Coordinated photodegradation and biodegradation of organic matter from macrophyte litter in shallow lake water: Dual role of solar irradiation

Abstract

Eutrophication and climate warming cause macrophytes to spread rapidly in shallow lakes. The accumulation of excess decayed litter in lake water can improve the particulate organic matter content and lead to ecological deterioration. While decomposition of macrophyte litters in aquatic ecosystems is generally assumed to be biologically driven, photodegradation has been largely overlooked. In this study, the potential contribution of photodegradation to the decomposition of organic matter from macrophyte litter in a shallow lake was investigated under two types of light radiation: laboratory incubation light and natural solar radiation. Results showed that the combined photo- and biodegradation treatment removed more litter mass compared to solely photodegradation or biodegradation for over 108 days. More reactive oxygen species (ROS) were produced during litter degradation in lake water under the combined photo- and biodegradation treatments in comparison to other treatments. Removal efficiency of litter was doubled in the presence of ROS compared when ROS were captured. Additionally, NMR analysis revealed that lignin guaiacyl units were preferentially lost and the litter became more refractory under the combined photo oxidation and microbial degradation after 108 days. High-throughput sequencing analysis further indicated that solar irradiation stimulates the growth of bacteria (Treponema and Magnetospirillum) and fungi (Aspergillus) that have the ability to degrade aromatic compounds. Altogether, both ROS and specific microorganisms played important role in promoting litter degradation in lake water under light irradiation. Therefore, the role of solar radiation in particulate organic matter decomposition needs to be considered for a more accurate prediction of carbon dynamics in aquatic ecosystems.