Background
The authors write that "Gracilaria lemaneiformis (Bory) Weber-van Bosse is an economically important red seaweed that is cultivated on a large scale in China due to the quantity and quality of agar in its cell walls." In addition, they say that "much attention has been paid to the biofiltration capacity of the species (Yang et al., 2005, 2006; Zhou et al., 2006)," and that it has thus been suggested to be "an excellent species for alleviating coastal eutrophication in China (Fei, 2004)."

What was done
Plants were grown from thalli -- which were collected at 0.5 m depth from a farm located in Shen'ao Bay, Nanao Island, Shantou (China) -- for a period of 16 days in 3-L flasks of natural seawater maintained at either natural (0.5 µM) or high (30 µM) dissolved inorganic phosphorus (Pi) concentrations in contact with air of either 370 or 720 ppm CO2, while their photosynthetic rates, biomass production, and uptake of nitrate and phosphate were examined.

What was learned
As best we can determine from Xu et al.'s graphical representations of their results, algal photosynthetic rates in the natural Pi treatment were only increased by a non-significant 5% due to the 95% increase in the air's CO2 concentration, while in the high Pi treatment they were increased by approximately 41%. In the case of growth rate or biomass production, on the other hand, the elevated CO2 treatment exhibited a 48% increase in the natural Pi treatment, while in the high Pi treatment there was no CO2-induced increase in growth, due to the fact that the addition of the extra 29.5 µM Pi boosted the biomass production of the low-CO2 natural-Pi treatment by approximately 83%, and additional CO2 did not increase growth rates beyond that point.

What it means
The three Chinese researchers say that "elevated levels of CO2 in seawater increase the growth rate of many seaweed species despite the variety of ways in which carbon is utilized in these algae," noting that "some species, such as Porphyra yezoensis Ueda (Gao et al., 1991) and Hizikia fusiforme (Harv.) Okamura (Zou, 2005) are capable of using HCO3-, but are limited by the current ambient carbon concentration in seawater," and that "enrichment of CO2 relieves this limitation and enhances growth." With respect to the results they obtained with Gracilaria lemaneiformis, on the other hand -- which they say "efficiently uses HCO3- and whose photosynthesis is saturated at the current inorganic carbon concentration of natural seawater (Zou et al., 2004)" -- they write that "the enhancement of growth could be due to the increased nitrogen uptake rates at elevated CO2 levels," which in their experiment were 40% in the natural Pi treatment, because "high CO2 may enhance the activity of nitrate reductase (Mercado et al., 1999; Gordillo et al., 2001; Zou, 2005) and stimulate the accumulation of nitrogen, which could contribute to growth." So whatever strategy may be employed, these several marine macroalgae appear to be capable of benefiting greatly from increased atmospheric CO2 concentrations.

References
Fei, X.G. 2004. Solving the coastal eutrophication problem by large scale seaweed cultivation. Hydrobiologia 512: 145-151.

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: 356-362.

Gordillo, F.J.L., Niell, F.X. and Figueroa, F.L. 2001. Non-photosynthetic enhancement of growth by high CO2 level in the nitrophilic seaweed Ulva rigida C. Agardh (Chlorophyta). Planta 213: 64-70.

Mercado, J.M., Javier, F., Gordillo, L., Niell, F.X. and Figueroa, F.L. 1999. Effects of different levels of CO2 on photosynthesis and cell components of the red alga Porphyra leucosticia. Journal of Applied Phycology 11: 455-461.

Yang, H., Zhou, Y., Mao, Y., Li, X., Liu, Y. and Zhang, F. 2005. Growth characters and photosynthetic capacity of Gracilaria lemaneiformis as a biofilter in a shellfish farming area in Sanggou Bay, China. Journal of Applied Phycology 17: 199-206.

Yang, Y.F., Fei, X.G., Song, J.M., Hu, H.Y., Wang, G.C. and Chung, I.K. 2006. Growth of Gracilaria lemaneiformis under different cultivation conditions and its effects on nutrient removal in Chinese coastal waters. Aquaculture 254: 248-255.

Zhou, Y., Yang, H., Hu, H., Liu, Y., Mao, Y., Zhou, H., Xu, X. and Zhang, F. 2006. Bioremediation potential of the macroalga Gracilaria lemaneiformis (Rhodophyta) integrated into fed fish culture in coastal waters of north China. Aquaculture 252: 264-276.

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.

Zou, D., Xia, J. and Yang, Y. 2004. Photosynthetic use of exogenous inorganic carbon in the agarophyte Gracilaria lemaneiformis (Rhodophyta). Aquaculture 237: 421-431.