Gao et al. (1993) grew cultures of the red macroalgae Gracilaria sp. and G. chilensis in vessels enriched with nitrogen and phosphorus that were continuously aerated with normal air containing 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. 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 the Gracilaria sp.

With respect to these findings, the researchers comment that "in their natural habitats, 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 macroalgae should grow ever better in the years ahead. Such should also be the case with many other macroalgae, for Gao et al. 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.

In a subsequent study, Kubler et al. (1999) grew Lomentaria articulata, a red seaweed common to the Northeast Atlantic intertidal zone, for three weeks in hydroponic cultures subjected to various atmospheric CO2 and O2 concentrations. In doing so, they found that oxygen concentrations ranging from 10 to 200% of ambient had no significant effect on either the seaweed's daily net carbon gain or its total wet biomass production rate. In contrast, CO2 concentrations ranging from 67 to 500% of ambient had highly significant effects on these parameters. At twice the ambient CO2 concentration, for example, daily net carbon gain and total wet biomass production rates were 52 and 314% greater than they were at ambient CO2.

More recently, Zou (2005) collected specimens of the brown seaweed Hizikia fusiforme from intertidal rocks along the coast of Nanao Island, Shantou, China, and maintained them in glass aquariums that contained filtered seawater enriched with 60 µM NaNO3 and 6.0 µM NaH2PO4, while continuously aerating the aquariums with air of either 360 or 700 ppm CO2 and periodically measuring seaweed growth and nitrogen assimilation rates, as well as nitrate reductase activities. By these means they determined that the slightly less than a doubling of the air's CO2 concentration increased the seaweed's mean relative growth rate by about 50%, its mean rate of nitrate uptake during the study's 12-hour light periods by some 200%, and its nitrate reductase activity by approximately 20% over a wide range of substrate nitrate concentrations.

As a subsidiary aspect of the study, Zou notes that "the extract of H. fusiforme has an immunomodulating activity on humans and this ability might be used for clinical application to treat several diseases such as tumors (Suetsuna, 1998; Shan et al., 1999)." He also reports that the alga "has been used as a food delicacy and an herbal ingredient in China, Japan and Korea." In fact, he says that it "is now becoming one of the most important species for seaweed mariculture in China, owing to its high commercial value and increasing market demand." As a result, the ongoing rise in the air's CO2 content bodes well for all of these applications. In addition, Zou notes that "the intensive cultivation of H. fusiforme would remove nutrients more efficiently with the future elevation of CO2 levels in seawater, which could be a possible solution to the problem of ongoing coastal eutrophication," suggesting that rising CO2 levels may also assist in the amelioration of this environmental problem.

In light of these several observations, there is reason to believe that just as with freshwater macrophytes, the ongoing rise in the air's CO2 content should help marine macroalgae to become ever more productive with the passage of time.

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.

Gao, K., Aruga, Y., Asada, K. and Kiyohara, M. 1993. Influence of enhanced CO2 on growth and photosynthesis of the red algae Gracilaria sp. and G. chilensis. Journal of Applied Phycology 5: 563-571.

Kubler, J.E., Johnston, A.M. and Raven, J.A. 1999. The effects of reduced and elevated CO2 and O2 on the seaweed Lomentaria articulata. Plant, Cell and Environment 22: 1303-1310.

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.

Shan, B.E., Yoshida, Y., Kuroda, E. and Yamashita, U. 1999. Immunomodulating activity of seaweed extract on human lymphocytes in vitro. International Journal of Immunopharmacology 21: 59-70.

Suetsuna, K. 1998. Separation and identification of angiotensin I-converting enzyme inhibitory peptides from peptic digest of Hizikia fusiformis protein. Nippon Suisan Gakkaishi 64: 862-866.

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.

Zou, D. 2005. Effects of elevated atmospheric CO2 on growth, photosynthesis and nitrogen metabolism in the economic brown seaweed, Hizikia fusiforme (Sargassaceae, Phaeophyta). Aquaculture 250: 726-735.