Dissolved organic matter dominating the photodegradation of free DNA bases in aquatic environments

Free DNA bases are widely present in the environments, and can be utilized by bacteria for their nucleic acids synthesis or as nutrition sources. In sunlit natural waters, these free bases probably undergo photodegradation which would change the bioavailable bases contents. Though the photodegradation of DNA has been investigated, the photodegradation behaviors of free bases may be quite different from those of DNA-confined bases in consideration of their different chemical environments. Herein, the photodegradation of four free bases (guanine, adenine, thymine and cytosine) was investigated. Results show that direct photodegradation of free bases in phosphate buffer caused by UV was slow. However, the photodegradation of these free bases were greatly enhanced in dissolved organic matter (DOM) solution. In the presence of 10-50 mg/L DOM, the photodegradation rates of free bases were increased by 1.85-14.6 times compared to the controls without DOM. DOM could result in indirect photodegradation by producing hydroxyl radical (•OH) and singlet oxygen (¹O₂) under irradiation, and this indirect photodegradation enhanced and dominated the free bases photodegradation. The •OH was involved in all four bases photodegradation, while the ¹O₂ only participated in guanine photodegradation. In phosphate buffer, the fastest photodegradation bases were pyrimidine, however, guanine became the fastest photodegradation base in DOM solution due to the selective oxidation of guanine by ¹O₂. In summary, DOM may be a determinant for free bases photodegradation in natural waters and thereby deeply influence free bases fates in aquatic environments.