Exploring eutrophic effects of marine sediments underneath fish cage farms: Insights from changes in eukaryotic and bacterial communities and volatile organic compounds.

The roles of bacteria and eukaryotes in the sediments of fish farms have received considerable attention. High concentrations of volatile organic compounds (VOCs) in eutrophic sediments pose significant problems in the marine environment. However, the identification of VOCs and their association with bacteria and eukaryotes in marine sediments from fish farms remain unexplored. By using third-generation 18 s/16 s sequencing with bacterial absolute quantity and headspace solid-phase microextraction coupled with gas chromatography mass spectrometry (HS-SPME-GC-MS), we investigated benthic community structure and VOCs composition in the sediments from five large yellow croaker farms in China (DJ, DC, DT, NJ, and ND), as well as geological and chemical changes. The ND sediments, characterized as mud substrates with the highest moisture and nutrient levels, were dominated by ciliates and flagellates, whereas typical benthic organisms such as echinoderms, annelids, and cnidarians were absent in other farming areas. These sediments had higher bacterial density and increased proportions of Desulfuromonadia and Desulfobacterota but lower proportions of Campylobacterota compared to other areas. Additionally, ND sediments exhibited the highest VOC content, with 2-Octen-1-ol being the most abundant compound, characterized by mushroom-like, earthy, fishy, rancid, and metallic odors that may negatively influence the flavor of large yellow croaker. We identified 76 differential VOCs, most of which showed a positive correlation with bacteria, ciliates, and flagellates, while some VOCs showed a positive correlation with the annelid Aurospio foodbancsia and the cnidarian Diadumene cincta. Our study is the first to elucidate the complex interactions of benthic organisms and VOCs during the eutrophication process in sediments from cage fish farms, providing potential biomarkers for ecosystem monitoring.