Plants grown in elevated atmospheric CO2 environments typically exhibit increased rates of photosynthesis and biomass production. Most of the studies that have established this fact have historically utilized CO2 concentration increases on the order of 300-400 ppm, which represents an approximate doubling of the air's current CO2 concentration; and they have been conducted on terrestrial plants. So what happens to aquatic plants if the air's CO2 concentration is super-enriched, to a value one to two (or even three) orders of magnitude more than it is currently? Are the consequences of the massive elevation of the atmosphere's CO2 concentration positive? Or are they negative? In what follows, we attempt to answer these questions by summarizing what we know about the subject via a brief review of pertinent scientific literature we have previously discussed on our website.
Kubler et al. (1999) grew a red seaweed common to the Northeast Atlantic intertidal zone, Lomentaria articulata, for three weeks in hydroponic cultures subjected to various atmospheric CO2 and O2 concentrations to determine the effects of these gases on growth. In doing so, they found that oxygen concentrations ranging from 10 to 200% of ambient had no significant effects on daily net carbon gain or total wet biomass production rates in this particular seaweed. In contrast, CO2 concentrations ranging from 67 to 500% of ambient had highly significant effects on these parameters. At twice the current ambient CO2 concentration, for example, daily net carbon gain and total wet biomass production rates were 52 and 314% greater than they were under ambient CO2 conditions. Likewise, Tisserat (2001) grew water mint (Mentha aquatica) plants for four weeks at ambient and enriched atmospheric CO2 conditions, finding that compared to plants exposed to air of 350 ppm CO2, those grown in air of 3,000 ppm CO2 produced 220% more fresh weight.
Noting that "the function and structure of the photosynthetic apparatus of many algal species resembles that of higher plants," 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)," but that "little is known about the changes to their photosynthetic apparatus during exposure to high CO2," Logothetis et al. (2004) sought to learn more about the latter aspect of the subject via an experiment wherein they grew batches of the unicellular green alga Scenedesmus obliquus in liquid culture medium for several days in a temperature-controlled water bath of 30°C at low (55 µmol m-2 s-1) and high (235 µmol m-2 s-1) light intensity, while the culture media were continuously percolated with air of either 300 or 100,000 ppm CO2. This work revealed that algal exposure to air of a whopping 100,000 ppm CO2 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 just five days under low light conditions, for example, the super-CO2-induced increase in the green alga's biomass was approximately 300%, while under high light conditions it was approximately 600%.
In search of a simple method for removing CO2 from high-CO2-concentration stack gases, Yue and Chen (2005) isolated and cultured a freshwater microalga of the genus Chlorella that they dubbed strain ZY-1, which they grew for periods of six days in vessels filled with growth media through which air of a variety of different CO2 concentrations was continuously bubbled. This protocol revealed that algal growth rates some 200% greater than those observed in ambient air were common at 100,000 ppm CO2. Thereafter, however, at higher CO2 concentrations, algal growth rates began to slowly decline; but they continued to remain greater than the growth rate observed in ambient air. Relative to that baseline, for example, the algal growth rate at 200,000 ppm CO2 was 170% greater, that at 300,000 ppm was 125% greater, and that at 500,000 ppm was about 40% greater. In addition, the two researchers report that similar results were obtained by Watanabe et al. (1992) for another Chlorella alga, by Hanagata et al. (1992) for both Chlorella and Scenedesmus species, and by Kodama et al. (1993) for the marine microalga Chlorococcum littorate.
In describing the significance of their findings, Yue and Chen say "the isolated ZY-1 strain can contribute to the simplification of a CO2 fixing system by microalgal cultivation" that may have an advantage over non-biological carbon scavenging systems in that "carbon fixed by microalgae is incorporated into carbohydrates and lipids, so energy, chemicals, or foods can be produced from microalgae biomass." On the further horizon, their research also presages the potential for genetic manipulation to enable higher land plants to also become more responsive to ultra-high CO2 concentrations.
Concentrating on isoetids (small slow-growing evergreen perennials that live submerged along the shores of numerous freshwater lakes and rely primarily on sediment-derived CO2 for their photosynthesis), Andersen et al. (2006) grew specimens of Littorella uniflora in sediment cores removed from Lake Hampen (Denmark) in 75-liter tanks with an overburden of 10 cm of filtered lake water for a period of 53 days, while measuring several plant, water and sediment properties, after which they harvested the plants and determined their biomass. Throughout the experiment, one set of plants had normal ambient air bubbled through the water covering the tank sediments, while an equivalent set of plants had a mixture of ambient air and CO2 (sufficient to lower the water's pH by one unit) bubbled through the tank water to simulate a 10-fold increase in atmospheric CO2 concentration. The three researchers report that the end result of these manipulations was that the ultra-CO2-enriched water led to an approximate 30% increase in plant biomass, as well as "higher O2 release to the sediment which is important for the cycling and retention of nutrients in sediments of oligotrophic softwater lakes."
Last of all, working with six 1.5-m-diameter flexible plastic cylinders placed in the littoral zone of the same Lake Hampen (three of which were maintained at ambient CO2 and three of which were enriched to ten times the ambient CO2 concentration), Anderson and Anderson (2006) measured the CO2-induced in situ growth response of a mixture of several species of filamentous freshwater algae (dominated by Zygnema species, but containing some Mougeotia and Spirogyra), as well as an isoetid community of macrophytes (dominated by Littorella uniflora, but containing some Myriophyllum alterniflorum and a few other species). After one full growing season (May to November), they determined that the ten-fold increase in aquatic CO2 enhanced the biomass production of Littorella uniflora by approximately 78%. Simultaneously, the biomass of filamentous algae was also enhanced by the elevated CO2: by 220% in early July, by 90% in mid-August, and by a whopping 3,750% in mid-November.
In considering the results of the several studies described above, it would appear that super-elevated atmospheric CO2 concentrations are not detrimental to freshwater and marine microalgae and macrophytes. In fact, they suggest that huge increases in aquatic CO2 concentration can sometimes lead to equally huge increases in aquatic plant growth.
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.
Andersen, T., Andersen, F.O. and Pedersen, O. 2006. Increased CO2 in the water around Littorella uniflora raises the sediment O2 concentration. Aquatic Botany 84: 294-300.
Hanagata, N., Takeuchi, T. and Fukuju, Y. 1992. Tolerance of microalgae to high CO2 and high temperature. Phytochemistry 31: 3345-3348.
Kodama, M., Ikemoto, H. and Miyachi, S. 1993. A new species of highly CO2-tolerant fast growing marine microalga suitable for high density culture. Journal of Marine Biotechnology 1: 21-25.
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.
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.
Tisserat, B. 2001. Influence of ultra-high carbon dioxide concentrations on growth and morphogenesis of Lamiaceae species in soil. Journal of Herbs, Spices & Medicinal Plants 9: 81-89.
Watanabe, Y., Ohmura, N. and Saiki, H. 1992. Isolation and determination of cultural characteristics of microalgae which functions under CO2 enriched atmosphere. Energy Conversion and Management 33: 545-552.
Yue, L. and Chen, W. 2005. Isolation and determination of cultural characteristics of a new highly CO2 tolerant fresh water microalgae. Energy Conversion and Management 46: 1868-1876.
Last updated 20 September 2006