Effects of Salinity and Temperature on the Growth and Physiology of Chaetomorpha linum

Chaetomorpha linum exhibits a strong tolerance to fluctuations in salinity and temperature. To investigate its environmental adaptation strategies, a 12-day two-factor culture experiment was conducted at five salinities (0, 5, 10, 15, and 30) and two temperatures (15 and 25 ℃). Indicators related to growth, morphology, photosynthetic performance, osmotic regulation, key enzyme activity, and oxidative stress were systematically analyzed. The results showed that at 25 ℃ and salinities of 10 and 15, C. linum exhibited the highest relative growth rate, photosynthetic and respiratory activities, and ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) activity, leading to significantly enhanced carbon fixation and energy conversion efficiency. In contrast, under salinity 0 conditions, the alga died, while under salinity 5 and low temperature (15 ℃) conditions, its growth was significantly inhibited, exhibiting a combined stress effect. Low temperature induced cell wall thickening, a decline in maximum photochemical efficiency, and an increase in non-photochemical quenching, indicating the activation of photoprotective responses. Under optimal thermal conditions, cell volume expansion and reduced chlorophyll content reflected improved light energy utilization. Salinity reduction disturbed the osmotic homeostasis while the compatible solute synthesis gradually restored its stability. At combined low temperature and low salinity stress, nitrogen assimilation was restricted, which was accompanied by the accumulation of reactive oxygen species and malondialdehyde. Elevated catalase and glutathione peroxidase activities alleviated the oxidative damage. In conclusion, C. linum adapts to varying salinity and temperature through coordinated regulation of Rubisco activity, cellular morphology, pigment composition, osmotic balance, photoprotection, and antioxidant defense. Our findings revealed a multifaceted physiological response of the alga, provided an essential basis for evaluating the adaptive potential of C. linum in dynamic coastal environments and will support the future ecological management and resource utilization.