How does rising atmospheric CO2 affect marine organisms?

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Aquatic Plants (Freshwater - Algae) -- Summary
How do freshwater algae respond to increases in the air's CO2 content? The subject has not been thoroughly researched, but the results of the studies discussed below provide a glimpse of what the future may hold as the atmosphere's CO2 concentration continues its upward course.

Working with cells of the freshwater alga Chlorella pyrenoidosa, Xia and Gao (2003) cultured them in Bristol's solution within controlled environment chambers maintained at low and high light levels (50 and 200 Ámol/m▓/s) during 12-hour light periods that were followed by 12-hour dark periods for a total of 13 days, while the solutions in which the cells grew were continuously aerated with air of either 350 or 700 ppm CO2. When the cells were harvested (in the exponential growth phase) at the conclusion of this period, the biomass (cell density) of the twice-ambient CO2 treatment was found to be 10.9% and 8.3% greater than that of the ambient-air treatment in the low- and high-light regimes, respectively, although only the high-light result was statistically significant. The two scientists thus concluded from these observations that a "doubled atmospheric CO2 concentration would affect the growth of C. pyrenoidosa when it grows under bright solar radiation, and such an effect would increase by a great extent when the cell density becomes high." Their data also suggest that the same may well be true when the alga grows under not-so-bright conditions.

Working on a much larger scale "in the field" with six 1.5-m-diameter flexible plastic cylinders placed in the littoral zone of Lake Hampen in central Jutland, Denmark (three maintained at the ambient CO2 concentration of the air and three enriched to ten times the ambient CO2 concentration), Andersen and Andersen (2006) measured the CO2-induced growth response of a mixture of several species of filamentous freshwater algae dominated by Zygnema species, but containing some Mougeotia and Spirogyra. After one full growing season (May to November), they determined that the biomass of the microalgal mixture in the CO2-enriched cylinders was increased by 220% in early July, by 90% in mid-August, and by a whopping 3,750% in mid-November.

In another study of the subject, Schippers et al. (2004a) say "it is usually thought that unlike terrestrial plants, phytoplankton will not show a significant response to an increase of atmospheric CO2," but they note, in this regard, that "most analyses have not examined the full dynamic interaction between phytoplankton production and assimilation, carbon-chemistry and the air-water flux of CO2," and that "the effect of photosynthesis on pH and the dissociation of carbon (C) species have been neglected in most studies."

In an attempt to rectify this situation, Schippers et al. developed "an integrated model of phytoplankton growth, air-water exchange and C chemistry to analyze the potential increase of phytoplankton productivity due to an atmospheric CO2 elevation," and as a test of their model, they let the freshwater alga Chlamydomonas reinhardtii grow in 300-ml bottles filled with 150 ml of a nutrient-rich medium at enclosed atmospheric CO2 concentrations of 350 and 700 ppm that they maintained at two air-water exchange rates characterized by CO2 exchange coefficients of 2.1 and 5.1 m day-1, as described by Shippers et al. (2004b), while periodically measuring the biovolume of the solutions by means of an electronic particle counter. The results of this effort, as they describe it, "confirm the theoretical prediction that if algal effects on C chemistry are strong, increased phytoplankton productivity because of atmospheric CO2 elevation should become proportional to the increased atmospheric CO2," which suggests that algal productivity "would double at the predicted increase of atmospheric CO2 to 700 ppm." Although they note that "strong algal effects (resulting in high pH levels) at which this occurs are rare under natural conditions," they still predict that effects on algal production in freshwater systems could be such that a "doubling of atmospheric CO2 may result in an increase of the productivity of more than 50%."

In the last of the few papers we have reviewed in this area, Logothetis et al. (2004) note that "the function and structure of the photosynthetic apparatus of many algal species resembles that of higher plants (Plumley and Smidt, 1984; Brown, 1988; Plumley et al., 1993)," and that "unicellular green algae demonstrate responses to increased CO2 similar to those of higher plants in terms of biomass increases (Muller et al., 1993)." However, they also note that "little is known about the changes to their photosynthetic apparatus during exposure to high CO2," which deficiency they began to correct via a new experiment, wherein batches of the unicellular green alga Scenedesmus obliquus (wild type strain D3) were grown autotrophically in liquid culture medium for several days in a temperature-controlled water bath of 30░C at low (55 Ámol m-2 s-1) and high (235 Ámol m-2 s-1) light intensity while they were continuously aerated with air of either 300 or 100,000 ppm CO2. This protocol revealed that exposure to the latter high CO2 concentration produces, in their words, a "reorganization of the photosynthetic apparatus" that "leads to enhanced photosynthetic rates, which ... leads to an immense increase of biomass." After five days under low light conditions, for example, the CO2-induced increase in biomass was approximately 300%, while under high light conditions it was approximately 600%.

Based on these few observations, it is not possible to draw any sweeping conclusions about the subject. However, they do indicate there may be a real potential for the ongoing rise in the air's CO2 content to significantly stimulate the productivity of this freshwater contingent of earth's plants. Clearly, therefore, much more work should be conducted in this under-researched area.

References
Andersen, T. and Andersen, F.O. 2006. Effects of CO2 concentration on growth of filamentous algae and Littorella uniflora in a Danish softwater lake. Aquatic Botany 84: 267-271.

Brown, J.S. 1988. Photosynthetic pigment organization in diatoms (Bacillariophyceae). Journal of Phycology 24: 96-102.

Logothetis, K., Dakanali, S., Ioannidis, N. and Kotzabasis, K. 2004. The impact of high CO2 concentrations on the structure and function of the photosynthetic apparatus and the role of polyamines. Journal of Plant Physiology 161: 715-724.

Muller, C., Reuter, W. and Wehrmeyer, W. 1993. Adaptation of the photosynthetic apparatus of Anacystis nidulans to irradiance and CO2-concentration. Botanica Acta 106: 480-487.

Plumley, F.G., Marinson, T.A., Herrin, D.L. Ideuchi, M. and Schmidt, G.W. 1993. Structural relationships of the photosystem I and photosystem II chlorophyll a/b and a/c light-harvesting apoproteins of plants and algae. Photochemistry and Photobiology 57: 143-151.

Plumley, F.G. and Smidt, G.W. 1984. Immunochemical characterization of families of light-harvesting pigment-protein complexes in several groups of algae. Journal of Phycology 20: 10.

Schippers, P., Lurling, M. and Scheffer, M. 2004a. Increase of atmospheric CO2 promotes phytoplankton productivity. Ecology Letters 7: 446-451.

Schippers, P., Vermaat, J.E., de Klein, J. and Mooij, W.M. 2004b. The effect of atmospheric carbon dioxide elevation on plant growth in freshwater ecosystems. Ecosystems 7: 63-74.

Xia, J. and Gao, K. 2003. Effects of doubled atmospheric CO2 concentration on the photosynthesis and growth of Chlorella pyrenoidosa cultured at varied levels of light. Fisheries Science 69: 767-771.

Last updated 18 October 2006