Heavy-ion beam-induced mutants of Medakamo hakoo indicate potential associations between photosynthesis and cell size, cell cycle, and cell wall morphology.

Medakamo hakoo is an ultrasmall green alga with a simplified cellular structure, offering potential as a new model organism. To explore the genetic basis of cell morphology and its physiological implications, we applied carbon-ion beam irradiation to induce mutations in M. hakoo and successfully isolated two mutants: LRG (Large) and TTR (Tetra). LRG exhibited significantly enlarged cell size and increased chlorophyll content, but lower photosynthetic efficiency compared to the wild-type. Proteomic analysis of LRG revealed upregulation of photosynthesis-related proteins, stress response proteins, and ribosomal proteins, suggesting a link between increased cell size and disrupted photosynthetic homeostasis. TTR showed a higher proportion of dividing cells throughout the light-dark cycle and exhibited cell aggregation. Proteomic profiling revealed increased abundance of dynein and cell wall-modifying enzymes such as expansin and mannosidases, indicating that TTR may undergo cell cycle delay or dysregulation of cytoplasmic and cell wall dynamics. TTR also displayed decreased levels of photosynthetic proteins and reduced photosynthetic activity. Both mutants demonstrated slower growth compared to the wild-type. These findings highlight the close relationship between cell size and cell cycle/cell wall dynamics with photosynthetic activity, providing new insights into the cellular regulatory mechanisms of photosynthetic microalgae. Our study also demonstrates the utility of heavy-ion mutagenesis in dissecting microalgal physiology in species for which genetic transformation systems have not yet been developed.