Effects of salinity, irradiance and photoperiod on tetraspore germination and early development of four Gracilaria species (Rhodophyta)

Spore-based propagation offers a sustainable alternative to vegetative reproduction in Gracilaria aquaculture, mitigating germplasm degradation and supporting long-term cultivation. This study examines the effects of salinity (6–36 psu), irradiance (25–115 μmol photons·m⁻²·s⁻¹), and photoperiod (8L:16D to 14L:10D) on the early development of four underutilized Gracilaria species native to southern China— G. fanii , G. bailinae , G. hainanensis , and G. punctata . All species were significantly influenced by salinity ( p < 0.05), with G. fanii exhibited a broad salinity tolerance (18-36 psu) for tetraspore attachment and disc survival, achieving an optimal growth at 30 psu (RGR: 5.64 %·d⁻¹); while G. punctata was more sensitive to salinity extremes but showed improved survival at 30 psu (RGR: 4.75 %·d⁻¹). G. bailinae favored moderate salinity (24 psu), low irradiance (25 μmol photons·m⁻²·s⁻¹), and 14L:10D regime, achieving 76.4 % survival and 44.4 % erect thallus emergence; In contrast, G. hainanensis tolerated high irradiance (up to 115 μmol photons·m⁻²·s⁻¹) and performed best at 30 psu under a 12L:12D regime, with 86.3% survival and 95.4% thallus emergence. These findings reveal distinct photophysiological and salinity adaptation strategies among lesser-known Gracilaria spp. and support their application in sustainable reproduction.