Advancing Pyropia kinositae gametophyte cultivation: a study of environmental, physiological, and biochemical factors

Pyropia kinositae, a cold-water red algal species found in East Asian marine environments, thrives on aquaculture rafts and adapts to continuous submersion. Its robust growth and suitability for offshore and northern regions make P. kinositae ideal for evolving mariculture practices in China, aligning with the industry's shift towards these areas. This study extensively investigated the optimal cultivation conditions (temperature, light, salinity, and nutrient levels) for the gametophyte generation of P. kinositae, focusing on conchospore germination, conchosporeling growth, and thallus growth. Conchospore germination and conchosporeling growth were optimal at 17 °C and salinity of 25–30, with light intensity optimal at 40–70 μmol photons m⁻² s⁻¹ for conchospore germination and 100 μmol photons m⁻² s⁻¹ for conchosporeling growth. The thalli showed best growth at 2 mg L⁻¹ nitrogen, particularly at 12 °C with light intensity between 30–90 μmol photons m⁻² s⁻¹, where the peak photosynthetic rate and F_v/F_m value were observed, indicating optimal photosynthetic efficiency. Biochemical analysis of the thalli revealed significant variations in pigments such as chlorophyll a and carotenoid, alongside soluble proteins, ATP, and enzymes involved in photosynthesis, respiration, antioxidation, and nitrogen assimilation. Notably, higher nitrogen concentrations led to increased contents of soluble proteins, ATP, chlorophyll a, and carotenoid, as well as elevated activities of phosphofructokinase (PFK), nitrate reductase (NR) and malate dehydrogenase (MDH), suggesting increased nutrient availability enhanced the metabolic activities of the thalli. At a lower nitrogen concentration (0.1 mg L⁻¹), the highest SOD activity and H₂O₂ contents were observed, indicating increased ROS generation and scavenging under stress of nutrient levels. Activities of antioxidant enzymes, such as SOD and CAT, increased with temperature, peaking at 13 °C, highlighting the enhanced defense against oxidative stress at this temperature. Additionally, RuBisCO activity was the highest at a light intensity of 70 μmol photons m⁻² s⁻¹. The optimal environmental conditions for the growth of P. kinositae gametophyte generation were obtained based on the physiological and biochemical performance, offering crucial information for optimizing cultivation practices for this economically valuable macroalga.