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The previously held notion that unlike terrestrial plants, submerged plants like algae will not show any response to an increase of atmospheric carbon dioxide. This view may be biased by a neglect of the effects of the plants themselves on the water chemistry [33]. If this effect is included, productivity may double due to a doubling of the atmospheric carbon dioxide concentration. In practice productivity increase will usually be less, however, under nutrient rich conditions, doubling of atmospheric carbon dioxide may result in a productivity increase up to 40% in saltwater species and up to 50% in freshwater species [34]. These results indicate that the carbon uptake by fresh and saltwater systems may increase more than ex-pected, and that nuisance algal blooms may be aggra- vated at elevated atmospheric carbon dioxide concentra- tions.
Moreover, plants grown in elevated atmospheric CO2 environments typically exhibit increased rates of photo-synthesis and biomass production [35]. Most of the stud-ies that have established this fact have historically util-ized 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 [36,37].
Andersen et al. [38] grew specimens of Littorella uniflora, one of the isoetids (small slow-growing evergreen peren-nials that live submerged along the shores of numerous freshwater lakes and rely primarily on sediment-derived CO2 for their photosynthesis) in sediment cores removed from Lake Hampen (Denmark) in 75-liter tanks. The end result of these experiments was that the ultra-CO2-en- riched water led to an approximate 30% increase in plant biomass. In a different study Anderson and Anderson [39] measured the CO2-induced in situ growth response of a mixture of several species of filamentous freshwater al- gae (dominated by Zygnema species, but containing some Mougeotia and Spirogyra), as well as an isoetid commu- nity 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 Lit- torella uniflora by approximately 78%. Simultaneously, the biomass of filamentous algae was also enhanced by Copyright © 2011 SciRes. JEP
650 Microalgae Tolerance to High Concentrations of Carbon Dioxide: A Review
the elevated CO2: by 220% in early July, by 90% in mid- August, and by a whopping 3,750% in mid-November.
A red seaweed common to the Northeast Atlantic in-tertidal zone, Lomentaria articulata, was grown for three weeks in hydroponic cultures subjected to various at-mospheric CO2 and O2 concentrations to determine the effects of these gases on growth [40]. 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 rang- ing 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 condi-tions. Likewise, Tisserat [41] 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.
Microalgae response to varying CO2 concentrations has been widely investigated. Chlorella vulgaris was cultivated under various light intensities in a gas recy-cling photobioreactor. The light intensity affected the algal growth and the CO2 concentration in the exit gas. In the linear growth phase, CO2 concentration in the exit gas ranged between 4.6% to 6.0% (v/v) when 20% (v/v) CO2 balanced with 80 % (v/v) N2 was introduced into the photobioreactor [42].
In search of a simple method for removing CO2 from high-CO2-concentration stack gases, Yue and Chen [43] isolated and cultured a freshwater microalga of the genus Chlorella for periods of six days in vessels filled with growth media through which air of a variety of different CO2 concentrations was continuously bubbled. Data re-vealed 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 concentra-tions, 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. Similar results were obtained by Watanabe et al. [44] for another Chlorella alga, by Hanagata et al. [15] for both Chlorella and Scenedesmus species, and by Kodama et al. [16] for the marine microalga Chlorococcum littorate.
Euglena cells were cultured to determine the maxi-mum CO2 elimination rate under a high concentration of CO2 with stirring of the supplied gas by bubbling. It was found that the maximum CO2 elimination rate or gas ex-change performance under a 10% concentration of CO2 was 2.3 times higher than 0.03% of CO2 concentration. The results suggest that the CO2 concentration in the supplied gas rate limits the algal system performance [45].
The effect of increased CO2 concentration on the growth rate of three planktonic algae (Chlamydomonas reinhard- tii, Chlorella pyrenoidosa, and Scenedesmus obliquus) enhanced significantly [46]. Specific growth rates reached maximal values at 30, 100, and 60 μM CO2 in C. rein- hardtii, C. pyrenoidosa, and S. obliquus, respectively. Such significant enhancement of growth rate with en- riched CO2 was also confirmed at different levels of in- organic N and P. The maximal rates of net photosynthe- sis, photosynthetic efficiency and light-saturating point increased significantly in high-CO2-grown cells. The au- thors concluded that increased CO2 concentrations with decreased pH could affect the growth rate and photosyn-thetic physiology of the three algae species.
In a different study [15] Chlorella species showed much higher log phase growth rates, while Scenedesmus species was better able to tolerate very high CO2 concen-trations than Chlorella. However, both algae had about the same growth rate when the CO2 concentration was in the range 10% - 30%. Scenedesmus was completely in-hibited by 100% CO2. This inhibition was reversible since growth was resumed when CO2 concentration was returned to 20%. Other microalgae species, Chlorella minutissima, grown under extreme carbon dioxide con-centrations (0.036% - 100%), strongly increase the mi-croalgal biomass through photochemical and non-photochemi- cal changes in the photosynthetic ap-paratus [47]. In conclusion, these extreme CO2 concen-trations—about 1,000 times higher than the ambient one—can be easily metabolized from the unicellular green alga to biomass and can be used, on a local scale at least, for the future development of microalgal photobio-reactors for the mitigation of the point source-produced carbon dioxide. Similar conclusion was derived by Prof. Shiraiwa’s Laboratory [48] that some microalgae could grow very rapidly at a CO2 concentration higher than 40%, those cells being referred to as extremely high-CO2 cells.
Yun, et al., [42] cultivated Chlorella vulgaris in waste- water discharged from a steel-making plant with the aim of developing an economically feasible system to remove ammonia from wastewater and CO2 from flue gas simul- taneously (since no phosphorus compounds existed in wastewater, external phosphate (15.3 - 46.0 g·m–3) was added to the wastewater). After adaptation to 5% (v/v) CO2, the growth of C. vulgaris was significantly im-proved at a typical concentration of CO2 in flue gas of 15% (v/v). Growth of C. vulgaris in raw wastewater was Copyright © 2011 SciRes. JEP
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better than that in wastewater buffered with HEPES at 15% (v/v) CO2. CO2 fixation and ammonia removal rates were estimated as 260 g CO2 m–3·h–1 and 0.92 g NH3 m–3·h–1, respectively, when the alga was cultivated in wastewater supplemented with 460 g PO4–3·m–3 without pH control at 15% (v/v) CO2.