Disentangling the bioavailability change of dissolved organic matter in the Pearl River Estuary: Overlooked contribution from lifestyle-facultative bacteria

Bacteria-mediated dissolved organic matter (DOM) transformation is crucial to marine carbon sequestration, yet the underlying mechanisms remain elusive. Particle-attached (PA) and free-living (FL) bacteria divided by filter-size fractionation were reported to differ in DOM utilization, while this oversimplification overlooked the lifestyle transition of bacteria, potentially leading to biased ecological interpretations. Our study focused on a plume transect of the Pearl River Estuary (PRE) to investigate the DOM-bacteria relationship employing amplicon sequencing and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). An alternative classification considering the lifestyle preference of bacteria was applied besides the conventional dichotomy. DOM indices including degradation index and saturation degree were found to correlate with salinity, while endmember mixing models and hierarchical analyses suggested bacteria as non-negligible mediators in DOM transformation. Moreover, associations between DOM and PA/FL communities showed poor correspondences with the molecular changes across the transect. Two lifestyle-facultative groups, particle-attached-preferred (PA P ) and free-living-preferred (FL P ) bacteria capable of altering between PA and FL lifestyles were identified and exhibited distinct taxonomic diversity and DOM processing preferences. While PA P taxa preferentially consumed DOM of high molecular weight and H/C values, FL P groups appeared to convert labile DOM into relatively recalcitrant forms. Their different utilization patterns provided comprehensive understandings of DOM bioavailability fluctuations in the PRE. Collectively, our study revealed the overlooked diversity of lifestyle-facultative bacteria and their distinct potentials in DOM transformation, addressing a valuable angle for investigating the bacteria-DOM interactions within river-to-coast continuum.