Long-Term Photochemical\nand Microbial Alterations\nLead to the Compositional Convergence of Algal and Terrestrial Dissolved\nOrganic Matter

Lakes are expected to become more active in processing\ndissolved\norganic matter (DOM), but the fate of DOM with different origins remains\npoorly constrained. We conducted long-term incubation experiments\n(∼1 year) with sole light, sole microbial, and combined light\nand microbial treatments using DOM from algal and terrestrial sources\n(DOMa and DOMt, respectively). Fourier transform ion cyclotron resonance\nmass spectrometry and 16s rRNA were used to analyze the DOM molecular\ncomposition and bacterial community, respectively. We observed that\nDOMa and DOMt converged toward a similar composition under the combined\nlight and microbial treatment, driven by the removal of source-specific\ncompositions along with the production of refractory, carboxylic-rich\nalicyclic molecules (CRAM). For CRAM enrichment, microbial processes\nplayed a greater role for DOMa, while phototransformation was more\nimportant for DOMt. The co-occurrence patterns between DOM molecules\nand bacteria showed that DOM molecular composition influenced the\nbacterial community. More complex DOM-bacteria interactions were observed\nfor DOMt compared to DOMa, suggesting that greater bacterial cooperation\nwas necessary for transforming DOMt. Collectively, these findings\noffer new insights into the mechanisms underlying the uniformity of\nDOM from various sources through prolonged environmental transformations\nin lakes.