Short-term bacterioplankton responses to black carbon-derived dissolved organic matter additions in Mediterranean seawater

Rising atmospheric inputs of black carbon (BC) from fossil-fuel combustion, shipping and wildfires are increasing the delivery of combustion-derived organic matter to coastal oceans, yet the responses of marine free-living bacterioplankton to this material remain poorly constrained. Here, we investigated the short-term effects of bioavailable black carbon-derived dissolved organic matter (BC-DOM) on free-living coastal bacterioplankton using dark microcosms with northwestern Mediterranean surface seawater. Triplicate bottles received BC-DOM from (1) diesel particulate matter (DREF), (2) UV-oxidised diesel particulate matter (DREF-OX) or (3) heavy-fuel-oil ship soot (SHIP), each at ~200 µM dissolved organic carbon; controls received unamended seawater. Communities were sampled initially and after 72 h and were analysed via V4 16S rRNA gene sequencing. PERMANOVA revealed strong effects of incubation time at both phylum ( F 1,16 = 25.54, p = 0.001) and genus levels ( F 1,16 = 50.40, p = 0.001). Treatment effects were significant at the genus level ( F 3,16 = 3.00, p = 0.020) but not at the phylum level ( F 3,16 = 2.11, p = 0.082), with significant Treatment × Timepoint interactions at both taxonomic resolutions. Genus-level ALDEx2 modelling identified 7 significant taxa ( q < 0.05). Alcanivorax increased significantly in all BC-DOM treatments (Δ centred-log-ratio [CLR] +1.9 in SHIP), while SHIP also enriched the sulphur-cycling Roseobacter , Sulfitobacter and the gammaproteobacterial aromatic-degrader Pseudohongiella . In contrast, Acinetobacter declined in both diesel-derived DOM, and the copiotrophic genera Marinobacter and Haliea decreased overall. These results demonstrate that BC-DOM additions restructure free-living communities at the genus level within 72 h, favouring specialist alkane and polycyclic-aromatic degraders while repressing certain canonical diesel oxidisers. Although functional consequences cannot be inferred from 16S data, the identified taxa represent potential microbial responders to episodic BC-DOM inputs.