How does rising atmospheric CO2 affect marine organisms?

Click to locate material archived on our website by topic

Effect of Atmospheric CO2 Enrichment on Isoprene Emissions from a Globally Ubiquitous Grass
Scholefield, P.A., Doick, K.J., Herbert, B.M.J., Hewitt, C.N.S., Schnitzler, J.-P., Pinelli, P. and Loreto, F.  2004.  Impact of rising CO2 on emissions of volatile organic compounds: isoprene emission from Phragmites australis growing at elevated CO2 in a natural carbon dioxide spring.  Plant, Cell and Environment 27: 393-401.

Isoprene is a highly reactive Non-Methane Hydrocarbon (NMHC) that is emitted in copious quantities by vegetation and is responsible for the production of vast amounts of ozone (Chameides et al., 1988; Harley et al., 1999).  It has been calculated by Poisson et al. (2000), for example, that current concentrations of NMHC emissions -- the vast majority of which are isoprene (more than twice as much as all other NMHCs combined) -- increase surface ozone concentrations by 50-60% over land and by as much as 40% over the world's oceans.  In addition, biogenic NMHCs (with isoprene being the most important) play a major role in the global tropospheric chemistry of methane, one of the atmosphere's most powerful greenhouse gases, boosting methane's atmospheric lifetime by approximately 14% above what it would be in the absence of isoprene (Poisson et al., 2000).

What was done
In a study designed to see if rising concentrations of atmospheric CO2 might alter plant emissions of isoprene and thereby impact atmospheric concentrations of ozone and methane over the world, Scholefield et al. measured isoprene emissions from Phragmites australis plants (one of the world's most important natural grasses) growing at different distances from (and elevations above) a natural CO2 spring (Bossoleto) located in central Italy near the village of Rapolano Terme in Tuscany.  At the specific locations they chose to study, atmospheric CO2 concentrations of approximately 350, 400, 550 and 800 ppm had likely prevailed for the entire lifetimes of the plants.

What was learned
As long-term atmospheric CO2 concentrations rose ever higher, plant isoprene emissions dropped ever lower: over the first 50-ppm increase in the air's CO2 concentration, they were reduced to approximately 65% of what they were at ambient CO2, while for CO2 increases of 200 and 450 ppm, they were respectively reduced to only about 30% and 7% of what they were in ambient-CO2 air, as best we can determine from the bar graph of the authors' relevant data.  These isoprene emission reductions were likely caused by reductions in isoprene synthase activity in the leaves of the Phragmites plants, which were observed to be highly correlated with them.

What it means
Scholefield et al. say their results suggest that "elevated CO2 generally inhibits the expression of isoprenoid synthesis genes and isoprene synthase activity which may, in turn, limit formation of every chloroplast-derived isoprenoid," noting that the "basal emission rate of isoprene is likely to be reduced under future elevated CO2 levels."  If other plants behave similarly, as much evidence seems to suggest (Monson and Fall, 1989; Loreto and Sharkey, 1990; Sharkey et al., 1991; Loreto et al., 2001; Rosenstiel et al., 2003), we can expect that the ongoing rise in the air's CO2 content will likely (1) dramatically boost plant productivity, (2) reduce the concentration and mitigate the deleterious consequences of one of earth's worst air pollutants, and (3) provide a means for reducing the atmospheric lifetime of one of the planet's most powerful greenhouse gases.

And CO2 is a substance some people fret about?  Go figure.

Chameides, W.L., Lindsay, R.W., Richardson, J. and Kiang, C.S.  1988.  The role of biogenic hydrocarbons in urban photochemical smog: Atlanta as a case study.  Science 241: 1473-1475.

Harley, P.C., Monson, R.K. and Lerdau, M.T.  1999.  Ecological and evolutionary aspects of isoprene emission from plants.  Oecologia 118: 109-123.

Loreto F. and Sharkey, T.D.  1990.  A gas exchange study of photosynthesis and isoprene emission in red oak (Quercus rubra L.).  Planta 182: 523-531.

Loreto, F., Fischbach, R.J., Schnitzler, J.-P., Ciccioli, P., Brancaleoni, E., Calfapietra, C. and Seufert, G.  2001.  Monoterpene emission and monoterpene synthase activities in the Mediterranean evergreen oak Quercus ilex L. grown at elevated CO2 concentrations.  Global Change Biology 7: 709-717.

Monson, R.K. and Fall, R.  1989.  Isoprene emission from aspen leaves.  Plant Physiology 90: 267-274.

Poisson, N., Kanakidou, M. and Crutzen, P.J.  2000.  Impact of non-methane hydrocarbons on tropospheric chemistry and the oxidizing power of the global troposphere: 3-dimensional modeling results.  Journal of Atmospheric Chemistry 36: 157-230.

Rosentiel, T.N., Potosnak, M.J., Griffin, K.L., Fall, R. and Monson, R.K.  2003.  Increased CO2 uncouples growth from isoprene emission in an agriforest ecosystem.  Nature 421: 256-259.

Sharkey, T.D., Loreto, F. and Delwiche, C.F.  1991.  High carbon dioxide and sun/shade effect on isoprene emissions from oak and aspen tree leaves.  Plant, Cell and Environment 14: 333-338.

Reviewed 19 May 2004