Global Distribution and Biogeochemical Significance of Magnetotactic Bacteria in Deep-Sea Cold Seep Ecosystems

Three surface sediment samples (hereafter referred to as CS01, CS02, and CS03) were collected from two active cold seeps in the South China Sea. For magnetic measurements, about 500 mg of each sediment sample was air-dried at room temperature for 12 hours and sealed in a non-magnetic gelatin capsule. Room-temperature hysteresis loops and first-order reversal curves (FORCs) were measured with a vibrating sample magnetometer (VSM, Princeton MicroMag model 3900, sensitivity = 5.0 × 10−10 Am2). Briefly, hysteresis loops were obtained under a maximum field of 1 T, with a 500 ms averaging time and a 5 mT field increment. Saturation magnetization, saturation remanent magnetization, and coercivity values were derived after slope correction in the high-field range of 0.7–1 T. Remanence coercivity was determined from a DC backfield remanence curve. FORC measurements involved acquiring 200 curves under a positive saturation field of 1 T, using a field step (δH) of 1.44 mT with a 500 ms averaging time.Low-temperature magnetic measurements were performed on a Quantum Design Magnetic Property Measurement System (MPMS XP-5, sensitivity 5.0 × 10⁻¹⁰ Am²) . Zero-field-cooled (ZFC) and field-cooled (FC) curves were obtained by cooling the sample from 300 to 10 K in zero field and in a 2.5-T field, respectively. Saturation isothermal remanent magnetization (SIRM) at 2.5 T and 10 K (SIRM10K-2.5T) was then measured upon warming in zero field. The Verwey transition temperature was determined from the maximum slope on the FC warming curve.