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Do FACE Studies Significantly Underestimate the Growth Responses of Plants to Atmospheric CO2 Enrichment?
Holtum, J.A.M. and Winter, K.  2003.  Photosynthetic CO2 uptake in seedlings of two tropical tree species exposed to oscillating elevated concentrations of CO2Planta 218: 152-158.

Free-Air CO2 Enrichment (FACE) studies of the effects of elevated levels of atmospheric CO2 on the growth and development of plants have long been considered to be the most realistic experimental route to determining the likely biological consequences of the ongoing rise in the air's CO2 content, providing minimal alterations (it was thought!) to the natural environment of the plants being studied.  Now, however, this assumption is being called into question, as concerns are being raised about the possible physiological impacts of rapidly fluctuating CO2 concentrations that occur in response to the over- and under-shooting of targeted plot CO2 concentrations as the FACE apparatus continually adjusts to counteract the concentration-perturbing consequences of variations in wind speed and direction.

What was done
"In order to dispel uncertainty on the effects of short-term fluctuations in CO2 concentrations on carbon gain," Holtum and Winter say they "tested whether the responses of net CO2 exchange by seedlings or leaves of two tropical tree species, teak (Tectona grandis L. f.) and Pseudobombax septenatum (Jacq.) Dug., to an increase in CO2 concentration from ca. 370 to 600 ppm CO2 are affected by symmetric oscillations around 600 ppm, with half-cycles of considerably less than 1 minute."  The seedlings they studied were germinated from locally-collected seeds found in the vicinity of the Smithsonian Tropical Research Institute in the Republic of Panama and grown in pots with soil supplied with half-strength Johnson's solution (Winter, 1973) while maintained in environmentally-controlled growth chambers.

What was learned
In air of constant 600 ppm CO2 concentration, the net CO2 uptake rates of shoots and leaves of seedlings of T. grandis and P. septenatum increased by approximately 28 and 52%, respectively.  In the presence of CO2 oscillations with a half-cycle of 20 seconds and amplitude of 170 ppm about a mean of 600 ppm, however, the researchers report that "the CO2 stimulation in photosynthesis associated with a change in exposure from 370 to 600 ppm CO2 was reduced by a third in both species."

What it means
Holtum and Winter conclude their paper with these words: "our observations raise the possibility that FACE systems may underestimate the potential fertilizing effects of above-ambient CO2 concentrations on plants."  If their conclusion is correct, and their data suggest that it is, this finding, together with its significant magnitude, suggests that earth's biosphere may be far more responsive to the ongoing rise in the air's CO2 content than most of us have imagined.

Winter, K.  1973.  CO2-Fixierungsreaktoinen bei der Salzpflanze Mesembryanthemum crystailinum unter variierten Aussenbedingungen.  Planta 114: 75-85.

Reviewed 28 January 2004