The relative roles of cell size and phylogeny in driving dissimilarity of algal functional trait

Algae have attracted extensive attention for their potential in photosynthetic carbon fixation and the synthesis of high-value bioproducts. To reveal the intrinsic regulatory mechanisms by which cell size and phylogeny influence algal physiological functions and resource allocation strategies, three cyanobacteria, three green algae, and two diatoms were selected for this study. Cell size was characterized by the median particle size (D50), and multidimensional functional traits were measured, including growth characteristics, photosynthetic performance, elemental stoichiometry, and key biomacromolecules. The underlying regulatory pathways were further analyzed using partial least squares structural equation modeling (PLS-SEM). The results revealed that cell size predominantly governs growth strategy differentiation. Small-sized algae (D50 < 10 μm) adopted a proliferation-oriented growth strategy, enabled by high specific surface area (SSA, 0.9 m²·g^-1) and cell density (> 7.00 × 10⁶ cells·mL^-1). In contrast, large-sized algae (D50 > 10 μm) shifted toward an accumulation-oriented growth strategy, enhancing unicellular biomass and unicellular biomacromolecular concentration. Photosynthetic strategies exhibited clear phylum-specific. PLS-SEM further quantitatively confirmed these drivers. Cell size negatively regulated growth rate, whereas phylogeny was the dominant factor shaping photosynthetic performance. Moreover, a significant negative correlated between growth rate and C/P and N/P ratios was observed in proliferative small-sized algae. This relationship was weakened in accumulative large-sized algae due to stronger phosphorus storage. This study provides a comprehensive framework demonstrating that cell size and phylogeny play dominant roles in shaping the differentiation of algal physiological traits and ecological strategies s, providing theoretical and practical guidance for targeted algal resource utilization.