## 3 Antibiotic Degradation and Fate Kinetics in Surface Water
### Biodegradation  
Biolysis is the biological degradation of antibiotics caused by enzyme-mediated transformation, primarily due to bacteria, human and animal metabolism, and microorganisms, such as fungi, microalgae, and protozoa [44]. The biological removal of antibiotics by an organism is through three forms: (1) through modifying the antibiotics compound (biotransformation); (2) by cleaving the antibiotics (biodegradation); and (3) through mineralizing the compound (subsistence) [39]. Generally, in terms of removal mechanism, biodegradation encompasses distinct processes, including mineralization (conversion of an antibiotic compound to an inorganic by-product), detoxication (alteration of antimicrobial substance to a harmless product), and co-metabolism (the simultaneous metabolism of two compounds that act as substrates to each other). Other transformation processes include activation (transforming a non-toxic compound to a toxic compound by microbial action) and defusing (converting the antibiotic compound to a harmless metabolite before its potential is realized). Uptake and metabolism in humans, animals, and microorganisms may degrade or transform antibiotic compounds into other byproducts. Some microorganisms degrade antibiotics as they endure, using substances, including antibiotics, as sources of carbon and energy sources [39]. Therefore, the intracellular and extracellular enzymes of bacteria and fungi partially or fully cleave the ring in the compound and transform it into other substances. Bacteria, such as phyla Proteobacteria, Actinobacteria, Bacteroidetes, Gammaproteobacteria, and fungi, such as white-rot fungi, Trametes versicolor, Pycnoporus, Tremellomycetes, and Cerrena unicolor, play a significant role in metabolizing various antibiotics [39]. For instance, white-rot fungi secrete lignin peroxidase, manganese peroxidase, and lactose which degrade 58 to 78 % of tetracyclines in 15 days [27; 45]. In the presence of nitrogen, microorganisms transform sulfamethoxazole into 3-amino-5-methylisoxazole, which lacks antibiotic activity [39]. Bio enzymes initiate degradation through demethylation and N-oxidation of macrolactone ring and sugar loss of macrolides (azithromycin, erythromycincin, and clarithromycin) [33; 46]. Further, bacterial populations that are resistant to antibiotics can degrade antibiotic concentrations. For instance, the resistant bacterial population removes 40% of recalcitrant ciprofloxacin over 104 days in a natural river [40]. Microorganisms in the water and sediment-water interface partially or fully decompose degradable antibiotics into non-toxic compounds, inorganic by-products, and metabolites. Depending on the compound's chemical structure and the degrader type, compounds can be biodegraded into stable solutes. Further, the microbial-mediated transformation of antibiotics may cause the parent compound to detoxify, mineralize, or activate potential toxins. For instance, in the presence of nitrate as a nitrogen source, Gammaproteobacteria metabolomic sulfamethoxazole into 4-nitro-sulfamethoxazole and des amino-sulfamethoxazole. However, 4-nitro-sulfamethoxazole is a more toxic compound than the parent compound, sulfamethoxazole. Consequently, upon the total consumption of available nitrate, it reverts to the parent compound form [39]. In such circumstances, only minor molecular change occurs with no loss of antibiotic activity [47]. Therefore, particular consideration should be set to such biotransformation processes when evaluating the removal rates of a specific antibiotic. Biological degradation of antibiotics is influenced by various factors, such as the antibiotic's physical and chemical properties (degradability and concentration of the compound), environmental parameters (sediment carbon content, retention time, and seasonality), microbial metabolism (density, growth, and the type of microorganisms), and water quality (dissolved oxygen concentration, PH, nutrient level and temperature). A higher concentration of antibiotics constrains microbial activity.