Coagulation promotes the spread of antibiotic resistance genes in secondary effluents.

Wastewater treatment plants (WWTPs) are biological hotspots receiving the residual antibiotics and antibiotic resistant bacteria/genes (ARB/ARGs) that greatly influence the spread of antibiotic resistance in the environment. A common method used in WWTPs for the purification of secondary effluent is coagulation. Notwithstanding the increasing health concern of antibiotic resistance in WWTPs, the impact of coagulation on the emergence and spread of antibiotic resistance remains unclear. To shed light on this, our study investigated the behavior of four representative ARB types (tetracycline, sulfamethoxazole, clindamycin, and ciprofloxacin resistance) during the coagulation process in a model wastewater treatment plant. Our search showed a significant reduction in the presence of ARBs after either PAC or FeCl3 coagulation, with removal efficiencies of 95% and 90%, respectively. However, after 4 days of storage, ARB levels in the coagulated effluent increased by 6-138 times higher than the original secondary effluent. It suggests a potential resurgence and spread of antibiotic resistance after coagulation. Detailed studies suggest that coagulants, particularly PAC, may facilitate the transfer of ARGs among different bacterial species by the enhanced cell-cell contact during coagulation-induced bacterial aggregation. This transfer is further enhanced by the factors such as auxiliary mixing, longer incubation time and ideal operating temperatures. In addition, both PAC and FeCl3 affected gene expression associated with bacterial conjugation, leading to an increase in conjugation efficiency. In conclusion, while coagulation serves as a purification method, it might inadvertently boost the spread of ARGs during tertiary wastewater treatment. This underscores the importance of implementing subsequent measures to mitigate this effect. Our findings provide a deeper understanding of the challenges posed by bacterial antibiotic resistance in wastewater and pave the way for devising more effective ARB and ARG management strategies.

Chemotactic movement and zeta potential dominate Chlamydomonas microsphaera attachment and biocathode development.

Microalgal cell attaching and biofilm formation are critical in the application of microalgal biocathode, which severs as one of the hopeful candidates to an original cathode in bioelectrochemical systems. Many efforts have been put in biofilm formation and bioelectrochemical systems for years, but the predominant factors shaping microalgal biocathode formation are sketchy. We launched a pair of researches to investigate microalgal attachment and biofilm formation in the presence/absence of applied voltages using Chlamydomonas microsphaera as a model unicellular motile microalga. In this study, we presented how microalga attached and biofilm formed on a carbon felt surface without applied voltages and try to manifest the most important aspects in this process. Results showed that while nutrient sources did not directly regulate cell attachment onto the carbon felt, limited initial nutrient concentration nevertheless promoted cell attachment. Specifically, nutrient availability did not influence the early stage (20-60 min) of microalgal cell attachment but did significantly impact cell attachment during later stages (240-720 min). Further analysis revealed that nutrient availability-mediated chemotactic movements and zeta potential are crucial to facilitate the initial attachment and subsequent biofilm formation of C. microsphaera onto the surfaces, serving as an important factor controlling microalgal surface attachment. Our results demonstrate that nutrient availability is a dominant factor controlling microalgal surface attachment and subsequent biofilm formation processes. This study provides a mechanistic understanding of microalgal surface attachment and biofilm formation processes on carbon felts surfaces in the absence of applied voltages.

Electrotaxis-mediated cell motility and nutrient availability determine Chlamydomonas microsphaera-surface interactions in bioelectrochemical systems.

Cell attachment onto electrode-forming biocathodes is a promising alternative to expensive catalysts used for electricity production in bioelectrochemical systems (BESs). Though BESs have been extensively studied for decades, the processes, underlying mechanisms, and determinant driving forces of microalgal biocathode formation remain largely unknown. In this study, we employed a model unicellular motile microalga, Chlamydomonas microsphaera, to investigate the microalgal attachment processes onto the electrode surface of a BES and to identify the determinant factors. Results showed that the initial attachment of C. micrrosphaera cells is determined by the applied external voltage rather than nutrient availability and occurs via electrotaxis-mediated cell motility. The subsequent development of the C. microsphaera biofilm is then increasingly determined by nutrient availability. Our results revealed that, in the absence of an external voltage, nutrient availability remains a dominant factor controlling the fate of the microalgal surface attachment and subsequent biofilm formation processes. Thus, our results show that electrotactic and chemotactic movements are crucial to facilitate the initial attachment and subsequent biofilm formation of C. microsphaera onto the electrode surfaces of BES. This study provides new insights into the mechanisms of microalgal surface attachment and biofilm formation processes on microalgal biocathodes, which hold great promise for improving the electrochemical properties of cathodes.