Seasonal variability in community structure and metabolism of active deep-sea microorganisms

Learning about the metabolic activities and adaptations of deep-sea microbes is challenging, as sample collection and retrieval often cause RNA degradation and microbial community shifts. Here, we employed an in situ DNA/RNA co-extraction device to collect 18 time-series nucleic acid samples during winter and summer in the South China Sea, minimizing sampling perturbation for metatranscriptome and metagenome analyses. Between the two seasons, the prokaryotic microbiota showed seasonal variations in species composition. Burkholderiales dominated in summer, whereas Pseudomonadales, Bacillales, and Rhodobacterales were enriched in winter. However, the dominant transcriptionally active taxa affiliated with Nitrososphaerales, MGIII, SAR324, UBA11654, Marinisomatales, and Poseidoniales remained largely stable across seasons. Among eukaryotes, Ciliophora were the most active, whereas Retaria were abundant but inactive. Despite the stable active prokaryotic community, metabolic profiles differed significantly between seasons. In the winter, autotrophic microorganisms, particularly Nitrososphaerales, exhibited higher CO2 fixation activity via the 3HP/4HB cycle, accompanied by enhanced ammonia oxidation for energy generation. In addition, CO oxidation activity was also elevated. In the summer, the primary source of energy originated from heterotrophic microorganisms capable of utilizing fatty acids, benzoate, and H2, likely relying on anaerobic respiration within organic particles. This may relate to nutrient source variations as reflected by the different levels of microbial network complexity between the two seasons. Altogether, our in situ metatranscriptomes revealed the metabolic activities and adaptations of active microbial groups across seasons, providing a basis for identifying the microbial contributors to elemental cycles in the deep ocean.