Different photosynthetic responses to heat and light favour green and red over brown macroalgae in the mediterranean sea

Global change is rapidly altering temperature and light regimes in marine environments, yet the physiological responses of benthic primary producers to these shifts remain poorly understood. Red (Phyllophora), brown (Cystoseira) and green (Flabellia) macroalgae dominate distinct assemblages in the Mediterranean Sea and act as ecosystem engineers that overgrow rocky substrates or seagrass meadows. Their taxon-specific physiological responses to changing temperature and light levels may have long-lasting consequences on algal community composition and overall ecosystem functioning, but remain poorly understood. Here, we experimentally test how water temperature and light availability affect photosynthesis and respiration in these macroalgae using specimens collected on Giglio Island, Italy. We exposed different macroalgae to a full-factorial combination of three temperatures (21, 26, and 30 °C) and light intensity levels (180, 320, and 760 µM photons m^{- 2} s^{- 1}) and measured oxygen fluxes to determine net photosynthesis (P_n) and respiration (R). Temperature and light both significantly influenced net photosynthesis, with marked taxon-specific responses. In Cystoseira, photosynthesis nearly ceased in high light and high temperature conditions, also indicated by a critically low photosynthesis to respiration (P:R) ratio. Flabellia and Phyllophora maintained more stable photosynthetic rates under medium light and temperature conditions and showed a moderate decline in their P: R ratios. Flabellia sustained slightly higher photosynthesis rates in high light and temperature conditions than Phyllophora and both significantly outperformed Cystoseira under maximal light and temperature conditions. Our findings suggest that under increasing temperature and light regimes, Cystoseira may decline, Phyllophora may persist, and Flabellia may even thrive. This may in turn degrade habitat complexity and affect coastal community assembly, as Flabellia, unlike the architecturally complex and persistent Cystoseira, does not form stable, three-dimensional habitats for associated organisms.