Molecular-level insights into dissolved organic matter during Ulva prolifera degradation and its regulation on the environmental behaviour of the organic pollutant tributyl phosphate.

Macroalgal blooms have frequently occurred in coastal waters, and a large amount of algogenic dissolved organic matter (DOM) is input into seawater as macroalgae degraded. It undergoes continuous changes under microbial degradation; however, the impact of microbially-modified marine DOM on the environmental behaviour of organic pollutants remains underexplored. This study focused on Ulva prolifera, the dominant species in green tides, and investigated the molecular diversity of DOM from U. prolifera degradation over a 100-day period and the role of DOM at different time points in the adsorption of organophosphate flame retardants onto goethite. Our findings revealed that dissolved organic carbon (DOC) in seawater increased sharply during the first two weeks, followed by a rapid decline, and eventually reached a stable level within 100 days. Multi-spectrum analysis and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), along with the results of spectral analysis, indicated an increase in the humification and aromaticity of DOM, accompanied by the transformation of protein components, suggesting a decrease in DOM bioavailability. Microbial abundance aligned with DOC trends, and 16S rRNA results revealed significant shifts in the microbial community during DOM transformation. A strong correlation between fluorescent DOM groups and microbial diversity was observed, and co-occurrence network analysis further identified Alteromonas and Vibrio as major contributors to DOM chemical diversity. The introduction of U. prolifera DOM reduced available surface sites on goethite, inhibiting the adsorption rate of tributyl phosphate (TnBP) in batch sorption experiments. However, this competitive sorption effect was mitigated by co-sorption, as DOM could bind with TnBP, explaining the observed increase in adsorption capacity. Redundancy analysis and verification tests suggested protein-like DOM components play a crucial role in sorption, and microbial transformation of DOM-proteins could diminish this effect. These findings underscore the importance of macroalgal DOM in influencing the environmental behaviour of organic pollutants and could supply a supplement about the ecological effect about macroalgal blooms.