Sulfated Flavonoids from <i>Phyllospadix</i> (Zosteraceae) Taxa from Baja California, Mexico

Sulfated flavonoids, a class of polyphenols integral to plant secondary metabolism and chemical defense, exhibit notable pharmacological potential. Seagrasses, marine angiosperms with critical ecological and socioeconomic roles, often accumulate these compounds in high concentrations. However, their complex chemical profiles-including closely related sulfated flavonoids-are challenging to characterize due to potential degradation during extraction. This study provides the first comprehensive analysis of sulfated flavonoids in <i>Phyllospadix scouleri</i> alongside a comparative analysis of <i>P. torreyi</i>. The <i>Phyllospadix</i> genus, known for forming productive intertidal meadows on rocky Pacific coastlines of North America, serves as a valuable model for understanding flavonoid diversity and adaptation in marine environments. From <i>P. scouleri</i> foliar tissues, we isolated and identified 1 phenolic acid and 15 sulfated flavonoids (HPLC-DAD, NMR, LC-MS), including previously undescribed 6-hydroxyflavonoid disulfates and monosulfates, and flavonoids not earlier reported for the genus. Lower amounts of sulfated glycosides were also tentatively identified in both species for the first time. The flavonoid profiles showed clear species-specific patterns: <i>P. scouleri</i> primarily produced 6-hydroxyflavonoids (70%), while <i>P. torreyi</i> favored 5- and 6-methoxyflavonoids (60 and 70%). Samples collected from nearby locations in May 2024 from both species showed similar flavonoid concentrations (∼20 mg/g DW) and comparable ratios between total flavonoids and rosmarinic acid (∼6:1). <i>P. torreyi</i> exhibited more disulfated flavonoids (84.3%) than monosulfated types (11.9%), whereas <i>P. scouleri</i> had 25.2% disulfated and 66.5% monosulfated flavonoids. Given the proven link between phenolic compounds and the physiological acclimation of surfgrasses to emersion during intertidal periods, as well as to marine heatwaves, this study provides a robust baseline for further research into the basic chemical ecology of these compounds and their responses to climate change.