A high CO2 world is likely to favor some marine macroalgae over others.  Marguerite Koch and coworkers recently published a meta-analysis, Climate change and ocean acidification effects on seagrasses and marine macroalgae (Global Change Biology 24 SEP 2012), which examines previous research on over 100 species of marine algae and their responses to high CO2 / low pH conditions.  They concluded that  calcareous macroalgae, like many corals and shellfish, are highly vulnerable to future ocean acidification and predict that fleshy macroalgae will take advantage of increased carbon assimilation rates at high CO2 levels to dominate in higher CO2 oceans.