Molecular composition characteristics of marine Synechococcus reflect the highly environmental heterogeneity in the shelf seas

Synechococcus, a dominant group of marine photosynthetic picophytoplankton, play a crucial role in primary productivity and carbon cycling within shelf seas. Its distribution patterns are highly sensitive to environmental changes, rendering it a valuable indicator for assessing ecosystem responses to both climate change and anthropogenic pressures. Nonetheless, the relationships between the genetic diversity of Synechococcus and environmental heterogeneity in the Bohai Sea and Yellow Sea have not been thoroughly explored. Here, we employed the high-throughput sequencing method to investigate the phylogenetic and phenogenetic diversity of Synechococcus in the Bohai and Yellow seas, revealing distinct genotype compositions depending on typical habitats. Regarding phylogenetic distribution, both S5.1 clades I and VI dominated in the Bohai Sea with an average proportion of 25.47 % and 26.20 %, respectively; while clades I and II became predominant in the Yellow Sea accounting for 47.72 % and 14.82 %, respectively. Phenogenetically, type 2 dominated in the Bohai Sea occupying 53.15 %; types 3a and 3dA were dominant in the Yellow Sea accounting for 49.51 % and 40.88 %, respectively. Temperature and salinity were identified as key environmental constraints influencing the distribution patterns of Synechococcus. At the sub-group level, clade IX and type 3dA showed distributions closely related to salinity, whereas types 3a were primarily influenced by dissolved oxygen. The findings underscored the significant correlations between the distribution patterns of Synechococcus and environmental properties, which were further supported by cluster results demonstrating similar patterns obtained through principal component analysis. These findings highlight the application potential of spatial niche analysis based on Synechococcus molecular markers in reflecting environmental changes in shelf seas. Future studies should focus on elucidating the molecular regulatory mechanisms governing the distribution patterns of Synechococcus, thereby providing a critical foundation for understanding the responses of keystone species to environmental stresses in shelf ecosystems.