The effect of increased CO2 concentration on the green, Ulva lactuca, Acrosiphonia arcta, and A. sonderii, and the brown, Laminaria digitata, Saccharina latissima and Pylaiella littoris, seaweeds was investigated in the laboratory. We designed to aerate algal discs of sheet-like Ulva, filaments of Acrospihonia and Pylaiella and young sporophytes of thick-blade kelps in 1L bottles with pre-industrial (280 ppm), ambient (≈ 380 ppm), and elevated (700 ppm) carbon dioxide for 15 days under 15°C and saturating photosynthetically active radiation (PAR). Routine measurements of CO2 using LICOR gas analyser showed the concentration in compressed ambient air was severely reduced down to 160 ppm effectively altering our desired experimental treatment to the lower spectrum compared to the pre-mixed bottled CO2. Water chemistry showed our CO2 concentrations were negatively correlated with pH and dissolved CO3, and positively correlated with dissolved CO2, and HCO3. Increasing CO2 concentration and consequent acidification was observed to exhibit no significant effect on photosynthesis and growth of all algae examined except for the higher growth rate in U. lactuca under 700 ppm compared to 160 ppm. In retrospect, our 160 ppm treatment may have no ecological significance relative to pre-industrial and the climate change-projected increase in CO2 concentration but showed the ability of some non-calcareous seaweed to cope up with extremely low and high CO2 concentrations to support photosynthesis and growth; an indication of a possible induction and suppression of carbon-concentrating mechanism at low and high CO2 concentrations, respectively, without extra energetic cost in most of the macroalgae investigated.