How does rising atmospheric CO2 affect marine organisms?

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Algae -- Summary
How do algae respond to increases in the air's CO2 content?  This question has broad implications, in view of the fact that algae inhabit both freshwater and marine environments, as well as a number of terrestrial habitats, some as far removed from lakes and oceans as the cryptobiotic crusts of desert soils.  Nevertheless, the subject is notoriously under-researched, although the studies reviewed below provide a tantalizing picture of what the future may hold as the atmosphere's CO2 concentration continues to climb.

Working with cells of the freshwater alga Chlorella pyrenoidosa, Xia and Gao (2003) cultured them in Bristol's solution within controlled environment chambers maintained at low and high light levels (50 and 200 Ámol/m▓/s) during 12-hour light periods that were followed by 12-hour dark periods for a total of 13 days, while the solutions in which the cells grew were continuously aerated with air of either 350 or 700 ppm CO2.  When the cells were harvested (in the exponential growth phase) at the conclusion of this period, the biomass (cell density) of the twice-ambient CO2 treatment was found to be 10.9% and 8.3% greater than that of the ambient-air treatment in the low- and high-light regimes, respectively, although only the high-light result was statistically significant.  The two scientists concluded from these observations that a "doubled atmospheric CO2 concentration would affect the growth of C. pyrenoidosa when it grows under bright solar radiation, and such an effect would increase by a great extent when the cell density becomes high."  Their data also suggest that the same may be true when the alga grows under not-so-bright conditions.

In the case of marine algae, atmospheric CO2 enrichment also appears to be beneficial to growth.  Gao et al. (1993), for example, grew cultures of the red macroalgae Gracilaria sp. and G. chilensis in vessels enriched with nitrogen and phosphorus that were continuously aerated with normal air containing 350 ppm CO2, air enriched with an extra 650 ppm CO2, and air enriched with an extra 1250 ppm CO2 for a period of 19 days.  Compared to the control treatments, the relative growth enhancements in the + 650-ppm and +1250-ppm CO2 treatments were 20% and 60%, respectively, for G. chilensis, and 130% and 190%, respectively, for the Gracilaria sp.

With respect to these findings, the researchers comment that, "in their natural habitats, photosynthesis and growth of Gracilaria species are likely to be CO2-limited, especially when the population density is high and water movement is slow."  Hence, as the air's CO2 content continues to rise, these marine macroalgae should grow ever better in the years ahead.  Such should also be the case with many other macroalgae, for Gao et al. note that "photosynthesis by most macroalgae is probably limited by inorganic carbon sources in natural seawater," citing the studies of Surif and Raven (1989), Maberly (1990), Gao et al. (1991) and Levavasseur et al. (1991) as evidence for this statement.

Marine microalgae also appear to be benefited by elevated concentrations of atmospheric CO2 (Riebesell et al., 1993; Hein and Sand-Jensen, 1997; Wolf-Gladrow et al., 1999).  In a study of the unicellular marine diatom Skeletonema costatus, for example, which is widely distributed in coastal waters throughout the world and is a major component of most natural assemblages of marine phytoplankton, Chen and Gao (2004) grew cell cultures of the species in filtered nutrient-enriched seawater maintained at 20░C under a light/dark cycle of 12/12 hours at a light intensity of 200 μmol m-2 s-1, while continuously aerating the culture solutions with air of either 350 or 1000 ppm CO2 and measuring a number of physiological parameters related to the diatom's photosynthetic activity.  They report that cell numbers of the alga "increased steadily throughout the light period and they were 1.6 and 2.1 times higher after the 12 h light period for the alga grown at 350 and 1000 ppm CO2, respectively."  They also say that chlorophyll a concentrations in the bulk of the two CO2 cultures "increased 4.4- and 5.4-fold during the middle 8 h of the light period for the alga grown at 350 and 1000 ppm CO2, respectively," and that "the contents of cellular chlorophyll a were higher for the alga grown at 1000 ppm CO2 than that at 350 ppm CO2."  In addition, they report that the initial slope of the light saturation curve of photosynthesis and the photochemical efficiency of photosystem II "increased with increasing CO2, indicating that the efficiency of light-harvesting and energy conversion in photosynthesis were increased."  The end result of these several responses, in the words of the scientists who observed them, was that "S. costatum benefited from CO2 enrichment."

We come at last, then, to algae of terrestrial habitats, where we begin with the study of Brostoff et al. (2002), who measured net photosynthesis rates of cryptobiotic desert-surface crusts (composed of cyanobacterial algae living on dunes and playas of the Mojave Desert) over a range of atmospheric CO2 concentrations while maintaining light intensity, crust moisture content, and antecedent crust moisture content within ranges conducive to maximizing net photosynthesis.  Their measurements revealed that the photosynthetic rates of the crusts increased in linear fashion with increasing atmospheric CO2 to a concentration of at least 1000 ppm, which was the upper limit of reliability of the instrumentation used in their study.  Compared to contemporary atmospheric CO2 values, the scientists report that photosynthetic rates at 1000 ppm CO2 were doubled for the playa crusts and tripled for the dune crusts.

Although cryptobiotic crusts have often been overlooked in studies of desert ecosystems, Brostoff et al. say that the substantial photosynthetic rates they observed "reiterate the ecosystem-wide importance of their carbon fixation."  Specifically, they note that "the ability of the cryptobiotic crusts to take up CO2 at much higher than normal levels calls attention to their potentially important role in global warming studies."  Because the crusts reduce wind and water erosion (Evans and Johansen, 1999), help preserve soil moisture (Yair, 1990), and provide much-needed nitrogen for larger vascular plants by means of their nitrogen-fixing activities (Evans and Belnap, 1999), these inconspicuous little plants help to stabilize shifting desert sands and provide an opportunity for higher plants to gain a foothold in regions that without them would sometimes be virtually uninhabitable.  In addition, and in consequence of the benefits they produce, it can be appreciated that the CO2-induced increases in the crusts' photosynthetic rates may significantly enhance the abilities of deserts to sequester ever greater amounts of carbon as the air's CO2 content rises ever higher.

Truly, the impacts of simple algae on freshwater, marine and terrestrial ecosystems is of monumental importance; and that importance will likely grow ever larger as the air's CO2 content rises ever higher.

References
Brostoff, W.N., Sharifi, M.R. and Rundel, P.W.  2002.  Photosynthesis of cryptobiotic crusts in a seasonally inundated system of pans and dunes at Edwards Air Force Base, western Mojave Desert, California: laboratory studies.  Flora 197: 143-151.

Chen, X. and Gao, K.  2004.  Characterization of diurnal photosynthetic rhythms in the marine diatom Skeletonema costatum grown in synchronous culture under ambient and elevated CO2Functional Plant Biology 31: 399-404.

Evans, R.D. and Belnap, J.  1999.  Long-term consequences of disturbance on nitrogen dynamics in an arid ecosystem.  Ecology 80: 150-160.

Evans, R.D. and Johansen, J.R.  1999.  Microbiotic crusts and ecosystem processes.  Critical Reviews in Plant Sciences 18: 183-225.

Gao, K., Aruga, Y., Asada, K., Ishihara, T., Akano, T. and Kiyohara, M.  1991.  Enhanced growth of the red alga Porphyra yezoensis Ueda in high CO2 concentrations.  Journal of Applied Phycology 3: 355-362.

Gao, K., Aruga, Y., Asada, K. and Kiyohara, M.  1993.  Influence of enhanced CO2 on growth and photosynthesis of the red algae Gracilaria sp. and G. chilensisJournal of Applied Phycology 5: 563-571.

Hein, M. and Sand-Jensen, K.  1997.  CO2 increases oceanic primary production.  Nature 388: 988-990.

Levavasseur, G., Edwards, G.E., Osmond, C.B. and Ramus, J.  1991.  Inorganic carbon limitation of photosynthesis in Ulva rotundata (Chlorophyta).  Journal of Phycology 27: 667-672.

Maberly, S.C.  1990.  Exogenous sources of inorganic carbon for photosynthesis by marine macroalgae.  Journal of Phycology 26: 439-449.

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

Surif, M.B. and Raven, J.A.  1989.  Exogenous inorganic carbon sources for photosynthesis in seawater by members of the Fucales and the Laminariales (Phaeophyta): ecological and taxonomic implications.  Oecologia 78: 97-103.

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.

Yair, A. 1990. Runoff generation in a sandy area of the Nizzana Sands, western Negev, Israel.  Earth Surface Processes and Landforms 15: 597-609.

Xia, J. and Gao, K.  2003.  Effects of doubled atmospheric CO2 concentration on the photosynthesis and growth of Chlorella pyrenoidosa cultured at varied levels of light.  Fisheries Science 69: 767-771.