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How Do Microalgae Respond to Rising Carbon Dioxide in the Atmosphere?

Considering that microalgae can be found in a wide range of habitats, including freshwater and marine habitats, as well as in several land-based ecosystems, the above question has wide implications. Yet, very little research has been conducted on this topic. The studies that have been done provide some insight into the impacts that rising atmospheric CO2 levels may have on microalgae in the future.

Freshwater Algae
In a 2003 study(1) conducted on Chlorella pyrenoidosa, algal cells were cultured in Bristol's solution and exposed to both high and low levels of light over a 12-hour light period, followed by a 12-hour period of darkness, in environmentally controlled chambers for 13 days. The cultures were aerated by pumping air containing either 350ppm or 700ppm carbon dioxide into the solutions. After 13 days the algae was harvested (during the peak growth phase). The researchers found that biomass of algal cells exposed to high levels of CO2 were 8.3% and 10.9% greater than the biomass of algal cells exposed to lower concentrations of CO2 in both low-light and high-light scenarios respectively. The authors conclude that rising atmospheric CO2 levels would cause C. Pyrenoidosa cells to multiply rapidly (and thus grow rapidly) under high light conditions, and the same may apply when exposed to low-light conditions.

Marine Algae
Various studies (2,3,4) show that marine microalgae also benefit from higher atmospheric CO2 concentrations. In a similar study(5) conducted on Skeletonema costatum — a single-celled marine diatom that is ubiquitous in marine coastal ecosystems worldwide, and which forms a major component of naturally occurring marine phytoplankton — scientists cultured the diatom cells in nutrient-enriched filtered seawater at a controlled temperature of 20°C under a similar 12-hour light/dark cycle while continually aerating the cultures with air containing CO2 at concentrations of either 350ppm or 1000ppm, and monitoring the photosynthetic activity of the diatoms. Their results showed that diatom numbers increased at a steady rate when exposed to light, with algae grown at CO2 concentrations of 350ppm 1.6 times higher and algae grown at 1000ppm 2.1 times higher after being exposed to light for 12 hours. Chlorophyll-a concentrations (particularly cellular chlorophyll-a) were higher in diatoms cultured in solutions exposed to CO2 concentrations of 1000ppm than those cultured in solutions exposed to the lower 350ppm concentrations of CO2. They also reported that light-harvesting and energy conversion during the process of photosynthesis increases as CO2 levels increased. They conclude that S. costatum do indeed benefit from increased atmospheric CO2 concentrations.

Terrestrial Algae
A 2002 study (6) measured the rate of photosynthesis of cyanobacteria (blue-green algae) inhabiting desert sand dunes in the Mojave Desert when exposed to varying atmospheric CO2 concentrations under environmental conditions (light intensity, crust- and antecedent crust moisture content) that were conducive to high net photosynthetic rates. Their results show that photosynthetic rates of desert crust increased linearly as atmospheric CO2 increased, and at CO2 concentrations of 1000ppm photosynthetic rates of playa crusts doubled, while those of the dune crusts tripled.

According to the authors, the substantially high rates of photosynthesis observed in this study highlights the importance that desert crust dwelling algae play in carbon fixation at an ecosystem-wide level. They point out that the ability of desert crusts to absorb CO2 at such high levels draws attention to the potential they may offer in climate change studies.

Desert crusts play an important role in reducing soil erosion from both strong winds and water, help retain soil moisture, and provide nitrogen to desert plants due to their nitrogen-fixing activity. Consequently, these seemingly insignificant algal cells help stabilize desert dunes and in doing so they give higher vascular plants the opportunity to grow in areas that would be otherwise inhospitable.

In addition to the above benefits, due to their CO2 uptaking abilities, an increase in desert crust photosynthetic rates fueled by an increase in atmospheric CO2 concentrations may substantially enhance the carbon sequestration capabilities of deserts as atmospheric CO2 concentrations rise.

Microalgae play a vital and intricate role in freshwater, marine and terrestrial environments. It is clear that the importance of this role will become even more apparent as atmospheric CO2 levels continue to increase.

References & Further Reading

[1] 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
[2] Hein, M. and Sand-Jensen, K. 1997. CO2 increases oceanic primary production. Nature 388: 988-990
[3] Riebesell, U., Wolf-Gladrow, D.A. and Smetacek, V. 1993. Carbon dioxide limitation of marine phytoplankton growth rates. Nature 361: 249-251
[4] Wolf-Gladrow, D.A., Riebesell, U., Burkhardt, S. and Bijma, J. 1999. Direct effects of CO2 concentration on growth and isotopic composition of marine plankton. Tellus 51B: 461-476
[5] Chen, X. and Gao, K. 2004. Characterization of diurnal photosynthetic rhythms in the marine diatom Skeletonema costatum grown in synchronous culture under ambient and elevated CO2. Functional Plant Biology 31: 399-404
[6] Brostoff, W.N., Sharifi, M.R. and Rundel, P.W. 2002. Photosynthesis of cryptobiotic soil crusts in a seasonally inundated system of pans and dunes at Edwards Air Force Base, western Mojave Desert, California: laboratory studies. Flora 197: 143-151
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