How does rising atmospheric CO2 affect marine organisms?

Click to locate material archived on our website by topic

Phytoplankton (Growth Response to CO2) -- Summary
The potential impacts of atmospheric CO2 enrichment on marine and freshwater phytoplankton have been little studied.  Nevertheless, when Wolf-Gladrow et al. (1999) reviewed the subject several years ago, they concluded that increases in the air's CO2 content likely would lead to significant increases in phytoplanktonic growth rates but decreases in biogenic calcification.  In this short summary, we briefly review what has been learned about these subjects over the following years.

Even though photosynthesis in many marine and freshwater phytoplankton appears to be saturated under current environmental conditions, Raven (1991) suggested that these species, many of which employ carbon-concentrating mechanisms, could well decrease the amount of energy they expend in this activity in a CO2-enriched world of the future, which metabolic adjustment would leave a larger proportion of their captured energy available for fueling enhanced growth.  To explore this possibility more thoroughly, Gordillo et al. (2003) studied the CO2-induced growth response of the microalgal chlorophyte Dunaliella viridis, which possesses a carbon concentrating mechanism and has been used as a model species for the study of inorganic carbon uptake.  Specifically, they batch-cultured the chlorophyte, which is one of the most ubiquitous eukaryotic organisms found in hypersaline environments, in 250-ml Perspex cylinders under controlled laboratory conditions at high (5 mM) and low (0.5 mM) nitrate concentrations, while continuously aerating the cultures with air of either 350 or 10,000 ppm CO2.  So what did they find?

Atmospheric CO2 enrichment had little effect on dark respiration in both N treatments.  Likewise, it had little effect on photosynthesis in the low-N treatment.  In the high-N treatment, on the other hand, the extra CO2 increased photosynthesis by 114%.  In the case of biomass production, the results were even more divergent: in the low-N treatment elevated CO2 had no effect at all, but in the high-N treatment it nearly tripled the cell density of the culture solution.

In discussing their findings, Gordillo et al. note that "it has long been debated whether phytoplankton species are growth-limited by current levels of CO2 in aquatic systems, i.e. whether an increase in atmospheric CO2 could stimulate growth (Riebesell et al., 1993)."  Their results clearly indicate that it can ... if sufficient nitrogen is available.  But that was not all that Gordillo et al. learned.  In the high-N treatment, where elevated CO2 greatly stimulated photosynthesis and biomass production, once the logarithmic growth phase had run its course and equilibrium growth was attained, approximately 70% of the carbon assimilated by the chlorophyte was released to the water, while in the low- CO2 treatment only 35% was released.

With respect to these latter observations, Gordillo et al. state that "the release of organic carbon to the external medium has been proposed as a mechanism for maintaining the metabolic integrity of the cell (Ormerod, 1983)," and that "according to Wood and Van Valen (1990), organic carbon release would be a sink mechanism protecting the photosynthetic apparatus from an overload of products that cannot be invested in growth or stored."  They additionally state that stores of photosynthetic products "are reduced to avoid overload and produce a high demand for photosynthates."  Under these conditions, they conclude that the process would "divert assimilated C to either the production of new biomass, or the release to the external medium once the culture conditions do not allow further exponential growth."  Interestingly, these ideas are closely analogous to the concept of CO2-induced acclimation in terrestrial plants, which is a very beneficial process that also provides for continued enhanced growth in high CO2 environments.

A second consequence of CO2-enhanced organic carbon release is that the internal C:N balance of the phytoplankton is maintained within a rather tight range.  Also of significance is the fact that this phenomenon has been observed in the green seaweed Ulva rigida (Gordillo et al., 2001) and the cyanobacterium Spirulina platensis (Gordillo et al., 1999).  Hence, what the study of Gordillo et al. (2003) reveals about the response of Dunaliella viridis to atmospheric CO2 enrichment may well be widely applicable to many, if not most, aquatic plants, not the least of which may be the zooxanthellae that by this means (enhanced organic carbon release) provide their coral hosts with the source of extra energy they need to continue building their skeletons at a non-reduced rate in the face of the negative calcification pressure produced by the changes in seawater chemistry that have been predicted to result from the ongoing rise in the air's CO2 concentration (see our Editorial of 25 Feb 2004).

Writing about still another concern, Yu et al. (2004) state that "oxidative stress is potentially experienced by all aerobic life when exposed to UV-B radiation," but they note that "elevated CO2 can enhance the capacity of plants to resist stress-induced oxidative damage," citing the study of Ren et al. (2001) who worked with terrestrial plants.  To see if this is also the case with marine phytoplankton, which they describe as "the single most important ecosystem on our planet," Yu et al. grew the marine microalgae Platymonas subcordiformis in the laboratory at ambient levels of atmospheric CO2 concentration and UV-B radiation flux density, as well as at elevated levels of 5000 ppm CO2 and UV-B radiation characteristic of that anticipated to result from a 25% stratospheric ozone depletion under clear-sky conditions in summer.

By itself, the Chinese scientists report that the elevated UV-B treatment "significantly decreased [microalgal] dry weight, photosynthetic rate, chlorophyll a and carotenoid contents," while the elevated CO2 treatment, by itself, "enhanced dry weight and photosynthetic rate, but chlorophyll a content and carotenoid content had no major difference compared with those of ambient UV-B and ambient CO2."  They also report that elevated UV-B, by itself, significantly increased the production of the toxic superoxide anion and hydrogen peroxide, as well as malonyldialdehyde, which is an end product of lipid peroxidation, whereas elevated CO2, by itself, did just the opposite.  In addition, in the treatment consisting of both elevated UV-B and elevated CO2, the concentrations of these three deleterious substances were lower than those observed in the elevated UV-B and ambient CO2 treatment.  Finally, they report that elevated CO2 decreased the levels of several antioxidative enzymes found in the microalgae, reflective of the algae's reduced need for detoxification of reactive oxygen species in the elevated CO2 treatment.

Yu et al. say their results suggest that "CO2 enrichment could reduce oxidative stress of reactive oxygen species to P. subcordiformis, and reduce the lipid peroxidation damage of UV-B to P. subcordiformis."  They also say that "CO2 enrichment showed a protective effect against the oxidative damage of UV-B-induced stress," and that "elevated CO2 can be [in] favor of enhancing the capacity of stress resistance."  Put more simply, they say in their concluding paragraph that "algae grown under high CO2 would better overcome the adverse impact of environmental stress factor[s] that act via generation of activated oxygen species."

In another study of atmospheric CO2 enrichment effects on phytoplanktonic productivity, Schippers et al. (2004a) grew the freshwater alga Chlamydomonas reinhardtii in 300-ml bottles filled with 150 ml of a nutrient-rich medium at enclosed atmospheric CO2 concentrations of 350 and 700 ppm, which they maintained at two air-water exchange rates characterized by CO2 exchange coefficients of 2.1 and 5.1 m day-1, as described by Shippers et al. (2004b), while periodically measuring the biovolume of the solutions by means of an electronic particle counter.

The three scientists report that their experimental results "confirm the theoretical prediction that if algal effects on C chemistry are strong, increased phytoplankton productivity because of atmospheric CO2 elevation should become proportional to the increased atmospheric CO2," which means, in their words, that "productivity would double at the predicted increase of atmospheric CO2 to 700 ppm."  Although they note that "strong algal effects (resulting in high pH levels) at which this occurs are rare under natural conditions," they still predict "a potential productivity increase of up to 40%, at observed pH levels for marine species with low affinity for HCO3-," and that effects on algal production in freshwater systems could potentially be larger, such that a "doubling of atmospheric CO2 may result in an increase of the productivity of more than 50%."

In a major review of what climate alarmists claim is a negative consequence of elevated atmospheric CO2 concentrations, 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," and 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."

As a hint of what the future might hold for calcifying organisms, however, Riebesell presented a preview of some 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 air of 190, 370 and 710 ppm CO2, simulating glacial, present-day, and predicted atmospheric CO2 concentrations 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 also reports that "CO2-related differences in primary production continued after nutrient exhaustion, leading to higher production of transparent exopolymer particles under high CO2 conditions," a phenomenon 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."

One last item in this Subject Index category concerns the paradox of the plankton, i.e., why the number of coexisting species is typically greater than what would be expected in light of the number of limiting resources in most oceanic mixed-water environments.  Attacking this question via a computer model study of sets of simultaneous equations describing the dependencies of various species of plankton upon different limiting resources, Hulsman and Weissing (1999) discovered that competition often induces a set of non-equilibrium species oscillations that readily allow for the coexistence of more species than would normally be expected on the basis of the number of limiting resources.  These results, in their words, show that "competition is not necessarily a destructive force," and that "competitive interactions that generate oscillations and chaos may allow for the persistence of a great diversity of competitors on only a few limiting resources."  And with the ongoing rise in the air's CO2 concentration stirring up the environmental mix even more, we might possibly see an even greater tendency for ecosystem biodiversity enhancement, particularly in environments of few and scarce resources.

In light of these several observations, we can only conclude that the ongoing rise in the air's CO2 concentration bodes well for the future vitality of the world's marine and freshwater phytoplankton, which also bodes well for everything above them in various marine and freshwater food chains.

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

Gordillo, F.J.L, Jimenez, C., Figueroa, F.L. and Niell, F.X.  1999.  Effects of increased atmospheric CO2 and N supply on photosynthesis, growth and cell composition of the cyanobacterium Spirulina platensis (Arthrospira).  Journal of Applied Phycology 10: 461-469.

Gordillo, F.J.L, Jimenez, C., Figueroa, F.L. and Niell, F.X.  2003.  Influence of elevated CO2 and nitrogen supply on the carbon assimilation performance and cell composition of the unicellular alga Dunaliella viridisPhysiologia Plantarum 119: 513-518.

Gordillo, F.J.L., Niell, F.X. and Figueroa, F.L.  2001.  Non-photosynthetic enhancement of growth by high CO2 level in the nitrophilic seaweed Ulva rigida C. Agardh (Chlorophyta).  Planta 213: 64-70.

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

Hulsman, J. and Weissing, F.J.  1999.  Biodiversity of plankton by species oscillations and chaos.  Nature 402: 407-410.

Ormerod, J.G.  1983.  The carbon cycle in aquatic ecosystems.  In: Slater, J.H., Whittenbury, R. and Wimpeny, J.W.T. (Eds.).  Microbes in Their Natural Environment.  Cambridge University Press, Cambridge, UK, pp. 463-482.

Raven, J.A.  1991.  Physiology of inorganic carbon acquisition and implications for resource use efficiency by marine phytoplankton: Relation to increased CO2 and temperature.  Plant, Cell and Environment 14: 774-794.

Ren, H.X., Chen, X. and Wu, D.X.  2001.  Effects of elevated CO2 on photosynthesis and antioxidative ability of broad bean plants grown under drought condition.  Acta Agronomica Sinica 27: 729-736.

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

Riebesell, U., Wolf-Gladrow, D.A. and Smetacek, V.  1993.  Carbon dioxide limitation of marine phytoplankton growth rates.  Nature 361: 249-251.

Schippers, P., Lurling, M. and Scheffer, M.  2004a.  Increase of atmospheric CO2 promotes phytoplankton productivity.  Ecology Letters 7: 446-451.

Schippers, P., Vermaat, J.E., de Klein, J. and Mooij, W.M.  2004b.  The effect of atmospheric carbon dioxide elevation on plant growth in freshwater ecosystems.  Ecosystems 7: 63-74.

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.

Wood, A.M. and Van Valen, L.M.  1990.  Paradox lost? On the release of energy rich compounds by phytoplankton.  Marine Microbial Food Webs 4: 103-116.

Yu, J., Tang, X-X., Zhang, P-Y., Tian, J-Y. and Cai, H-J.  2004.  Effects of CO2 enrichment on photosynthesis, lipid peroxidation and activities of antioxidative enzymes of Platymonas subcordiformis subjected to UV-B radiation stress.  Acta Botanica Sinica 46: 682-690.

Last updated 9 March 2005