Objective The oceans are the origin of life and the largest gene pool on Earth. However, human exploration and research on the oceans remain very limited. Based on relevant data, only 5% of the ocean area has been explored, and merely 0.01% has been sampled. The abyssal ocean, with a depth greater than 6 000 m, accounts for 45% of the total ocean depth. The high-pressure, low-temperature, and dark ecological conditions of the abyssal zone give rise to a unique ecosystem that contains the largest microbial reservoir in the entire aquatic system, encompassing nearly 75% of the prokaryotic biomass and 50% of its production. Numerous studies demonstrate that in situ filtration represents the most effective approach to obtaining microbial samples from seawater. However, most existing sampling devices function only at depths shallower than 6 000 m, and very few operate reliably for extended periods in abyssal waters. This study introduces a seawater microbial sampling device applicable at all depths, featuring in situ filtration and preservation capabilities. The degradation of microbial RNA is effectively prevented by adding RNAlater preservation solution to the filtered container, ensuring sample quality and supporting subsequent genetic material research. The device is successfully employed for in situ enrichment of microorganisms at a sea depth exceeding 10 000 m in the Mariana Trench. Methods The principal scheme design and detailed design of each component of the in situ filtration, sampling, and preservation system for abyssal microorganisms were conducted. First, a multi-stage membrane system was adopted to enhance filtration efficiency, and the inlet and outlet check valves, along with volume compensator structures, were utilized to achieve contamination-free and pressure-surge-free sampling. Second, the advantages and disadvantages of the membrane-based and cartridge-based schemes were compared, and flat filters and microporous membranes were selected as the filtration components. Then, a deep-water motor and a deep-water pump were designed for operation in abyssal environments, and an in situ fixed preservation structure was developed to process the collected microbial samples and improve sample quality. Results and discussions The study independently designed and developed a microbial sampling system with in situ filtration and in situ preservation functions suitable for 10 000 m abyssal environments by referring to and summarizing domestic and international research on deep-sea microbial samplers. The work included the conceptual scheme and detailed design of each component. For practical application, high-pressure environment tests were performed for both the deep-water motor and the complete system, followed by field sea trials during the TS15 and TS21 voyages. The entire system was successfully tested in the field, verifying its capability to operate in 10 000 m-class abyssal environments. Compared to existing deep-sea microbial samplers worldwide, the system designed and developed in this study demonstrated the following advantages: The entire system was designed for 10 000 m abyssal operations and was capable of functioning at all sea depths; the multi-stage membrane filtration system was successfully applied in actual sea trials, and the combination of filter membranes with different pore sizes enabled the preliminary screening of microorganisms in seawater, reduced clogging in small-pore membranes, and enhanced filtration efficiency. The design of the inlet and outlet one-way valve structures and the volume compensator structure ensured that the system did not come into contact with seawater at non-target depths during the sampling process, fully achieving contamination-free and pressure-stable operation. In addition, the in situ fixed preservation structure injected RNAlater preservation solution into the samples immediately after filtration to effectively prevent RNA degradation during recovery and improve overall sample quality. Conclusions This study develops a set of in situ microbial multi-stage filtration sampling devices for operation across the entire sea depth. The system primarily utilizes a multi-stage membrane structure and an in situ preservation structure to enable the collection of enriched, high-quality microbial samples. The plunger pump, deep-sea oil-filled motors, RNAlater syringe pumps, and other functional components are designed and developed to meet the actual working requirements. High-voltage equipment is employed to test the oil-filled motors, and current values are obtained under different pressures, providing a foundation for structural adjustment and improvement. In addition, high-pressure testing of the oil-filled motor is conducted using specialized high-pressure equipment, and the current values under varying pressures are recorded to support further optimization of the sampling device. The device was successfully deployed twice in the Mariana Trench at depths exceeding 10 000 m and effectively obtained abyssal microbial samples, establishing a reliable basis for building an abyssal microbial gene pool and supporting potential application research. Future improvements of the sampling system can focus on structural optimization and weight reduction.
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