Unveiling the role of tropinone reductase I in biodegradation of oxytetracycline: Identifying antibiotic deactivation enzymes to enhance engineering feasibility of microalgae-based biotechnologies

The global challenge of antibiotic contamination calls for a comprehensive approach that integrates various technological, economic, and social factors to design effective, and sustainable solutions. Microalgae-based biotechnologies offer a sustainable and efficient approach to mitigating residual antibiotics while curbing the spread of antibiotic resistance genes. In this study, we found that the widely distributed microalgal species Chlamydomonas reinhardtii can efficiently degrade tetracycline, oxytetracycline (OTC), and doxycycline. Despite this promising result, the enzymatic mechanisms underlying this degradation process remain largely unknown. Taking OTC as a model chemical, we demonstrated that neither cytochrome P450 enzymes nor primary amine oxidase in C. reinhardtii were essential for OTC degradation using enzyme inhibition assays. Further identification and characterization of a novel tropinone reductase I (TRI) demonstrated that this enzyme catalyzes the demethylation of OTC, achieving a remarkable removal efficiency of 93.10% within 24 h. Additionally, informative analysis of metagenomic and metatranscriptomic databases revealed a widespread distribution and transcriptional activity of genes encoding TRI homologs across diverse environmental settings. The widespread distribution and transcriptional activity of TRI homologs suggest potential for TRI-mediated OTC degradation in diverse marine settings. These fundamental understandings of enzymatic catalysis processes not only provide critical insights into improving environmental management strategies, but also offer new lights on the unknown global impacts of novel phytoplankton enzymes.