Long-Term Photochemical and Microbial Alterations Lead to the Compositional Convergence of Algal and Terrestrial Dissolved Organic Matter

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