Phenolic Characterization and Comparative Antioxidant Profiling of Australian Asparagopsis armata and A. taxiformis Across Their Developmental Stages

<i>Asparagopsis</i> has gained global attention for its chemical properties and environmental applications. However, its two main species, <i>Asparagopsis armata</i> and <i>Asparagopsis taxiformis</i>, remain understudied, with limited information available regarding their bioactive potential, especially across their development. In this study, we examined the phenolic profiles and antioxidant potentials of gametophyte and tetrasporophyte life stages and compared differences between conventional solvent extraction (CSE) and ultrasound-assisted extraction (UAE), including total phenol content, total flavonoid content, determination of condensed tannins, and seven types of antioxidant activity detections such as DPPH and ABTS. In general, the phenolic compounds and antioxidant potential of the <i>Asparagopsis</i> species vary significantly at different life stages and under different extraction techniques. Among them, the phenolic profile and antioxidant capacity of <i>A. armata</i> were recorded as significantly higher than those of <i>A. taxiformis</i>, as reflected by its greater relative antioxidant capacity index scores. In our study, while UAE did not universally outperform CSE, species- and life stage-specific improvements were recorded. Moreover, LC-ESI-QTOF-MS/MS tentatively identified 24 phenolic compounds (17 in <i>A. armata</i> and 14 in <i>A. taxiformis)</i>, pointing to a diverse bioactive profile. Overall, <i>Asparagopsis</i> species demonstrated marked variability in phenolic and antioxidant potentials across life stages and extraction techniques.