$) conditions (1, 2 and 4 Pa), to chemical (benzalkonium chloride – BAC, glutaraldehyde – GLUT and sodium hypochlorite – SHC) and mechanical (20 Pa) treatments (alone and combined). The biofilms formed under different ${\mathrm{\tau }}_{\mathrm{w}}$ showed different structural characteristics. Those formed under a higher ${\mathrm{\tau }}_{\mathrm{w}}$ were invariably more tolerant to chemical and mechanical stresses. SHC was the biocide which caused the highest biofilm killing and removal, followed by BAC. The sequential exposure to biocides and mechanical stress was found to be insufficient for effective biofilm control. A basal layer containing biofilm cells mostly in a viable state remained on the surface of the cylinders, particularly for the 2 and 4 Pa-generated biofilms. Graphical Abstract

Phototrophic and heterotrophic microorganisms coexist in complex and dynamic structures called periphyton. These structures shape the biogeochemistry and biodiversity of aquatic ecosystems. In particular, microalgae-bacteria interactions are a prominent focus of study by microbial ecologists and can provide biotechnological opportunities for numerous applications (i.e. microalgal bloom control, aquaculture, biorefinery and wastewater bioremediation). In this review, we analyze the species dynamics (i.e. periphyton formation and factors determining the prevalence of one species over another), coexisting communities, exchange of resources and communication mechanisms of periphytic microalgae and bacteria. We extend periphyton mathematical modelling as a tool to comprehend complex interactions. This review is expected to boost the applicability of microalgae-bacteria consortia, by drawing out knowledge from natural periphyton.

Anthropogenic activities have increased the dispersal of emerging contaminants (ECs), particularly of parabens, causing an escalation of their presence in wastewater (WW). Current WW technologies do not present satisfactory efficiency or sustainability in removing these contaminants. However, bioremediation with microalgae-based systems is proving to be a relevant technology for WW polishing, and the use of microalgae-bacteria consortia can improve the efficiency of WW treatment. This work aimed to study dual cultures of selected bacteria (Raoultella ornithinolytica, Acidovorax facilis, Acinetobacter calcoaceticus, Leucobacter sp. or Rhodococcus fascians) and the microalga Chlorella vulgaris in microbial growth and WW bioremediation - removal of methylparaben (MetP) and nutrients. The association with the bacteria was antagonistic for C. vulgaris biomass productivity as a result of the decreased growth kinetics in comparison to the axenic microalga. The presence of MetP did not disturb the growth of C. vulgaris under axenic or co-cultured conditions, except when associated with R. fascians, where growth enhancement was observed. The removal of MetP by the microalga was modest (circa 30 %, with a removal rate of 0.0343 mg/L.d), but increased remarkably when the consortia were used (> 50 %, with an average removal rate > 0.0779 mg/L.d), through biodegradation and photodegradation. For nutrient removal, the consortia were found to be less effective than the axenic microalga, except for nitrogen (N) removal by C. vulgaris w/ R. fascians. The overall results propose that C. vulgaris co-cultivation with bacteria can increase MetP removal, while negatively affecting the microalga growth and the consequent reduction of sludge production, highlighting the potential of microalgae-bacteria consortia for the effective polishing of WW contaminated with parabens.

Parabens are emerging contaminants due to their abundant use as preservatives and inefficient treatment in wastewater (WW) treatment plants. To overcome the limitations of WW treatment plants in removing parabens, microalgae-based bioremediation has aroused great interest as an effective and sustainable process. Nevertheless, several factors affect the WW bioremediation capacity, which must be studied to achieve an effective biological treatment. The main objective of the present work was to evaluate the effects of photoperiod and WW composition, specifically organic carbon concentration (sourced as glucose), on the ability of the microalga Chlorella vulgaris to remove methylparaben (MetP). For that, two photoperiods (12/12 h light/dark and 24 h light) were studied and the composition of synthetic WW (SWW) was manipulated to have a glucose concentration of 0, 3, 30, or 300 mg/L. It was observed that the photoperiod significantly affects the bioremediation process. For a 12/12 h photoperiod, MetP was not removed. Removal was effective for a 24 h photoperiod, with percentages of MetP removal over 88% for glucose concentrations lower than 30 mg/L, decreasing for higher glucose levels. MetP photodegradation was negligible. For the 24 h photoperiod, it was possible to verify that higher glucose concentrations decreased microalga growth, with a decrease in the specific growth rate and the production of photosynthetic pigments. Furthermore, with the increase of the glucose concentration, the MetP constant rate of degradation decreased and its half-life time increased, taking longer to degrade the contaminant (through diauxic growth). Also, C. vulgaris exhibited strong growth ability and removed over 80% of nitrogen and phosphorous, unaffected by the presence of MetP and proportional to glucose levels, underscoring its potential for treating WW contaminated with high concentrations of parabens.