What was done
The authors grew cultures of the red algae Gracilaria sp. and G. chilensis in the laboratory in vessels enriched with nitrogen and phosphorus that were continuously aerated with normal air of 350 ppm CO2, air enriched with an extra 650 ppm CO2, and air enriched with an extra 1250 ppm CO2 for a period of 19 days.

What was learned
Compared to the control treatments, the relative growth enhancements in the + 650-ppm and +1250-ppm CO2 treatments were 20% and 60%, respectively, for G. chilensis, and 130% and 190%, respectively, for Gracilaria sp.

What it means
The authors say their study "suggests that, in their natural habitats or cultivation sites, photosynthesis and growth of Gracilaria species are likely to be CO2-limited, especially when the population density is high and water movement is slow."  Hence, as the air's CO2 content continues to rise, these marine marcoalgae should grow ever better in the years ahead.  Such should also be the situation with respect to most other macroalgae, for the authors note that "photosynthesis by most macroalgae is probably limited by inorganic carbon sources in natural seawater," citing the studies of Surif and Raven (1989), Maberly (1990), Gao et al. (1991) and Levavasseur et al. (1991) as evidence for this statement.

References
Gao, K., Aruga, Y., Asada, K., Ishihara, T., Akano, T. and Kiyohara, M.  1991.  Enhanced growth of the red alga Porphyra yezoensis Ueda in high CO2 concentrations.  Journal of Applied Phycology 3: 355-362.

Levavasseur, G., Edwards, G.E., Osmond, C.B. and Ramus, J.  1991.  Inorganic carbon limitation of photosynthesis in Ulva rotundata (Chlorophyta).  Journal of Phycology 27: 667-672.

Maberly, S.C.  1990.  Exogenous sources of inorganic carbon for photosynthesis by marine macroalgae.  Journal of Phycology 26: 439-449.

Surif, M.B. and Raven, J.A.  1989.  Exogenous inorganic carbon sources for photosynthesis in seawater by members of the Fucales and the Laminariales (Phaeophyta): ecological and taxonomic implications.  Oecologia 78: 97-103.