Carbon isotope discrimination (Δ¹³C) in seaweeds serves as a powerful tool in algal biotechnology to understand CO2 concentrating mechanisms (CCMs) and predict growth rates. By measuring the ¹³C/¹²C ratio in algal tissue (δ¹³C), researchers can infer whether seaweeds rely more on dissolved inorganic carbon sources, like CO₂ or bicarbonate (HCO₃⁻), which influences their photosynthetic efficiency. This knowledge helps select species for enhanced biomass production, bioenergy, and climate change mitigation, as well as optimizing cultivation conditions. In this research, by systematically collecting seaweed samples from diverse coastal habitats of Oman sea, including intertidal zones, subtidal regions, and lagoons, and analyzing them using an Isotope Ratio Mass Spectrometer (IRMS), significant variations in δ¹³C values, ranging from -8.42‰ to -19.71‰ were revealed. Carbon acquisition strategies among Oman sea macroalgae show clear distinctions based on habitat, morphology, and environmental conditions. Most species adopt mixed bicarbonate and dissolved CO₂ strategies to optimize carbon uptake under varying environmental gradients. However, specialized strategies, such as exclusive reliance on bicarbonate, are observed in taxa like Padina australis, particularly in shallow intertidal environments. Species with hollow or blade-like morphologies, such as Colpomenia sp. and Sargassum sp., demonstrate flexibility in their carbon uptake mechanisms, with δ¹³C values reflecting both bicarbonate and CO₂ utilization. In contrast, tubular forms like Scytosiphon sp. and siphonous morphologies like Caulerpa sp. display slightly more negative δ¹³C values, indicating increased reliance on CO₂ diffusion. In subtidal environments, membranous and cartilaginous species such as Hypnea cornuta and Gracilaria sp. exhibit lower δ¹³C values due to their increased dependence on passive CO₂ diffusion. These patterns suggest that morphological features, combined with habitat driven factors, play a pivotal role in determining carbon uptake efficiency. The findings highlight the diverse carbon fixation pathways employed by seaweeds in response to environmental stressors unique to the Oman Sea.
