Responses of macroalgae to CO enrichment cannot be inferred solely from their inorganic carbon uptake strategy.

Increased plant biomass is observed in terrestrial systems due to rising levels of atmospheric CO, but responses of marine macroalgae to CO enrichment are unclear. The 200% increase in CO by 2100 is predicted to enhance the productivity of fleshy macroalgae that acquire inorganic carbon solely as CO (non-carbon dioxide-concentrating mechanism [CCM] species-i.e., species without a carbon dioxide-concentrating mechanism), whereas those that additionally uptake bicarbonate (CCM species) are predicted to respond neutrally or positively depending on their affinity for bicarbonate. Previous studies, however, show that fleshy macroalgae exhibit a broad variety of responses to CO enrichment and the underlying mechanisms are largely unknown. This physiological study compared the responses of a CCM species () with a non-CCM species () to CO enrichment with regards to growth, net photosynthesis, and biochemistry. Contrary to expectations, there was no enrichment effect for the non-CCM species, whereas the CCM species had a twofold greater growth rate, likely driven by a downregulation of the energetically costly CCM(s). This saved energy was invested into new growth rather than storage lipids and fatty acids. In addition, we conducted a comprehensive literature synthesis to examine the extent to which the growth and photosynthetic responses of fleshy macroalgae to elevated CO are related to their carbon acquisition strategies. Findings highlight that the responses of macroalgae to CO enrichment cannot be inferred solely from their carbon uptake strategy, and targeted physiological experiments on a wider range of species are needed to better predict responses of macroalgae to future oceanic change.