Deep-sea water columns are enriched with SAR202 that may conduct detrital matter degradation. There are several subclusters in SAR202, but their subtle differences in geochemical cycles are largely unknown, particularly for their in situ activities in the marine deep zone. Deep-sea DNA/RNA samples obtained from 12 continuous time periods over two days by in situ nucleic acid collection apparatus were used to re-evaluate the ecological functions of each SAR202 subcluster at a depth of similar to 1000 m in the South China Sea (SCS). Phylogenomics of 32 new SAR202 genomes from the SCS and western Pacific revealed their distribution in five subclusters. Metatranscriptomics analysis showed that the subclusters II and III were the dominant SAR202 groups with higher transcriptional activities in the SCS deep-sea zone than other subclusters. The analyses of functional gene expression further indicated that SAR202 subclusters II and III might be involved in different metabolic pathways in the deep-sea environment. The SAR202 subcluster III might take part in the degradation of deep-sea aromatic compounds. Time-course metagenomics and metatranscriptomics data did not show metabolic correlation of subclusters II and III over two days, suggesting diversified ecological functions of SAR202 subclusters under different organic inputs from the overlying water column. Collectively, our results indicate that the SAR202 subclusters play different roles in organic degradation and have probably undergone subtle and gradual adaptive evolution in the dynamic environment of the deep ocean.