Lakes transform a globally relevant amount of carbon due to complex interactions between organic matter and microbes. In every lake, thousands of organic compounds and thousands of species form a complex network of interactions that dictates how carbon is transformed. However, the role of most organic compounds and microbes in aquatic ecosystems remains poorly understood. Furthermore, multiple environmental factors linked to climate change and anthropogenic activities are constantly acting on the composition of both organic matter and microbes and it is challenging to identify which factor ultimately affects the functioning of an aquatic ecosystem. Here, I aimed to improve our fundamental understanding of organic matter and microbes in lakes by combining laboratory experiments isolating the transformation of organic matter by a few microbes and large scale mesocosms experiments testing the effects of multiple environmental stressors under present and potential future scenario. I focused on light availability and nutrients, as both are prominent drivers of many ecosystem functions and important factors for controlling the production of organic matter and its consumption by microbes. In Chapter 1, I found that organic matter transformation by microbes is even more complex than originally thought, with fungi degrading and producing complex organic compounds. In Chapter 2, I found that the compositional dynamics of organic compounds can be used to disentangle the effects of multiple stressors in lakes. I apply the latter concept to isolate the effects of two environmental stressors with opposite effects on organic matter composition, i.e. brownification and eutrophication. In Chapter 3 and 4, I further tested the interaction between organic matter and microbes in the presence of artificial light at night. Chapter 3 revealed that artificial light at night affects the vertical migration of photosynthetic microbes as well as the quality of organic matter that is produced and consumed using bulk measurements. I confirmed these conclusions in Chapter 4 and identified microbes and organic compounds causing numerous changes observed. Furthermore, the ultra-high-resolution methods used in Chapter 4 allowed me to detect and characterize the effect of artificial light at night at irradiances that are below those of a full moon. Overall, this thesis improves our general understanding of dissolved organic matter and microbes in lakes and provides the first evidence that low light pollution affects pelagic microbes.
Jeremy Fonvielle (Thu,) studied this question.