Concentrations of dissolved carbon dioxide (CO2) in the ocean are predicted to increase by 200% by the year 2100 due to anthropogenic activity, causing ocean acidification. The responses of seaweeds to increased CO2 is difficult to predict but may depend on whether their growth/productivity is currently limited by dissolved inorganic carbon (DIC) supply, and the mechanism by which they take up DIC. In natural seawater, DIC is primarily available as bicarbonate which cannot pass through the seaweed plasmalemma via diffusion; 65% of seaweeds possess mechanisms which aid its uptake, termed CO2 concentrating mechanisms (CCMs). However, 35% of seaweeds do not possess CCMs and solely use CO2 as their source of DIC (non-CCM species). As DIC levels increase in the ocean, seaweeds which are currently limited for DIC may utilise this additional carbon for growth. Once DIC is assimilated into organic carbon via photosynthesis, seaweeds may release dissolved organic carbon (DOC) into the surrounding seawater depending on local environmental conditions such as light and nitrogen. Diaz-Pulido et al. (2020) found a greater DOC release with an increase in seawater DIC i.e. increasing ocean acidification. We tested the effect of ocean acidification on DOC release by three temperate seaweed species with differing DIC uptake mechanisms: Hemineura frondosa (Rhodophyta, non-CCM), Lenormandia marginata (Rhodophyta, CCM) and Ecklonia radiata (Ochrophyta, CCM). I will discuss the experimental design, results and implications of our findings for future ocean acidification experiments.