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Photosynthesis in the seagrass Zostera muelleri remains poorly understood. We investigated the effect of reduced irradiance on the incorporation of 13C, gene expression of photosynthetic, photorespiratory and intermediates recycling genes as well as the enzymatic content and activity of Rubisco and PEPC within Z. muelleri. Following 48 h of reduced irradiance, we found that i) there was a ∼7 fold reduction in 13C incorporation in above ground tissue, ii) a significant down regulation of photosynthetic, photorespiratory and intermediates recycling genes and iii) no significant difference in enzyme activity and content. We propose that Z. muelleri is able to alter its physiology in order to reduce the amount of C lost through photorespiration to compensate for the reduced carbon assimilation as a result of reduced irradiance. In addition, the first estimated rate constant (K cat) and maximum rates of carboxylation (V cmax) of Rubisco is reported for the first time for Z. muelleri. • Photosynthetic carbon fixation is reduced in Zostera muelleri when exposed to low irradiance. • Photosynthesis, photorespiration and intermediates recycling genes are down regulated. • No significant changes in Rubisco content or phosphoenol pyruvate carboxylase activity. [ABSTRACT FROM AUTHOR]