Hydrothermal influence reduces the nutritional quality of the Antarctic seaweed Monostroma hariotii (Chlorophyta)

Seaweeds are foundational primary producers that support marine food webs. Their resilience in fluctuating environments is partially facilitated by their ability to synthesize diverse biochemical compounds with key physiological roles, many of which also offer nutritional benefits to consumers. Because seaweed biochemical composition is shaped by environmental conditions, it is potentially sensitive to global change stressors such as ocean warming and acidification. Yet, species-specific biochemical responses of Antarctic seaweeds remain poorly understood so far. We studied fatty acid, carbon, nitrogen, and pigment levels in the green seaweed Monostroma hariotii Gain, 1911 (Chlorophyta) from two Antarctic stations representing ambient Antarctic conditions (Rongé and Amsler Islands), and compared them with specimens naturally exposed to elevated temperatures (⁓2.7–3.2 °C higher) and reduced pH (⁓1.2 units lower) at a fumarole vent site on Deception Island (South Shetland Islands). Specimens from Rongé and Amsler Islands exhibit high nutritional quality, with elevated polyunsaturated fatty acids (PUFA) n-3 (39.39 ± 1.11 and 37.37 ± 1.05% TFA, respectively), low C:N ratios (6.93 ± 0.05 and 7.20 ± 0.08, respectively), and abundant pigments. By contrast, specimens from the fumarole vent site had reduced PUFA n-3 (29.01 ± 2.48% TFA) and pigments, along with higher PUFA n-6 levels (⁓96–145% higher) and elevated C:N ratios (8.97 ± 0.30), suggesting a decrease in nutritional value under warmer and acidic conditions. Our findings suggest that ocean warming and acidification may alter the biochemical profile of M. hariotii , potentially leading to a decline in its nutritional value under future climate scenarios. • Hydrothermal influence decreases PUFA n-3 relative to PUFA n-6 in the Antarctic green seaweed Monostroma hariotii. • Monostroma hariotii from fumarole vents exhibit decreased N and increased C:N ratios. • Exposure to hydrothermal influence decreases β -carotene and chlorophyll b levels in this species. • Shifting biochemical composition of M. hariotii from fumarole vents may suggest a decrease in nutritional value under ocean warming and acidification.