Abstract:

The unicellular picocyanobacteria, represented by Prochlorococcus and Synechococcus, are dominant primary producers in the ocean and play essential roles in nutrient cycling. In natural environments, they are continuously exposed to biological and metabolic pressures, including cyanophage infection and fluctuations in dissolved amino acids as organic nitrogen sources. Although these pressures influence cyanobacterial survival and ecological fitness, the molecular mechanisms underlying their adaptive responses remain incompletely understood. This thesis investigates the evolutionary molecular mechanisms of cyanobacteria under cyanophage and amino acid stresses by integrating metagenomics, whole-genome sequencing and physiological analyses. For cyanophage-driven selection, laboratory evolution identified 18 phage-resistant Synechococcus mutants containing 128 genomic mutations. Further, metagenomic analyses across 4 representative Chinese estuaries recovered 83 cyanophage viral operational taxonomic units and 77 cyanobacterial metagenome-assembled genomes, revealing diverse cyanophage communities and auxiliary metabolic genes related to host metabolism. Notably, mutations in rfbA, related to lipopolysaccharide biosynthesis, and cpeT, associated with photosynthetic energy transfer, were detected in both laboratory-evolved mutants and field-derived metagenomes. For amino acid stress, growth responses of Prochlorococcus MED4, Prochlorococcus MIT9313, and Synechococcus WH8102 were examined, and 60 spontaneous mutant substrains were isolated. Whole-genome analyses showed that adaptive mutations were enriched in membrane-associated functions, amino acid biosynthesis, ABC transporters, redox-related proteins, and polysaccharide biosynthesis. Together, these results show that picocyanobacteria adapt to cyanophage infection and amino acid stresses through coordinated changes in cell-surface structures, membrane transport systems, energy metabolism, redox balance, and genome evolution.