Abstract
Macrophytes play a key role in coastal environments, acting to transform inorganic carbon into biologically available organic matter. This process supports the marine food web at large, however, the dynamics behind macrophyte carbon acquisition are not fully understood with factors influencing their ability to utilize different carbon forms (HCO3− and/or CO2) and subsequent release mechanics of this carbon remaining rather poorly understood. This study aims to investigate the physiological responses of two important Baltic Sea macrophytes, Ulva intestinalis and Cladophora glomerata. By examining the effects of pH drift inhibitors, coupled with carbon-concentrating mechanisms (CCMs) and dissolved organic carbon (DOC) dynamics, we provide insights into the complex adaptations of these macroalgae to changing environmental conditions. The results demonstrate that both species exhibit distinct capabilities to adapt their carbon concentration mechanisms (CCMs) but suggest that C. glomerata may potentially gain a photosynthetic advantage in future high CO2. The observed differences between pH and water motion highlight species-specific nuances in the regulation of dissolved organic carbon (DOC) release, aligning with current theories on DOC dynamics. This research underscores the importance of understanding macroalgal adaptation and fitness in both present and future coastal ecosystems, particularly as environmental changes continue to evolve. By examining these factors, the study contributes valuable insights into how macroalgae may respond to future climate shifts.