## 2 The Degradation of Antibiotics in the Natural Environment
### Biodegradation  
Microbial degradation is one of the important biological pathways for the removal of antibiotics from the environment. The degradation process is relatively complex and can degrade antibiotics through direct or indirect mechanisms, offering advantages such as high efficiency, environmental friendliness, and low costs. The direct actions of the microbial degradation of antibiotics primarily include co-metabolism and enzyme-catalyzed reactions, such as hydrolysis and redox reactions. Co-metabolism, in particular, involves the breakdown of antibiotics into substances that can be utilized by microorganisms through non-specific degrading enzymes. This process requires the addition of carbon and nitrogen sources essential for microbial growth [34; 35]. In enzyme-catalyzed reactions, the degrading enzymes produced by antibiotic-resistant bacteria primarily include the following: \(\beta\)-lactamase [36], aminoglycosoxide-modifying enzymes [37], macrolide inactivating enzymes [38], and chloramphenicol acetyltransferase-ferase [39]. They degrade antibiotics by modifying or hydrolyzing their molecular structures, resulting in the breakdown of these compounds. Research has found that both fungi and bacteria may be involved in the direct degradation of antibiotics. Bacillus cereus P41 is capable of completely degrading the \(\beta\)-lactam antibiotic cefotaxime in a tryptone soybean broth medium at 35 \({}^{\circ}\)C under anaerobic conditions [40]. The genus G1 of the yellow rod bacteria can degrade 80% of the macrolide antibiotic ampicillin in an MH culture medium at a pH of 7, using avermectin as the sole carbon and nitrogen source [41]. Under the conditions of a liquid culture medium containing 50 mg\(\cdot\)L\({}^{-1}\) of oxytetracycline (OTC) and 3% malt extract, the rough-skinned side ear completely degraded the tetracycline antibiotic oxytetracycline [42]. The indirect mechanism of the microbial degradation of antibiotics involves microorganisms promoting the removal of antibiotics by altering environmental conditions or producing non-specific substances. This process includes microbial adsorption and redox reactions. Bioadsorption refers to the phenomenon in which substances are adsorbed onto the surfaces of organisms through covalent, electrostatic, or intermolecular interactions, such as utilizing activated sludge as a carrier for antibiotics. Research has demonstrated that activated sludge can provide adsorption sites for zwitterionic ions, with tetracycline (TC) ions binding to the activated sludge through cation exchange and electrostatic attraction mechanisms. The competition for adsorption sites between low-valent cations and TC ions can diminish the adsorption of TC by activated sludge [43]. During the process of antibiotic removal, microorganisms produce extracellular polymeric substances (EPSs), which are key components responsible for antibiotic adsorption. These substances act as a reservoir, allowing antibiotics to embed within the EPS matrix and accumulate, thereby enabling microorganisms to access growth materials more effectively. For instance, the EPS of microalgae has been demonstrated for the first time to play a significant role in bioadsorption, with the maximum absorption of ciprofloxacin (CIP) by the EPS being comparable to the total bioadsorption [44]. Redox reactions refer to the metabolic processes of microorganisms that produce electron donors or free radicals, which trigger the chemical degradation of antibiotics. Dissimilatory iron reduction (DIR) can induce the formation of iron redox species, thereby promoting metabolic activities and stimulating a series of beneficial reactions, such as biological Fenton reactions. Shewanella is one of the earliest isolated and thoroughly studied dissimilatory iron-reducing bacteria (DIRB), recognized for its ability to drive the Fenton reaction under neutral conditions that are conducive to its respiratory metabolism.