Salt-tolerant marine sludge is more suitable for stable and efficient high-salinity wastewater treatment than activated sludge

• AOB in marine sludge showed better salt tolerance and ammonia oxidation ability. • Activated sludge bacteria adapted to high salt by enriching betaine intracellularly. • Marine sludge bacteria exhibited Na + extrusion to adapt to high-salinity environment. • Marine sludge can be used as seed sludge for high-salinity wastewater treatment. The increasing production and discharge of high-salinity wastewater exert a negative impact on the ecological environment, and restricts the treatment of such wastewater by activated sludge (AS) in sewage plants, making the treatment of high-salinity wastewater both challenging and costly. Marine bacteria are naturally adapted to high-salinity environments, offering an ideal solution as seed sludge for such treatment. Here, activated sludge and marine sludge (MS) were adopted as seed sludge to start up partial nitrification (PN) reactors. At a salinity of 10 g NaCl/L, both PN reactors successfully started up within 21 days and remained stable for 38 days. As the salinity increased, the activity of AS was gradually inhibited and collapsed at 70 g NaCl/L salinity, with ammonia removal efficiency (ARE) of l.77 %. In contrast, MS demonstrated better activity and stability as salinity increased. Even at 70 g NaCl/L, the MS reactor retained an ARE of 73.76 %, revealing higher activity under high salinity conditions. AS and MS bacteria use entirely different strategies to adapt to the high-salinity environment. AS bacteria accumulate betaine intracellularly as an adaptation strategy, with betaine biosynthetic genes ( bet A), and genes of betaine transporters (ProX and BCCT family) significantly up-regulated under hyperosmotic conditions. Moreover, MS bacteria extruded Na + through active transport to protect against the harmful effects of high-salinity environments. The ATP-binding cassette transporter (ABC transporter) related to Na + extrusion was up-regulated 8.8-fold. The result demonstrated that MS can better adapt to high osmotic pressure environments, making it a suitable seed sludge for such applications. This work provided a practical and effective solution for high-salinity wastewater treatment.