Background
"It has been suggested," in the words of the authors, "that increases in CO2 may lead to an increase in algal biomass, which would in turn lead to more CO2 being removed from the atmosphere by these algae." In addition, because "oceanic primary production constitutes about 46% of the total primary production on earth (Field et al., 1998)," they say that "experiments examining how carbon uptake by microalgae responds to rising CO2 are needed to understand how oceanic primary production will change in the future."

What was done
Exploring this subject in a study that allowed for the occurrence of evolutionary changes in algal primary productivity that could possibly be induced by rising atmospheric CO2 concentrations, Collins et al. propagated ten replicate lines from each of two clones of the freshwater microalga Chlamydomonas reinhardtii within a phytotron by batch culturing them in flasks through which either air of 430 ppm CO2 was continuously bubbled or air of gradually increasing CO2 concentration was bubbled over the course of development of 600 generations of the microalga, at which point in time a concentration of 1050 ppm was reached and maintained throughout the development of 400 more algal generations. Each of these sets of plants (low-CO2-adapted and high-CO2-adapted) was then grown for a short period of time at both 430 and 1050 ppm CO2 and their steady-state CO2 uptake rates determined.

What was learned
For the algae whose atmospheric CO2 concentration had been continuously maintained at 430 ppm, abruptly increasing it to a value of 1050 ppm led to a 143% increase in steady-state CO2 uptake rate, while for the algae that had experienced the gradual CO2 increase from 430 to 1050 ppm, there was a 550% increase in CO2 uptake rate when the rate in the 1050-ppm air was compared to the rate that prevailed when the air's CO2 concentration was abruptly lowered to 430 ppm. For the algae experiencing the most realistic scenario of all, however, i.e., gradually going from a state of continuous 430-ppm CO2 exposure to one of 1050-ppm exposure over a period of 600 generations and then maintaining that higher CO2 level for a further 400 generations, the increase in steady-state CO2 uptake rate due to the long-term 620-ppm increase in atmospheric CO2 concentration was a more modest 50%, which roughly translates to a 25% increase in growth for the more typical 300-ppm increase in atmospheric CO2 concentration that is employed in numerous CO2 enrichment studies of terrestrial plants.

What it means
If the results obtained by Collins et al. for the freshwater Chlamydomonas reinhardtii are typical of what to expect of marine microalgae - which Field et al. suggest may provide nearly half of the primary production of the planet - the totality of earth's plant life may well provide a significant brake upon the rate at which the air's CO2 content may increase in the future, as well as the ultimate level to which it may rise. A rough indication of just how powerful this phenomenon may be is provided by Collins et al., when they note that "mathematical simulations have estimated that pre-industrial levels of CO2 would have been as high as 460 ppm" without the operation of the well-known "biological pump" (Sarmiento and Toggweiler, 1984), by which dying phytoplankton sink carbon into deep ocean sediments, "whereas pre-industrial atmospheric CO2 levels were [actually] around 280 ppm (Etheridge et al., 1996)," or 180 ppm less.

References
Etheridge, D.M., Steele, L.P., Langerfelds, R.L., Francey, R.J., Barnola, J.-M. and Morgan, V.I. 1996. Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn CO2. Journal of Geophysical Research 101: 4115-4128.

Field, C.B., Behrenfeld, M.J., Randerson, J.T. and Falkowski, P. 1998. Primary production of the biosphere: integrating terrestrial and oceanic components. Science 281: 237-240.

Sarmiento, J.L. and Toggweiler, J.R. 1984. A new model for the oceans in determining atmospheric pCO2. Nature 308: 621-624.