Algal organic matter inhibits methylmercury photodegradation in eutrophic lake water: A dynamic study

Algal organic matter (AOM) is a major component of dissolved organic matter (DOM) in eutrophic lakes and could impact the photodegradation of neurotoxic methylmercury (MeHg) in water. Predicting these effects, however, is challenging, largely due to the dynamic changes of AOM during algal decomposition. Here, we investigated the effects of AOM on MeHg photodegradation throughout the algal decomposition process and elucidated these effects by characterizing dynamic changes of AOM and exploring the respective roles of various reactive oxygen species (ROS). Our results reveal that AOM derived from algal decomposition significantly inhibits MeHg photodegradation, and the extent of this inhibition varies depending on the specific lakes (8-21 %, p < 0.05) and their eutrophication states (16-28 %, p < 0.05). The inhibitory effect gradually weakened as the decomposition progressed, which may be attributed to the dynamic changes in the quantity and quality of AOM. Moreover, hydroxyl radical (center dot OH) was found to be the main contributor in driving MeHg photodegradation (15-23 %) during the early stages of decomposition (day 0-3), while in the later stage (day 12-24), the role of singlet oxygen ((1)O2, 15-20 %) and ((DOM)-D-3*, 21-30 %) gradually strengthened and these three ROS jointly drove MeHg photodegradation. Based on our findings and recent studies, we propose that AOM derived from algal decomposition plays a vital role in increasing the risk of MeHg in eutrophic lakes. It promotes MeHg formation while simultaneously inhibiting its photodegradation. Integrating AOM-MeHg interactions into Hg biogeo-chemical cycling models would reduce uncertainties when predicting MeHg risks.

Automated summary

Algal organic matter (AOM) hinders the photodegradation of neurotoxic methylmercury (MeHg) in eutrophic lakes due to dynamic changes in AOM during algal decomposition. The inhibitory effect of AOM on MeHg photodegradation varies among lakes and their eutrophication states. Reactive oxygen species (ROS), including hydroxyl radical, singlet oxygen, and DOM-D-3*, play different roles in driving MeHg photodegradation throughout the algal decomposition process. This study highlights the importance of considering AOM-MeHg interactions in predicting MeHg risks and suggests integrating them into Hg biogeochemical cycling models.