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Responses of Marine Phytoplankton to Elevated Atmospheric CO2
Volume 8, Number 6: 9 February 2005

In introducing one of the more controversial subjects of global change research, Riebesell (2004) notes that "a doubling [of] present-day atmospheric CO2 concentrations is predicted to cause a 20-40% reduction in biogenic calcification of the predominant calcifying organisms, the corals, coccolithophorids, and foraminifera."  On the other hand, he notes that "a moderate increase in CO2 facilitates photosynthetic carbon fixation of some phytoplankton groups," including "the coccolithophorids Emiliania huxleyi and Gephyrocapsa oceanica."  Hence, in what appears to us to constitute a major challenge to the climate-alarmist claim that atmospheric CO2 enrichment will definitely harm such marine organisms, Riebesell suggests that "CO2-sensitive taxa, such as the calcifying coccolithophorids, should therefore benefit more [our italics] from the present increase in atmospheric CO2 compared to the non-calcifying diatoms."  An additional fact of importance, according to Riebesell, is that "the mechanism of calcification by coccolithophores is not completely understood."  This being the case, he says "it is too early ... to make any predictions regarding the physiological or ecological consequences of a CO2-related slow down in biogenic calcification."

Most significant of all, Riebesell presents a preview of some as-yet-unpublished results of CO2 perturbation experiments conducted south of Bergen, Norway, where nine 11-m3 enclosures moored to a floating raft were aerated in triplicate with CO2-depleted, normal, and CO2-enriched air to achieve CO2 levels of 190, 370 and 710 ppm, simulating glacial, present day, and predicted conditions for the end of the century, respectively.  In the course of the study, a bloom consisting of a mixed phytoplankton community developed; and, in Riebesell's words, "significantly higher net community production was observed under elevated CO2 levels during the build-up of the bloom."  He further reports that "CO2-related differences in primary production continued after nutrient exhaustion, leading to higher production of transparent exopolymer particles under high CO2 conditions," something that has also been observed by Engel (2002) in a natural plankton assemblage and by Heemann (2002) in monospecific cultures of both diatoms and coccolithophores.  These particles, according to Riebesell, "accelerate particle aggregation and thereby enhance vertical particle flux," which he says may "provide an efficient pathway to channel dissolved and colloidal organic matter into the particulate pool."

Another important finding of this experiment was the fact that the community that developed under the high CO2 conditions expected for the end of this century was dominated by Emiliania huxleyi.  Hence, Riebesell finds even more reason to believe that "coccolithophores may benefit from the present increase in atmospheric CO2 and related changes in seawater carbonate chemistry," in contrast to the many negative predictions that have been made about rising atmospheric CO2 concentrations in this regard.  Finally, in further commentary on the topic, Riebesell states that "increasing CO2 availability may improve the overall resource utilization of E. huxleyi and possibly of other fast-growing coccolithophore species," and that "if this provides an ecological advantage for coccolithophores, rising atmospheric CO2 could potentially increase the contribution of calcifying phytoplankton to overall primary production."

In spite of these several compelling observations, Riebesell says "it seems impossible at this point to provide a comprehensive and reliable forecast of large-scale and long-term biological responses to global environmental change," and that "any responsible consideration aiming to regulate or manipulate the earth system in an attempt to mitigate the greenhouse problem is presently hindered by the large gaps in our understanding of earth system regulation," implying (we presume) that proposed programs such as deep-ocean CO2 injection should not be implemented any time soon.  We agree; and we further suggest that this warning should also be applied to plans designed to regulate CO2 emissions, for there currently is no hard evidence from the real world of nature to suggest that calcifying organisms will be harmed by even a century-long continuation of the historical and still-ongoing rise in the air's CO2 content, while there is considerable evidence to suggest they may be benefited thereby (see Coral Reefs (Calcification) in our Subject Index).  Clearly, we need to learn considerably more about these topics before we embark upon what may well be an ill-advised "course correction" that could actually work against our best interests ... and those of the rest of the biosphere as well.

Sherwood, Keith and Craig Idso

References
Engel, A.  2002.  Direct relationship between CO2 uptake and transparent exopolymer particles production in natural phytoplankton.  Journal of Plankton Research 24: 49-53.

Heemann, C.  2002.  Phytoplanktonexsudation in Abhangigkeit der Meerwasserkarbonatchemie.  Diplom.  Thesis, ICBM, University of Oldenburg, Germany.

Riebesell, U.  2004.  Effects of CO2 enrichment on marine phytoplankton.  Journal of Oceanography 60: 719-729.